Badreddine Barhoumi

Marc Tedetti

Andrea Peris

Catherine Guigue

Òscar Aznar-Alemany

Soufiane Touil

Mohamed Ridha Driss

Ethel Eljarrat

Halogenated flame retardants in atmospheric particles from a North African coastal city (Bizerte, Tunisia): Pollution characteristics and human exposure

Keywords:

In the present study, atmospheric particle samples were collected between September 2015 and February 2016 from Bizerte city (Tunisia) to investigate the occurrence, potential sources and impacts of a range of flame retardants (FRs), including hexabromocyclododecane diastereoisomers (HBCDDs), new halogenated flame retardants (NHFRs), as well as methoxylated polybrominated diphenyl ethers (MeO-BDEs). Among 19 examined compounds, 9 congeners (α-HBCDD, β-HBCDD, γ-HBCDD, HBB, DBDPE, syn-DP, anti-DP, 2′-MeO-BDE-68 and 6-MeO-BDE-47) were detected in atmospheric particles at different levels and frequencies. Overall, the average concentration of NHFRs (1.53 pg m -3 ) was 1.5 and 51 folds higher than that of HBCDDs (1.04 pg m -3 ) and MeO-BDEs (0.03 pg m -3 ), respectively. By comparison with other areas of the world, NHFRs and HBCDDs in Bizerte were at medium pollution level, while MeO-BDEs were at lower levels. No significant correlations were found between NHFR, HBCDD and MeO-BDE concentrations, suggesting different sources related on one hand to the biogenic origin of MeO-BDE, and on the other hand to various types and utilisations of imported NHFR-and HBCDD-containing products, as Tunisia does not manufacture FRs. DP and HBCDD diastereoisomer profiles appeared to be divergent from their commercial products, pointing out that a complex degradation or stereoselective transformation processes occurring in ambient air around Bizerte city. The estimated daily human exposure dose (DED) to particle-bound HBCDDs, NHFRs and MeO-BDEs via outdoor air inhalation was generally at low levels. Hence, this study reports for the first time the occurrence and potential impact of HBCDDs, NHFRs and MeO-BDEs in atmospheric particles from North Africa.

Introduction

Flame retardants (FRs) include a diverse group of chemicals, which are added to consumer and industrial products, such as furnitures, electronic and electrical equipments, textiles, plastics, antifoaming agents, hydraulic fluids, construction materials, to prevent and minimise fire hazards [START_REF] Alaee | An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release[END_REF][START_REF] Covaci | Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour[END_REF][START_REF] Chen | Determination of HFRs and OPFRs in PM 2.5 by ultrasonic-assisted extraction combined with multi-segment column purification and GC-MS/MS[END_REF]. Polybrominated diphenyl ethers (PBDEs), as well as 1,2,5,6,9,10-hexabromocyclododecane diastereosomers (HBCDDs) (used mainly in building insulation) and tetrabromobisphenol A (TBBPA) have historically been the most widely used brominated flameretardants (BFRs) [START_REF] Covaci | Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour[END_REF]UNEP, 2015;[START_REF] Rauert | Global atmospheric concentrations of brominated and chlorinated flame retardants and organophosphate esters[END_REF]. However, PBDEs and HBCDDs have been banned worldwide and added to the Stockholm Convention on Persistent Organic Pollutants (POPs) due to their persistence, bioaccumulation, and toxicity [START_REF] Renner | Government Watch: in US, flame retardants will be voluntarily phased out[END_REF]. Since then, a variety of new halogenated flame retardants (NHFRs) have been introduced as replacements. For example, the commercial Penta-and Octa-BDE formulations were primarily replaced by firemaster 550 and 1,2-bis (2,4,6-tribromophenoxy) ethane (BTBPE), and Deca-BDE formulation by decabromodiphenylethane (DBDPE) [START_REF] Covaci | Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour[END_REF]. In addition, Dechlorane Plus (DP), a nonregulated, highly chlorinated FR, was suggested by the European Union as a possible replacement for the insecticide Mirex [START_REF] Hoh | Dechlorane plus, a chlorinated flame retardant, in the Great Lakes[END_REF] and Deca-BDE formulation [START_REF] Meire | Seasonal and altitudinal variations of legacy and current-use pesticides in the Brazilian tropical and subtropical mountains[END_REF]. A few monocyclic aromatic compounds, with lower production volumes, such as pentabromoethyl benzene (PBEB) and hexabromobenzene (HBB), have T OH, USA) equipped with a precombusted (450 °C, 6 h) quartz fiber filters (QFFs) (Whatman QMA grade,20.3 × 25.4 cm) was used to collect samples for 48 h at an average flow rate of 0.66 m 3 min -1 , resulting in a total sample volume of ~1900 m 3 for each sample.

After collection, QFFs were individually wrapped in a clean aluminum foil, transported to the laboratory and stored in a freezer at -20 °C before processing. They were conditioned in a desiccator (25 °C) for 24 h, weighted using a microbalance, and then cut into two equal parts. One half was used for HFR and MeO-BDE analysis and the other half for the determination of total suspended particles (TSP), organic carbon (OC) and organic nitrogen (ON). Details regarding the sampling dates, meteorological conditions and collected air volumes, as well as TSP, OC and ON concentrations for each sample, are given in the SI (Table S1).

Analytical procedures

Details on analytical procedures (chemicals, extraction and instrumental analysis) are presented in the SI (Text S1). Briefly, QFF filters were cut into small pieces, spiked with a multi-standard mixture containing internal standards for PBDEs ( 13 C- , dechloranes ( 13 C-syn-DP), and hexabromocyclododecane (d 18 α-HBCDD), and extracted using a fully automated accelerated solvent extraction system (ASE 350). The extracts were then purified using solid phase extraction (SPE), and depending on the analytes, the analysis of compounds was performed using:

-A liquid chromatography coupled to tandem mass spectrometry (LC-MS-MS) using electrospray ionisation (ESI), for HBCDDs. -A gas chromatography coupled to tandem mass spectrometry (GC-MS-MS) working with negative ion chemical ionisation (NICI), for dechloranes. -A gas chromatography coupled to mass spectrometry working with negative ion chemical ionisation (GC-NCI-MS), for DBDPE. -And an agilent 7890 gas chromatography coupled to an Agilent 7000 A GC/MS triple quadrupole working with electron ionisation (EI) (GC-EI-MS-MS), for PBEB, HBB and MeO-BDEs.

The concentrations of TSP in the QFFs were determined according to [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF] while OC and ON concentrations were determined by high temperature combustion (CHN analyzer) [START_REF] Raimbault | Distribution of inorganic and organic nutrients in the South Pacific Ocean. Evidence for long-term accumulation of organic matter in nitrogen-depleted waters[END_REF].

2.3. Quality assurance/quality control (QA/QC) QA/QC was conducted by analysing field and laboratory blanks, calibrations of the high volume air sampler, and evaluation of the analytical performance of the instruments, which includes reproducibility, detection limits and standard spiked recoveries. Three field blanks were collected for the whole sampling period by placing a QFF in the air sampler and immediately removing without drawing air through the sampler. Laboratory and field blanks were processed in the same way as the samples to determine the contamination introduced during extraction and clean-up, and by handling, shipping and storage, respectively. Laboratory blanks were included with each batch of 5 samples. All target compounds measured in this study were detected in the blanks (field and laboratory blanks) with amounts ranging from 0.07 to 1.17 pg (Table S3). The air sampler was regularly calibrated (each month) following the manufacturer's manual (Tisch Environmental, HIGH VOL + ). Multilevel calibration curves, covering the whole concentration ranges found in the samples, were used to quantify the analytes, and R 2 values > 0.95 were found for all of the analytes (Table S3). Quantification was carried out by isotopic dilution method based on the use of isotopically labeled standards. Table S2 shows the target compounds analysed in this study with each corresponding internal standard. A blank clean QFF was spiked with 50 μL of standards also found new FR applications during this "FR revolution" [START_REF] Ma | Has the phase-out of PBDEs affected their atmospheric levels? Trends of PBDEs and their replacements in the Great Lakes atmosphere[END_REF]. Large amounts of these NHFRs have been produced with growing annual demands due to the rapid economic development. Generally, they are used as additives rather than chemically bonded to the product matrix, which facilitates their release into the environment [START_REF] Ali | Brominated and organophosphate flame retardants in indoor dust of Jeddah, Kingdom of Saudi Arabia: implications for human exposure[END_REF]. As a result, NHFRs are consistently detected in different environmental media [START_REF] Guerra | Transfer of hexabromocyclododecane from industrial effluents to sediments and biota: case study in Cinca river (Spain)[END_REF][START_REF] Zhao | Gridded field observations of polybrominated diphenyl ethers and decabromodiphenyl ethane in the atmosphere of North China[END_REF][START_REF] Mekni | Occurrence of halogenated flame retardants in sediments and sea urchins (Paracentrotus lividus) from a North African Mediterranean coastal lagoon (Bizerte, Tunisia)[END_REF][START_REF] Reche | Particlephase concentrations and sources of legacy and novel flame retardants in outdoor and indoor environments across Spain[END_REF]. They are also detected regularly in Arctic [START_REF] Liu | Brominated flame retardants and dechlorane plus on a remote high mountain of the eastern Tibetan Plateau: implications for regional sources and environmental behaviours[END_REF][START_REF] Vorkamp | Novel brominated flame retardants and dechlorane plus in Greenland air and biota[END_REF] and Antarctic (Aznar-Alemany et al., 2019a), indicating their potential to undergo long-range atmospheric transport. Although there are limited data about most NHFRs, especially production volumes, usages, toxicity, and physicalchemical properties, many of them show properties of persistency and toxicity (hormonal disruption, reproductive effects) leading to concerns over environmental levels [START_REF] Abdelouahab | Polybrominated diphenyl ethers and sperm quality[END_REF][START_REF] Zhang | Novel flame retardants: estimating the physical-chemical properties and environmental fate of 94 halogenated and organophosphate PBDE replacements[END_REF][START_REF] Li | New halogenated flame retardants in the atmosphere of nine urban areas in China: pollution characteristics, source analysis and variation trends[END_REF].

Methoxylated brominated diphenyl ethers (MeO-BDEs), which are structurally related to PBDEs, may be transformation products of the latter [START_REF] Zeng | Development and validation of a congener-specific photodegradation model for polybrominated diphenyl ethers[END_REF] or of natural (marine) origin, produced by sponges, mollusks, cyanobacteria and algae [START_REF] Yu | Metabolites of 2,4,4′-tribrominated diphenyl ether (BDE-28) in pumpkin after in vivo and in vitro exposure[END_REF]. MeO-BDEs have gained attention in recent years because they have toxic properties (effect on mRNA abundance of steroidogenic enzymes in the H295R cell line) and display a greater biomagnification potential than PBDEs in aquatic food webs [START_REF] He | Effects of 20 PBDE metabolites on steroidogenesis in the H295R cell line[END_REF][START_REF] Wiseman | Polybrominated diphenyl ethers and their hydroxylated/methoxylated analogs: environmentalsources, metabolic relationships, and relative toxicities[END_REF].

There is no record of FR production in African countries and, therefore, the possible sources of these pollutants in the air are assumed to be the release from imported FR-containing products and the longrange atmospheric transport. Electrical and electronic e-waste recycling coupled with poor disposal techniques (such as open burning and dumping at poorly or non-constructed landfill sites, or even in surface water bodies) has demonstrated to be an important emission source of FRs in Africa [START_REF] Robinson | E-waste: an assessment of global production and environmental impacts[END_REF][START_REF] Schluep | Where Are WEEE in Africa[END_REF][START_REF] Katima | A review of brominated flame retardants in the environment with emphasis on atmospheric levels, knowledge and information gaps in the African continent[END_REF]. According to [START_REF] Baldé | The Global E-Waste Monitor -2014: Quantities, Flows and Resources[END_REF] the total electrical and electronic waste equipment generation in Africa was 1.9 Mt in 2014.

Africa is one of the least studied continents in the world with respect to air quality [START_REF] Laakso | Hot-air balloon as a platform for boundary layer profile measurements during particle formation[END_REF]. Previous works in the atmosphere of North Africa have been focused on PBDEs and organophosphate ester (OPE) flame retardants and plasticizers [START_REF] Castro-Jiménez | Occurrence, loading, and exposure of atmospheric particle-bound POPs at the African and European edges of the western Mediterranean Sea[END_REF][START_REF] Castro-Jiménez | Atmospheric particle-bound organophosphate ester flame retardants and plasticizers in a North African Mediterranean coastal city (Bizerte, Tunisia)[END_REF]. However, until now, there have been no published studies on halogenated FRs (HFRs), i.e., NHFRs, HBCDDs and MeO-BDEs in North Africa atmosphere. Hence, the objectives of this study were first to investigate the occurrence and pollution levels of HFRs and MeO-BDEs in atmospheric particles from Bizerte (Tunisia), a coastal, industrial-port city of North Africa. Secondly, to assess their potential sources and make a comparison with the levels of regulated PBDEs, and thirdly, to estimate their atmospheric dry deposition and human exposure risk. To our knowledge, this is the first report of atmospheric concentrations of MeO-BDEs for the entire African continent.

Materials and methods

Atmospheric particle sampling

Seventeen atmospheric particle samples were collected from September 2015 to February 2016 on the roof of the Faculty of Sciences of Bizerte (37° 16′ 0.5802″ N, 9° 52′ 49.875" E; 8 m above ground level), ~1 km far from the Bizerte city center, between the Mediterranean Sea and the Bizerte Lagoon (Fig. S1 in the supplementary information (SI)). Detailed descriptions of the sampling site, sample collection and pretreatment procedures have been given elsewhere [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF]. Briefly, Bizerte is a medium-sized city (~127,000 inhabitants), with a humid climate and economic activity focused mainly on agriculture, fishing and operations of light and heavy industries [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF], inducing a considerable anthropogenic pressure. A high volume air sampler (Tisch Environmental Inc.,

= × F V C d P M (1)
where V d is the deposition velocity and C PM is the volumetric concentration of HFRs and MeO-BDEs in atmospheric particles (pg m -3 ). V d is known to be variable, depending on particle size and meteorological conditions. There are no measured velocities available for these compounds for the Mediterranean Sea conditions. Based on recent works on the dry deposition of POPs and OPE [START_REF] Castro-Jiménez | Occurrence, loading, and exposure of atmospheric particle-bound POPs at the African and European edges of the western Mediterranean Sea[END_REF][START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF][START_REF] Castro-Jiménez | Atmospheric particle-bound organophosphate ester flame retardants and plasticizers in a North African Mediterranean coastal city (Bizerte, Tunisia)[END_REF] we used in the present study a constant velocity of 0.2 cm s -1 . The results from this calculation only provide indicative estimation of fluxes that should be eventually validated by experimental measurements. The daily human exposure dose (DED, pg kg -1 bw day -1 ) to particle-bound HFRs and MeO-BDEs via outdoor air inhalation was calculated using Eq. ( 2) proposed by the U.S. Environmental Protection Agency (USEPA) Exposure Factor Handbook (USEPA, 2011):

= × × × DED C IR ED AF/BW (2)
where C PM is the measured concentration of particle-bound HFRs/MeO-BDEs in each sample (pg m -3 ); IR is the inhalation rate (m 3 day -1 ); ED is the exposure duration per day (h day -1 ); AF is the absorption fraction (the inhalation fraction was assumed to be 100% bioavailable) and BW is the body weight of human (kg). A mean daily inhalation rate of 16.0 m 3 day -1 was employed for adults and 7.6 m 3 day -1 for children [START_REF] Zhang | Brominated flame retardants in atmospheric fine particles in the Beijing-Tianjin-Hebei region, China: spatial and temporal distribution and human exposure assessment[END_REF]. It was assumed that a person spends 6 h/day in an outdoor environment (i.e., 1/4 of the day) [START_REF] Deng | Phosphorus flame retardants and Bisphenol A in indoor dust and PM 2.5 in kindergartens and primary schools in Hong Kong[END_REF]. The average body weight was considered to be 60 kg for adults and 16 kg for children [START_REF] Liu | Occurrence and concentrations of halogenated flame retardants in the atmospheric fine particles in Chinese cities[END_REF][START_REF] Deng | Phosphorus flame retardants and Bisphenol A in indoor dust and PM 2.5 in kindergartens and primary schools in Hong Kong[END_REF].

Results and discussion

From a total 19 target analytes, 3 HBCDDs (α-HBCDD, β-HBCDD, γ-HBCDD), 4 NHFRs (HBB, DBDPE, syn-DP, anti-DP) and 2 MeO-BDEs (2′-MeO-BDE-68, 6-MeO-BDE-47) were detected in samples with detection frequency ranging between 94 and 100% (Table S4). The concentrations of ƩHFRs and ƩMeO-BDEs ranged from 0.80 to 5.97 pg m -3 and from nq to 0.14 pg m -3 , with a mean (median) of 2.57 (2.52) pg m -3 and 0.03 (0.01) pg m -3 , respectively (Table S4, Fig. 1 and Fig. S2). Overall, the mean concentration of NHFRs was 1.5 and 51 times higher than that of HBCDDs and MeO-BDEs, respectively (Fig. 1).

Concentrations and composition profiles

Hexabromocyclododecanes

HBCDDs in air were largely particle-bound and rarely detected in the gas phase because of their low vapor pressure and high affinity for particles [START_REF] Jo | Matrix-specific distribution and diastereomeric profiles of hexabromocyclododecane (HBCD) in a multimedia environment: air, soil, sludge, sediment, and fish[END_REF]. In the present study, except for one sample for which β-HBCDD was not detected, the detection frequency of the three HBCDDs was 100% in all atmospheric particle samples (Table S4). The mean (median) concentrations of α-, β-, and γ-HBCDD were 0.60 (0.61), 0.13 (0.13), and 0.29 (0.18) pg m -3 , respectively (Table S4 and Fig. 1). ƩHBCDDs ranged from 0.16 to 3.02 pg m -3 with a mean (median) of 1.04 (0.92) pg m -3 , which was comparable to the values from Guangzhou and Shenzhen, China, and Central Brno, Czech Republic [START_REF] Okonski | Particle size distribution of halogenated flame retardants and implications for atmospheric deposition and transport[END_REF]Yu et al., 2008a;[START_REF] Zhu | Airborne particle-bound brominated flame retardants: levels, size distribution and indoor-outdoor exchange[END_REF]. Our concentrations were lower than those from Beijing, Shanghai (Zhabei) and Dalian in China, but much higher than those from North-East Greenland, Denmark and Stockholm, Sweden (Table 1) [START_REF] Hu | Levels of polybrominated diphenyl ethers and hexabromocyclododecane in the atmosphere and tree bark from Beijing, China[END_REF][START_REF] Li | Levels, isomer profiles and chiral signatures of particle-bound hexabromocyclododecanes in ambient air around Shanghai, China[END_REF][START_REF] Li | Levels and gas-particle partitioning of hexabromocyclododecanes in the urban air of Dalian, China[END_REF][START_REF] Newton | Emerging flame retardants, PBDEs, and HBCDDs in indoor and outdoor media in Stockholm, Sweden[END_REF][START_REF] Vorkamp | Novel brominated flame retardants and dechlorane plus in Greenland air and biota[END_REF].

The HBCDD composition profile along the whole sampling period is shown in Fig. 2. Overall, αand γ-HBCDDs were the most abundant diastereomers, contributing on average about 59 and 29% to the (100 pg/μL for each HFR and 50 pg/μL for each MeO-BDEs), and processed as real samples. Recoveries from spiked QFF were in the range of 54-104% and 61-81% for HFRs and MeO-BDEs, respectively, being always within the range established as acceptable for isotopic dilution methods (40-120%). Reproducibility of the method was also satisfactory with relative standard deviations (RSD) < 20%. Method detection limits (MDLs) defined as mean blank value + 3 × standard deviation (SD) (considering an average sampled air volume of 806 m 3 ), ranged from 0.1 to 1.5 fg m -3 (Table S3). For compounds that were not detected in blanks, instrument detection limits (IDLs), determined as a signal-to-noise ratio (S/N) of 3, were substituted. If the concentration of a given compound in a sample was below its MDL/IDL, thus this compound was considered as not dectected in the sample, and was assumed zero for deriving sums and calculating fluxes. All data reported here were blank corrected.

Data handling

The targeted compounds analysed in this study were classified into two groups, the HFRs (PBEB, HBB, DBDPE, Dechlorane 602 (DDC-DBF), Dechlorane 603 (DDC-Ant), Dechlorane 604 (HCTBPH), syn-DP, anti-DP, α-HBCDD, β-HBCDD, γ-HBCDD) and MeO-BDEs (5-MeO-BDE-47, 6-MeO-BDE-47, 4′-MeO-BDE-49, 2′-MeO-BDE-68, 5′-MeO-BDE-99, 5′-MeO-BDE-100, 4′-MeO-BDE-101 and 4′-MeO-BDE-103), defined as ƩHFRs and ƩMeO-BDEs, respectively. Among HFRs, eight compounds (PBEB, HBB, DBDPE, DDC-DBF, DDC-Ant, HCTBPH, syn-DP, anti-DP) named new halogenated flame retardants are defined as ΣNHFRs (or NHFRs), and three compounds (α-HBCDD, β-HBCDD, γ-HBCDD) are defined as ΣHBCDDs (or HBCDDs), which are considered as discontinued FRs. Dechlorane Plus represents the sum of syn-DP and anti-DP, and is defined as ΣDPs (or DPs).

Box-and-whisker plots and Spearman rank order correlations were performed using graph prism 5 software. Mann-Whitney nonparametric tests (U-tests), conducted with Stat-View 5.0, were used to compare the distributions of two groups. The significance level for the result was set at p ≤ 0.05. Principal component analysis (PCA) was performed using XLSTAT 2013.5.01.

The dry deposition fluxes (F pg m -2 day -1 ) of HFRs and MeO-BDEs associated with particles were calculated as follows:

Table 1 Comparison of concentrations of ƩHBCDD concentrations, respectively. β-HBCDD was the less abundant with an average contribution of ~13%. In the HBCDD technical formulation, the diastereomer contributions is generally 75-89% for γ-HBCDD with αand β-accounting for 10-13% and 1-12%, respectively (Fig. 3) [START_REF] Covaci | Hexabromocyclododecanes (HBCDs) in the environment and humans: a review[END_REF]. Therefore, the composition profile observed here is clearly divergent from that of the initial technical formulation (Fig. 3). The diastereomers can undergo thermal rearrangements at temperatures above 160 °C, resulting in profiles containing 78% α-HBCDD, 13% β-HBCDD and 9% γ-HBCDD [START_REF] Covaci | Hexabromocyclododecanes (HBCDs) in the environment and humans: a review[END_REF], which is more in line with the profile detected in Bizerte city (Fig. 3).

Other authors [START_REF] Newton | Emerging flame retardants, PBDEs, and HBCDDs in indoor and outdoor media in Stockholm, Sweden[END_REF][START_REF] Drage | Concentrations of legacy and emerging flame retardants in air and soil on a transect in the UK West Midlands[END_REF][START_REF] Rauert | Global atmospheric concentrations of brominated and chlorinated flame retardants and organophosphate esters[END_REF][START_REF] Li | Levels and gas-particle partitioning of hexabromocyclododecanes in the urban air of Dalian, China[END_REF]Yu et al., 2008a;[START_REF] Zhu | Airborne particle-bound brominated flame retardants: levels, size distribution and indoor-outdoor exchange[END_REF]) also found a similar profile with a predominance of α-HBCDD. By contrast, [START_REF] Abdallah | Hexabromocyclododecanes and tetrabromobisphenol-A in indoor air and dust in Birmingham, UK: implications for human exposure[END_REF], [START_REF] Hu | Levels of polybrominated diphenyl ethers and hexabromocyclododecane in the atmosphere and tree bark from Beijing, China[END_REF] and [START_REF] Hoh | Brominated flame retardants in the atmosphere of the eastcentral United States[END_REF] reported higher γ-HBCDD contributions to ƩHBCDD levels in atmospheric samples from Birmingham (UK), Beijing (China) and Chicago (US) (89, 61 and 83%, respectively) (Fig. 3). In our study, the average ratio of the air concentration of α-HBCDD to γ-HBCDD (2.74) was comparable with that of decorative polystyrene (α/γ = 2.1), but were not similar to those of the commercial mixtures, where the γ isomer is dominant, or construction polystyrene (α/γ = 0.07) [START_REF] Kukučka | BFRs Distribution in Air of Selected Indoor Environments from the Czech Republic. Platform Presentation[END_REF]. These results suggest that thermal isomer conversion and photodegradation could be an explanation for the higher abundances of α-HBCDD encountered in the Bizerte urban air [START_REF] Koppen | On the thermally induced isomerisation of hexabromocyclododecane stereoisomers[END_REF][START_REF] Covaci | Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour[END_REF][START_REF] Okonski | Particle size distribution of halogenated flame retardants and implications for atmospheric deposition and transport[END_REF], which is also supported by the significant positive correlation (p < 0.001) between the concentrations of γ-HBCDD and α-HBCDD or β-HBCDD (Table S5).

According to the Bizerte climatic conditions, the sampling period (autumn and winter) is considered as cold and humid [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF]. Thereby, no clear seasonal differences or correlations with temperature were observed for ƩHBCDD concentrations (Fig. S2A and Table S5), which was consistent with the results reported by [START_REF] Drage | Concentrations of legacy and emerging flame retardants in air and soil on a transect in the UK West Midlands[END_REF] in Birmingham (UK) air.

Alternative/new FRs

From 8 NHFRs analysed, only 4 compounds were detected in all samples with the detection frequency of 100% (Table S4). ƩNHFRs ranged from 0.11 to 4.46 pg m -3 with a mean (median) concentrations of 1.53 (0.90) pg m -3 . anti-/syn-DP were the most predominating compounds (64% together, 0.54 and 0.43 pg m -3 , respectively) followed by DBDPE (25%, 0.38 pg m -3 ) and HBB (12%, 0.18 pg m -3 ) (Table S4, Figs. 1 and2). In comparison to other studies, the average level of HBB reported here is close to that observed by [START_REF] Saini | Flame retardants in urban air: a case study in Toronto targeting distinct source sectors[END_REF] in Toronto, Canada (0.24 pg m -3 ), [START_REF] Khairy | Selected organohalogenated flame retardants in Egyptian indoor and outdoor environments: levels, sources and implications for human exposure[END_REF] in Alexandria, Egypt (0.92 pg m -3 ) and by [START_REF] Ma | Has the phase-out of PBDEs affected their atmospheric levels? Trends of PBDEs and their replacements in the Great Lakes atmosphere[END_REF] in Chicago and Cleveland, US (0.67 and 0.81 pg m -3 , respectively). However, it was significantly lower than that reported by [START_REF] Kurt-Karakus | Polybrominated diphenyl ethers (PBDEs) and alternative flame retardants (NFRs) in indoor and outdoor air and indoor dust from Istanbul-Turkey: levels and an assessment of human exposure[END_REF] in Istanbul, Turkey (510 pg m -3 ). DBDPE concentrations were similar to those measured in Eagle Harbor (1.0 pg m -3 ) [START_REF] Venier | Flame retardants in the atmosphere near the Great Lakes[END_REF] and lower than that detected in Southern China (158 pg m -3 ) [START_REF] Tian | Atmospheric deposition of halogenated flame retardants at urban, e-waste, and rural locations in southern China[END_REF]. The mean DP concentrations in this study were similar to those observed in the atmosphere of the Canadian High Arctic (< 0.05-2.1 pg m -3 ) [START_REF] Möller | Large-scale distribution of dechlorane plus in air and seawater from the Arctic to Antarctica[END_REF], Spain (0.7 ± 0.5 pg m -3 ) [START_REF] Reche | Particlephase concentrations and sources of legacy and novel flame retardants in outdoor and indoor environments across Spain[END_REF] and in samples from Beijing (2.0 ± 3.9 pg m -3 ) and Dalian (3 ± 6 pg m -3 ) [START_REF] Yu | Particle-bound Dechlorane Plus and polybrominated diphenyl ethers in ambient air around Shanghai, China[END_REF], but up to 10 times lower than those measured in air samples from 97 Chinese sites (15 ± 15 pg m -3 ) [START_REF] Ren | Levels and isomer profiles of dechlorane plus in Chinese air[END_REF]. As shown in Table 1, the ƩNHFR concentrations measured in this study generally fell within the lowermiddle ranges of the levels reported in previous studies around the world.

The isomeric composition of DPs is a useful tool to track the transport and fate of these chemicals. The fractional abundance (f anti )is defined as the amount of anti-DP divided by the total amount of both DP isomers. The f anti values observed here ranged from nq to 0.79, with a mean of 0.41 ± 0.28 (Fig. S3), which is lower than those in the commercial DP products Anpon and OxyChem (0.64-0.80 and 0.60, respectively) [START_REF] Wang | An Asia-specific source of dechlorane plus: concentration, isomer profiles, and other related compounds[END_REF]. Considering the preferential atmospheric photodegradation of the anti-isomer and its isomerisation over the syn-isomer, our f anti values may reflect degradation/transformation processes that may occurr during long-or short-range atmospheric transport [START_REF] Möller | Large-scale distribution of dechlorane plus in air and seawater from the Arctic to Antarctica[END_REF][START_REF] Yang | Historically and currently used dechloranes in the sediments of the Great Lakes[END_REF].

Temporal variations of the ƩNHFR concentrations are showed in Fig. S2B. Like ƩHBCDDs, there were no notable seasonal (autumn versus winter) differences in levels of NHFRs. The highest ƩNHFR concentrations, recorded in December 2015 and January 2016, did not co-occur with HBCDD peak (September 2015). This emphasises differences in emission patterns and possibly source regions for NHFRs and HBCDDs [START_REF] Vorkamp | Novel brominated flame retardants and dechlorane plus in Greenland air and biota[END_REF].

Methoxylated bromodiphenyl ethers

Two of the eight measured MeO-BDEs were detected in all atmospheric particle samples of Bizerte city, with concentrations ranging from nq to 0.14 pg m -3 and a mean (median) of 0.03 (0.01) pg m -3 (Table S4, Fig. 1 and Fig. S2D). In the same region, Ben Ameur et al. (2013[START_REF] El Megdiche | Anthropogenic (PBDE) and naturallyproduced (MeO-PBDE) brominated compound levels in Bizerte Lagoon clams (Ruditapes decussatus): levels and human health risk assessment[END_REF] and [START_REF] Mekni | Occurrence of halogenated flame retardants in sediments and sea urchins (Paracentrotus lividus) from a North African Mediterranean coastal lagoon (Bizerte, Tunisia)[END_REF] have also detected 6-MeO-BDE-47 and 2′-MeO-BDE-68 are the most abundant MeO-BDEs in fishes (Solea solea), clams (Ruditapes decussatus) and sea urchins (Paracentrotus lividus) from the Bizerte Lagoon. These two compounds have also been the most frequently observed MeO-BDEs in marine environment [START_REF] Barón | Bioaccumulation and biomagnification of classical flame retardants, related halogenated natural compounds and alternative flame retardants in three delphinids from Southern European waters[END_REF]Aznar-Alemany et al., 2019b;[START_REF] Routti | Polybrominated diphenyl ethers and their hydroxylated analogues in ringed seals (Phoca hispida) from Svalbard and the Baltic Sea[END_REF], where those have been found to be naturally produced by the metabolic activities of marine sponges, bacteria or red algae in the presence of bromine, bromoperoxidase and hydrogen peroxide [START_REF] Malmvarn | Hydroxylated and methoxylated brominated diphenyl ethers in the red algae Ceramium tenuicorne and blue mussels from the Baltic Sea[END_REF][START_REF] Nomiyama | Organohalogen compounds and their metabolites in the blood of Japanese amberjack (Seriola quinqueradiata) and scalloped hammerhead shark (Sphyrna lewini) from Japanese coastal waters[END_REF]. Interestingly, in the Baltic region, the highest levels of MeO-BDEs have been recorded in summer, and the lowest in winter [START_REF] Löfstrand | Seasonal variations of hydroxylated and methoxylated brominated diphenyl ethers in blue mussels from the Baltic Sea[END_REF][START_REF] Dahlberg | Hydroxylated and methoxylated polybrominated diphenyl ethers in long-tailed ducks (Clangula hyemalis) and theirmain food, Baltic blue mussels (Mytilus trossulus × Mytilus edulis)[END_REF], leading to the assumption that phytoplanktonic production is the main source of MeO-BDEs [START_REF] Löfstrand | Seasonal variations of hydroxylated and methoxylated brominated diphenyl ethers in blue mussels from the Baltic Sea[END_REF]. [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF] also found that the highest levels of the biogenic n-alkanes in the atmospheric particle samples of Bizerte city were observed in spring and summer due to the algae bloom occurring at this period in Bizerte Lagoon [START_REF] Grami | The plankton food web of the Bizerte Lagoon (South-western-Mediterranean): II. Carbon steady-state modelling using inverse analysis[END_REF]. In the same way, the MeO-BDE peak in September 2015 (Fig. S2D) could be related to phytoplankton activity occurring in the Bizerte Lagoon and/or marine coastal waters in late summer. Since our samples were obtained in fall and winter, we may assume that higher levels of MeO-BDEs could be detected during spring and summer, which warrants further investigations.

Data on MeO-BDEs, especially in the atmosphere, are extremely limited. [START_REF] Kim | Hydroxylated, methoxylated, and parent polybrominated diphenyl ethers (PBDEs) in the Inland environment, Korea, and potential OHand MeO-BDE source[END_REF] reported a higher mean concentration (6.9 pg m -3 ) in atmospheric particle samples from Busan (South Korea). On the other side, [START_REF] Bidleman | Sea-air exchange of bromoanisoles and methoxylated bromodiphenyl ethers in the Northern Baltic[END_REF] determined comparable levels (mean of 0.03 pg m -3 ) in Island Holmön (Northern Baltic) (Table 1). It is worth noting that the comparison of the HBCDD, NHFR and MeO-BDE concentrations recorded in this study with those reported in literature must be taken with caution, as the meteorological conditions, the number of samples collected, and the number of compounds analysed may not be the same.

Influence of air mass origin and meteorological parameters

The PCA loading and score plots of the whole dataset are shown in Fig. 4. The target chemicals were clearly separated into three clusters: HBCDDs were clustered together in top right corner of the plot, while NHFRs and MeO-BDEs were clustered in lower-right and upper-left of the plot, respectively (Fig. 4a). This may suggest that HBCDD, NHFR and MeO-BDE individuals originated from different sources or have different fates [START_REF] Deng | Phosphorus flame retardants and Bisphenol A in indoor dust and PM 2.5 in kindergartens and primary schools in Hong Kong[END_REF]. This result is consistent with that of the Spearman's rank correlation (Table S5). The weak correlation between HBCDDs, NHFRs and MeO-BDEs was expected due to their various physical-chemical properties and applications. The strong Fig. 3. Comparison between the HBCDD diastereomer profiles reported in Bizerte city (this work) with those of technical formulation, thermal rearrangements and other atmospheric studies.

Fig. 4. Spearman principal component analysis loading (A) and scores (B) plots for the first two components of TSP, OC, ON, HBCDDs, NHFRs, MeO-BDEs and meteorological (AT, WS, RH, P) variables for the atmospheric particle samples of Bizerte city. Green and blue colors indicate autumn and winter seasons, respectively.

Comparison of atmospheric PBDE and NHFR concentrations

DBDPE was introduced to the market as alternative/replacement for the discontinued BDE-209 [START_REF] Kierkegaard | Identification of the flameretardant decabromodiphenyl ethane in the environment[END_REF]. In order to evaluate the PBDE response to their bans by the EU, we compared DBDPE concentrations measured in this study with those of BDE-209 measured by [START_REF] Castro-Jiménez | Occurrence, loading, and exposure of atmospheric particle-bound POPs at the African and European edges of the western Mediterranean Sea[END_REF] in atmospheric particles collected from the same area and same period (Fig. S5). BDE-209 concentration was significantly higher (U test, p < 0.05) than DBDPE concentration (10 times higher on average), and no significant correlation was found between these two compounds, pointing out that the use of DBDPE was still low in Bizerte city during the sampling period (2015-2016) compared to the use of BDE-209. [START_REF] Mekni | Occurrence of halogenated flame retardants in sediments and sea urchins (Paracentrotus lividus) from a North African Mediterranean coastal lagoon (Bizerte, Tunisia)[END_REF] found a predominance of DBDPE over BDE-209 (BDE-209/DBDPE: < MDL-0.85) in sediment samples collected from the Bizerte Lagoon. The strongest degradation and debromination of BDE-209 in sediment relative to atmosphere can explain the predominance of DBDPE in sediments [START_REF] Raff | Deposition versus photochemical removal of PBDEs from lake superior air[END_REF][START_REF] Demirtepe | Degradation of decabromodiphenyl ether (BDE-209) in microcosms mimicking sediment environment subjected to comparative bioremediation strategies[END_REF].

Estimation of particle dry deposition fluxes and human exposure via inhalation

Dry particle deposition is a continuous mechanism of the input of particle-bound contaminants into the global ocean during periods without precipitation [START_REF] Dachs | Oceanic biogeochemical controls on global dynamics of persistent organic pollutants[END_REF]. For the study period, the dry deposition fluxes of ƩHBCDDs, ƩNHFRs and ƩMeO-BDEs ranged from 28.4 to 521 pg m -2 day -1 , 18.5-771 pg m -2 day -1 and from nq to 24.5 pg m -2 day -1 , with a mean (median) of 265 (155) pg m -2 day -1 , 179 (158) pg m -2 day -1 and 5.01 (1.46) pg m -2 day -1 , respectively (Fig. 5 and Table S6). Dry deposition fluxes of ƩHBCDDs and ƩNHFRs were slightly higher in winter than in autumn, while ƩMeO-BDEs were slightly higher in autumn. The seasonal fluctuation can be explained by the different ambient concentrations, phytoplankton activities and meteorological conditions (i.e., relative humidity, wind speed, temperature, and atmospheric stability) [START_REF] Tasdemir | Measurement of particle phase dry deposition fluxes of polychlorinated biphenyls (PCBs) with a water surface sampler[END_REF][START_REF] Löfstrand | Seasonal variations of hydroxylated and methoxylated brominated diphenyl ethers in blue mussels from the Baltic Sea[END_REF]. Very limited data are available for comparison with the HBCDDs, NHFRs and MeO-BDEs deposition fluxes measured in this study. Atmospheric dry deposition fluxes of ƩHBCDDs estimated in this work are similar to those reported in rural sites of Southern China (mean of 193 ng m -2 yr -1 ) [START_REF] Tian | Atmospheric deposition of halogenated flame retardants at urban, e-waste, and rural locations in southern China[END_REF], but 12 times lower than those estimated in an urban site in central Brno, Czech Republic (mean of 2.1 ng m -2 day -1 ) [START_REF] Okonski | Particle size distribution of halogenated flame retardants and implications for atmospheric deposition and transport[END_REF]. This may be due to significant correlation between syn-DP and anti-DP (r = 0.777, p < 0.001) and between HBCDD diastereoisomers (r = 0.820-0.870, p < 0.001) can be explained by their similar sources, which is obvious given their similar use, origin and chemical properties [START_REF] Saini | Flame retardants in urban air: a case study in Toronto targeting distinct source sectors[END_REF]. No evident temporal separation was observed from PCA score plots for most of samples (Fig. 4b). Relatively high scores of PC1 were found in some samples collected in autumn and winter, indicating that HBCDDs and NHFRs were dominant during both seasons.

The influence of each meteorological parameter on the levels of HBCDDs, NHFRs and MeO-BDEs measured in this study was assessed. Individual HBCDDs and NHFRs exhibited negative correlations with ambient temperature (AT) and precipitation (P), and positive correlations with relative humidity (RH) (Table S5 and Fig. 4). Inversely, MeO-BDEs had positive correlations with AT, and significant negative correlations with RH, suggesting that HBCDDs and NHFRs had high affinity for particles, while MeO-BDEs had high affinity to gaseous phase. Similar results were highlighted by [START_REF] Jo | Matrix-specific distribution and diastereomeric profiles of hexabromocyclododecane (HBCD) in a multimedia environment: air, soil, sludge, sediment, and fish[END_REF] and [START_REF] Löfstrand | Seasonal variations of hydroxylated and methoxylated brominated diphenyl ethers in blue mussels from the Baltic Sea[END_REF]. [START_REF] Masiol | Seasonal trends and spatial variations of PM10-bounded polycyclic aromatic hydrocarbons in Veneto Region, Northeast Italy[END_REF] shows that low temperatures may stimulate the transfer of pollutants from the gaseous to particulate phase, and inversely, while RH and precipitation promote pollutant condensation on particulate matter and deposition. Some positive and negative correlations appeared between individual HBCDDs/NHFRs/MeO-BDEs, and TSP, OC and ON (Table S5). This indicates that the levels of HBCDDs, NHFRs and MeO-BDEs in the Bizerte atmosphere were not driven by a single meteorological parameter, but by the combined effect of several meteorological conditions.

To investigate the influence of local atmospheric transport on atmospheric concentrations of HFRs and MeO-BDEs in Bizerte city, wind sector was applied (Fig. S4). As shown in Fig. S4, the dominant wind directions were from the West-North-West (WNW) and the North-West (NW) in autumn, and the South-East (SE) in winter. This suggests that local emissions from downtown Bizerte (high urbanisation), lead and cement factories, located NW of the site, and from Zarzouna city (municipal waste incineration) and Menzel Jemil city (mechanic and electronic factories, tire, plastic and textile industries), located SE of the site, are the most likely to influence the pollutant levels observed here. By using the NOAA HYSPLIT Backward Trajectory Model, [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF] found that the influence of inter-continental atmospheric transport was minor in the same area and same period, since the air masses reaching the sampling site were of oceanic origin with limited passage through the European continent. Indeed, the clean nature of the oceanic air masses and high wind speeds above the Mediterranean Sea can disperse pollutants before they reach the sampling site. The negative coorelations between wind speed (WS) and all individual HFRs and MeO-BDEs concentrations (Table S5), suggested the predominance of local sources, since strong winds flush pollutants out of the study area whereas weak winds allow pollutants to accumulate over time [START_REF] Barhoumi | Levels and risk assessment of hydrocarbons and organochlorines in aerosols from a North African coastal city (Bizerte, Tunisia)[END_REF].

The presence of HBCDDs in the air can be attributed to a variety of sources including, plastics, installed insulation boards in buildings, textiles, and municipal and hospital waste incineration [START_REF] Lassen | Brominated Flame Retardants Substance Flow Analysis and Assessment of Alternatives[END_REF][START_REF] Morf | Dynamic substance flow analysis as a valuable risk evaluation tool: a case study for brominated flame retardants as an example of potential endocrine disrupters[END_REF]. DPs is mainly used for coating electrical wires and cables, connectors in computers, and plastic decoration materials [START_REF] Qiu | Dechlorane Plus and other flame retardants in a sediment core from Lake Ontario[END_REF], which are widely used in urban areas. HBB is used mainly as an additive FR in papers, wood, textiles, electronics and plastics [START_REF] Li | New halogenated flame retardants in the atmosphere of nine urban areas in China: pollution characteristics, source analysis and variation trends[END_REF]. DBDPE is added to various polymeric materials, such as high-impact polystyrene, polypropylene acrylonitrile butadiene styrene (ABS), which is used mainly in televisions, computers and cable boxes [START_REF] Covaci | Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour[END_REF][START_REF] Pei | A Dissertation Submitted to Shanghai University for the Degree of Master in Engineering[END_REF]. In Tunisia, there is no FR manufacturing plant. Therefore, based on these applications, we assumed that air transportation from local sources and release from FRcontaining products are the most likely sources of particulate atmospheric HFRs in Bizerte city. different particle-bound concentrations of HBCDDs and different V d values used for the estimation. [START_REF] Xie | Brominated flame retardants in seawater and atmosphere of the Atlantic and the Southern Ocean[END_REF] reported for the HBB a dry deposition flux of 0.5 pg m -2 day -1 (median value) in atmosphere of the Atlantic and the Southern Ocean, which was lower than that in the present study (median value of 12.49 pg m -2 day -1 ). Dry deposition flux of DBDPE (mean of 65.25 pg m -2 day -1 ) in Bizerte city was significantly lower than those measured in rural sites of Southern China (mean of 850 ng m -2 yr -1 ) [START_REF] Tian | Atmospheric deposition of halogenated flame retardants at urban, e-waste, and rural locations in southern China[END_REF]. The total dry deposition fluxes of DPs were in the range of 28.32-485.22 pg m -2 day -1 , with an average value of 178.80 ± 149.30 pg m -2 day -1 , which is close to those observed by [START_REF] Okonski | Particle size distribution of halogenated flame retardants and implications for atmospheric deposition and transport[END_REF] in central Brno, Czech Republic (mean of 0.07 ng m -2 day -1 ), but significantly lower than that detected in urban areas of Southern China (14.5-83.77 ng m -2 day -1 ) [START_REF] Tian | Atmospheric deposition of halogenated flame retardants at urban, e-waste, and rural locations in southern China[END_REF][START_REF] Wu | Atmospheric deposition of PBDEs and DPs in dongjiang river basin, south China[END_REF]. Additionally, anti-DP was usually the less abundant isomer, contributing to 43% of the total DP deposition fluxes, suggesting that the anti-isomer is less persistent in the atmosphere than the syn-isomer. Considering the surface of the Bizerte Lagoon as 1.28 × 10 8 m 2 [START_REF] Barhoumi | Occurrence of polycyclic aromatic hydrocarbons (PAHs) in mussel (Mytilus galloprovincialis) and eel (Anguilla anguilla) from Bizerte lagoon, Tunisia, and associated human health risk assessment[END_REF], approximately 0.01 kg yr -1 of ƩHBCDDs/ƩNHFRs and 2.34 × 10 -4 kg yr -1 of ƩMeO-BDEs can be deposited to surface waters through atmospheric particles.

The human body can be exposed to FRs through various exposure pathways, of which breathing is thought to be important [START_REF] Li | Levels and gas-particle partitioning of hexabromocyclododecanes in the urban air of Dalian, China[END_REF]. Therefore, the daily exposure doses of children and adults to particle-bound HFRs and MeO-BDEs via outdoor air inhalation were estimated and are summarised in Fig. 6 and Table S7. The mean DED values for HBCDDs, NHFRs and MeO-BDEs were 0.07, 0.10 and 0.002 pg kg -1 bw day -1 for adults, respectively, and 0.12, 0.18 and 0.003 pg kg -1 bw day -1 for children, respectively. When the estimated exposures of DBDPE and were compared with corresponding reference dose (RfD) [START_REF] Ali | Occurrence of alternative flame retardants in indoor dust from New Zealand: indoor sources and human exposure assessment[END_REF], they were several orders of magnitude lower than the RfD for both children and adults (Table S7), indicating insignificant risk to local population. Limited data of DED are available in literature for particle-bound FRs. [START_REF] Reche | Particlephase concentrations and sources of legacy and novel flame retardants in outdoor and indoor environments across Spain[END_REF] found DED values for adults ranging from 2 × 10 -4 -8 × 10 -4 , and from nd-9 × 10 -4 ng kg -1 bw day -1 for DP and DBDPE, respectively; which were 5-15 folds higher than the DED values reported here. It is worth noting that DED values were calculated based on outdoor sources only. However HFRs are known to have higher concentrations in indoor than outdoor environments [START_REF] Melymuk | Seasonality and indoor/outdoor relationships of flame retardants and PCBs in residential air[END_REF]. Therefore the estimated DED must be taken with caution.

Conclusion

This study provided valuable baseline data on the occurrence, potential sources and impacts of HBCDDs, NHFRs and MeO-BDEs in a North African coastal city (Bizerte). Nine compounds (α-HBCDD, β-HBCDD, γ-HBCDD, HBB, DBDPE, syn-DP, anti-DP, 2′-MeO-BDE-68 and 6-MeO-BDE-47) were detected in all of the air samples. Tunisia does not manufacture FRs, therefore, their occurrence may have come from imported FR-containing products, which end up in landfills after their utilisation. sector and air mass backward trajectories suggested that the sources of most FRs were mainly originated from local emissions. No obvious autumn/winter variations were observed for most of the compounds and the estimated DED indicating insignificant risk to local population as a result of their exposure to these pollutants through inhalation. Further research is needed to elucidate precisely the potential sources and impacts of these new organic contaminants in the study area.

Fig. 1 .

 1 Fig. 1. Concentrations (pg m -3 ) of HBCDDs, NHFRs and MeO-BDEs in atmospheric particles of Bizerte city as shown in box-and-whisker plots across the whole sampling period (September 2015 to February 2016). Boxes represent the interquartile range, bound by the 75th and 25th percentiles. Line and cross markers indicate medians and averages, respectively, whereas error bars show minimum and maximum concentrations. Comparison between mean total concentrations (U test, p < 0.05) is shown; the concentration distributions sharing the same letter are not statistically different at p < 0.05.

  ΣHBCDDs, ΣNHFRs andΣMeO-BDEs (pg m -3 ) in atmospheric particles of Bizerte city (present study) with those reported in other areas. quantification limit, ND: not determined, HVAS: high-volume air sample, LVAS: low-volume air sample, GFF: glass fibre filter.

Fig. 2 .

 2 Fig. 2. Composition profiles of HBCDDs and NHFRs in atmospheric particle of Bizerte city from September 2015 to February 2016.

Fig. 5 .

 5 Fig. 5. Box-and-whisker plots of atmospheric particle dry deposition fluxes for ƩHBCDDs, ƩNHFRs and ƩMeO-BDEs in Bizerte city at the two seasons and for the whole sampling period (September 2015 to February 2016). Boxes represent the interquartile range, bound by the 75th and 25th percentile. Line and cross markers indicate medians and averages, respectively, whereas error bars show minimum and maximum dry deposition fluxes.

  CRediT author statementBadreddine Barhoumi: Conceptualization, Methodology, Writing-Original draft preparation, Writing-Reviewing and Editing; Marc Tedetti: Writing-Reviewing and Editing, finalizing the paper content; Andrea Peris and Òscar Aznar-Alemany: Data curation, Software and Methodology; Catherine Guigue: Initial data analysis; Soufiane Touil and Mohamed Ridha Driss: Supervision; Ethel Eljarrat: Funding acquisition, Methodology, Writing-Reviewing and Editing, Supervision and Validation.

Fig. 6 .

 6 Fig.6. Estimated daily exposure dose (DEDs) of adults and children to particlebound HBCDDs, NHFRs and MeO-BDEs via inhalation (ng kg -1 bw day -1 ). Boxes represent the interquartile range, bound by the 75th and 25th percentiles. Line and cross markers indicate medians and averages, respectively, whereas error bars show minimum and maximum daily exposure dose.

Acknowledgements

This study was supported by the Institut de Recherche pour le Développement (IRD), through the International Joint Laboratory (LMI) "COSYS-Med", the Tunisian Ministry of Higher Education and Scientific Research, the PACA region "Particule" project, the IRD-MIO Action Sud "AEROBIZ" project and the Generalitat de Catalunya (Consolidated Research Group Water and Soil Quality Unit 2017). P. Raimbault (MIO) is acknowledged for OC and ON analyses. We are grateful to two anonymous Reviewers for their relevant comments on the manuscript.

Declaration of competing interest

All authors declare no competing financial interest.


Walid Khodja

Dr Alexandre Leclair

Prof Jordi Rull-Barrull

Dr Françoise Zammattio

Dr Ksenia V Kutonova

Dr Marina E Trusova

François-Xavier Felpin
email: fx.felpin@univ-nantes.fr

Dr Mireia Rodriguez-Zubiri

F.-X Felpin

] W Khodja

The promoting effect of pyridine ligands in the Pd-catalysed Heck-Matsuda reaction

Keywords: Heck-Matsuda reaction, Arenediazonium salts, Palladium, N-ligands, Allylic alcohols 1. Introduction

An efficient Pd-catalyzed arylation reaction of challenging acyclic olefins, in the presence of an organic ligand, has been disclosed. Commercially available cheap pyridine-based ligands are able to promote good to excellent yields for poorly efficient Heck-Matsuda arylation reactions of several allylic alcohols. A wide range of electronically different arenediazonium salts bearing either electron-releasing or withdrawing groups have been used allowing the synthesis of a range of β-aryl-methoxy-lactols. The catalytic system has been optimised, along with the reaction conditions, in order to achieve remarkable yields in less than 1 h.

palladium oxidative addition step is easily accessible due to the high reactivity of the nitrogen function. 18 Once the C-N 2 + bond has been dissociated by reaction with the Pd(0) precursor, the olefin insertion step takes place more easily on the resulting cationic palladium intermediate when compared to the classical Heck process. In addition, the hazardous reputation of diazonium salts has been mitigated and several safe industrial processes have been developed recently. 19 Indeed, arenediazonium tetrafluoroborates, hexafluorophosphates and tosylates, among others, have revealed good solubility, high reactivity and, more importantly, they can be stable up to 600 °C and be handled safely on a large scale, as reported recently by Filimonov and co-workers. [20][21][22] The large range of widely accessible and inexpensive anilines, and the ease of the diazotization strategy, have enabled the coupling of olefins with arenediazonium salts exhibiting very different steric and electronic properties. Although a substantial amount of substrates have been arylated, this strategy seems essentially limited to acrylates and styrenes. Other olefins, less prone to arylation in the presence of arenediazonium salts, represent an important challenge to be addressed. 23,24 The reported inability to guide the regioselectivity of the β-hydride elimination and the migratory insertion step make the arylation of acyclic alkenes a challenging issue. 25 In this area, the development of ligand-based catalytic systems could afford a step forward towards a convenient model process with a broad scope of application. Palladium (II) species generated during the oxidative addition step exist in solution as coordination complexes weakly stabilized by coordinated solvent molecules and are prone to decomposition by formation of inactive Pd-black aggregates. Formal donation of one or more electron pairs from a chelating organic ligand generally involves stabilization of metal species. In spite of their usefulness in homogeneous catalysis, phosphanes have mostly been found unsuitable ligands for Pd-catalyzed Heck-Matsuda reactions, presumably because they are capable of transferring an electron to arenediazonium salts to form arene radicals. 26 On the other hand, several N-containing ligands such as the thiourea of Yang et al., 27 the carbenes of Song et al. 28 and Beller and coworkers, 29 and the macrocyclic triolefin of Roglans et al., 30 have proven to be compatible with Pd for the arylation of highly reactive acrylates in the presence of diazonium salts, although the catalytic performance could not be substantially improved.

It has only recently been proven that chiral oxazoline-type ligands are effective in the intermolecular enantioselective Heck-Matsuda reaction and remarkable results have, in this way, been reported for the asymmetric arylation of several allylic alcohols. [31][32][33][34] However, a careful outlook on the literature survey reveals the lack of a full study on the influence of an organic ligand in the catalytic activity of the Heck-Matsuda reaction so that the existing limitations, such as the arylation of resistant olefins, could find a valuable solution through a practical and efficient general protocol. It is thus of great interest to disclose cheaper organic ligands for non chiral arylation reactions.

With this in mind, we have attempted to develop simple and efficient catalytic systems containing N-ligands for the arylation of acyclic alkenes. We have examined very common, commercially available, cheap and tunable pyridine-based ligands in the challenging arylation of allylic alcohols in the presence of a variety of arenediazonium salts. Our preliminary results towards this end are reported herein.

Experimental Section

Instrumentation and materials

All commercial reagents were used as received. Extra pure methanol and dichloromethane were used without further purification. Purification of products was carried out by flash column chromatography using silica gel (40-30 m). Analytical thin layer chromatography (TLC) was performed on 0.25 mm silica gel 60-F254 plates. Visualization was accomplished with UV lamp (254 nm). 1 H and 13 C NMR spectra were recorded at 300 MHz and 75 MHz respectively, and they are reported as δ values (ppm) relative to residual CDCl 3 δ H (7.26 ppm), CDCl 3 δ C (77.16 ppm) as internal standard. Low resolution mass spectroscopy (LRMS) was performed using chemical ionization (CI). High resolution mass spectroscopy (HRMS) was recorded on an orbitrap spectrometer. The GC analyses were performed on a GC-FID Agilent 7820A chromatograph equipped with a 30 m HP5 capillary column. The GC-MS analyses were recorded on a TRACE GC Ultra (ThermoScientific) apparatus equipped with a 30 m TR-5MS silica capillary column and a DSQII quadrupole analyser (ThermoScientific).

Methods

Arenediazonium tetrafluoroborate salts (general procedure):

To a stirred solution of the corresponding substituted aniline (23.4 mmol) in Et 2 O (4.6 mL), was added an aqueous solution of tetrafluoroboric acid (50 %, 4.6 mL, 73.86 mmol) at 0 °C. A solution of sodium nitrite (1.64 g, 23.4 mmol) in water (4.6 mL) was then added dropwise at 0 °C. The reaction was kept under vigorous stirring for further 30 minutes before filtration. The crude solid was washed with cold EtOH (10 mL) and Et 2 O (3 x 15 mL). The resulting white solid was dissolved in approximately 10 mL of acetone and re-precipated by adding 80 mL of Et 2 O. After filtration, the resulting white solid (65-85 % yield) was dried under vacuum and kept in the fridge.

Pd-catalysed

Heck-Matsuda reaction (general procedure): In a 25 mL pressure tube equipped with a magnetic stirring bar were sequentially added: Pd(TFA) 2 (16.6 mg, 0.05 mmol), pyridine-based ligands (1 mL of a MeOH solution containing 0.11 mmol for bidentate ligands or 0.22 mmol for monodentate ligands) and methanol (10 mL). The reaction mixture was then heated (60 °C) and kept under stirring for 5 min. Next, base (0.5 mmol), olefin (1 mL of a 1 M MeOH solution, 1 mmol) and arenediazonium tetrafluoroborate (2 mmol) were sequentially added to the reaction mixture. The suspension was kept under vigorous stirring at 60 °C for further 1 h. The mixture was cooled and the solvent was removed under reduced pressure. The crude was purified (hexanes:ethyl acetate, 1:1, 6 × 50 mL) through a plug of silica gel (2.5 × 3 cm) and evaporated to dryness. The expected products along with other related isomers were identified by GC-MS and NMR techniques. All synthesised compounds are known and their characterisation data is according to the corresponding spectral data reported in the literature.

32, 35-39 n-Dodecane and 1,2,4-trimethylbenzene or 1,3,5-trimethylbenzene were used as internal standards respectively for GC and 1 H NMR yield determination.

Pd-catalysed

Heck-Matsuda reaction: kinetic study (general procedure): In a 25 mL pressure tube equipped with a magnetic stirring bar were sequentially added: Pd(TFA) 2 (16.6 mg, 0.05 mmol), L5

(1 mL of a 0.11 M MeOH solution, 0.11 mmol) and methanol (10 mL). The reaction mixture was then heated (at 25 °C, 40 °C or 60 °C) and kept under stirring for 5 min. Next, K 2 CO 3 (73 mg, 0.5 mmol), 2butene-1,4-diol (1 mL of a 1 M MeOH solution, 1 mmol) and 4-chlorobenzenediazonium tetrafluoroborate (453 mg, 2 mmol) were sequentially added to the reaction mixture. The suspension was kept under vigorous stirring and several aliquots of the reaction mixture (0.5 mL) were sequentially extracted for analysis at different reaction times. Immediately after extraction and prior to GC analysis, the aliquots were quenched with KCN (0.5 mL of a 0.05 M MeOH solution, 0.025 mmol) and filtrated. The yields were determined by GC using n-dodecane as internal standard.

Results and Discussion

In this work we were led to study the Heck-Matsuda arylation of diol 1 with arenediazonium 2, as a model reaction, in the presence of a catalytic system made of Pd(TFA) 2 , a pyridine-based ligand and a base (Scheme 2).

Scheme 2. Heck-Matsuda arylation of but-2-ene-1,4-diol, 1.

Screening of Suitable Ligands

Owing the very few existing reports on the use of ligands in the Heck-Matsuda reactions when compared to classical Heck transformations, our first experiments were aimed at evaluating the effect of an organic ligand on a model reaction (Scheme 2).

Interestingly, our study evidenced the essential role of an organic ligand since the already reported catalytic activity on the model reaction (Scheme 2) was strongly inhibited (<1% of coupling product 3) when using a ligand-free catalytic system whereas, keeping the same operational conditions (Pd(TFA) 2 5 mol%, ZnCO 3 1 mmol, 0.27 M, 60 °C, 15 min) but in the presence of a bis-oxazoline ligand, up to 93% yield of the expected product had been pointed out elsewhere. 32 On the basis of these results we were prompt to conduct some studies to identify a compatible class of ligands able to afford good catalytic activities. We were led to investigate the effect of several classical and simple pyridine-type ligands (Figure 1) in terms of their chelating effect by means of the nature of the heteroatoms binding the metal and the size of the resulting metalligand chelate. At first it should be recalled that, under our reaction conditions (depicted in Table 1) and in the absence of ligand, the corresponding arylation product 3 is produced in only 24% yield (entry 1, Table 1). All the pyridine-based ligands studied have been shown to improve the efficiency of the catalytic process (entries 3, 4 and 6, Table 1) to the exception of L3, 1,10-phenanthroline, a very rigid bidentate ligand (entry 8, Table1).

Table 1. Effect of pyridine type ligands in the efficiency of the arylation of diol 1. [a] Entry Ligand (equiv.) Yield [b] [%] product 3 Results from Table 1 have revealed several important features. Indeed, simple monodentate pyridine L1 has appeared as an efficient ligand affording 56% of arylated products (entry 3, Table 1). Increasing structural rigidity through the chelating ability seems, nevertheless, to have a detrimental effect on the catalytic efficiency since bipyridine, L2, and phenanthroline, L3, ligands are less performant than L1 (entries 6 and 8, Table 1). As expected, a ligand ratio L/Pd = 2 was the optimal ratio found for monodentate ligands such as L1 (entry 3, Table 1), and L/Pd = 1 for bidentate ligands L2 and L3 (entries 6 and 8, Table 1), so the active palladium species are stabilized by coordination of two pyridine nitrogen atoms to the metal center while keeping two free coordination sites for the catalytic reaction to take place. Consequently, 4 equiv. of monodentate L1 (entry 2, Table 1) and 2 equiv. of bidentate L2 and L3 (entries 2, 5 and 7, Table 1) inhibit completely the reaction as the coordination sphere of the palladium center becomes completely saturated with four σ-donor/π-acceptor pyridine ligands, indicating, moreover, that the pyridine nitrogen atom seems to be a compatible and robust ligand for palladium under the studied reaction conditions.

The selectivity of the reaction is also an important issue to be addressed here since the expected arylation product 3 was often produced along with 4, an unprecedented arylated product which has been identified, on the basis of a GC-MS analysis, as an aryl dihydrofuran (Scheme 3). Due to its instability, the latter could not be isolated preventing us from its NMR characterization and yield quantification. Product 4 may correspond to the elimination product either from the acetal 3 or the lactol 5, precursor of 3 (Scheme 3). Formation of 4 could have been catalyzed by HBF 4 formed as a byproduct of the C-C bond formation reaction under the thermal reaction conditions used (60 °C).

Scheme 3. Formation of 4 by elimination reaction from either 3 or 5.

Although the expected arylated compound 3 is the major product of the reaction, the formation of 4 is also enhanced by the use of a promoting ligand and could represent up to 20-30% of the total yield when using pyridine ligands.

Motivated by our results on the pyridine ligand L1 and conscious of the need to further develop it in order to tune its structural and electronic properties, we have also preliminary explored a family of pyridine type ligands bearing a second harder coordinating oxygen heteroatom. Indeed, hybrid chelating ligands containing at least two chemical groups such as a soft and a hard donor atoms have been of great interest. Amongst them, heterodentate hemilable ligands with a reversible labile functionality and an irreplaceable group have shown very interesting catalytic performances. Nitrogen and oxygen containing ligands are an important class of hemilabile mixed ligands. The π-acceptor character of the (σ-donor) pyridine moiety can stabilize the metal centre in a low-oxidation state and the (σ/π)-donor ability of the oxygen renders the metal more susceptible to oxidative reactions. series of N-O ligands, L4-L8 (Figure 1), which were assumed to be compatible with arenediazonium salts due to their reduced Lewis basicity compared to phosphine ligands, were thus studied (Table 2).

Table 2. Effect of pyridine-based ligands in the efficiency of the arylation of diol 1. [a] Entry Ligand (equiv.) Yield [b] [%] product 3 Gratifyingly, the N,O-pyridine ligands studied promoted the formation of the Heck adduct 3 (entries 2-9, Table 2) to the exception of ligand L8 (entries 10-11, Table 2). This hydroxyquinoline can exists as its tautomeric form by intermolecular proton transfer (Scheme 4), 40 and be deprotonated by the carbonate base present in the reaction media.

1 - 24 2 L4 (2) 46 3 L4 (
Scheme 4. Tautomeric pair from L8, 8-hydroxyquinoline, and their deprotonated analogues.

Ligand L8 could, in this way, behave as a rigid bidentate π-acceptor ligand through the N sp 2 atom, and as a σ-donor through the O -function. The rigid structure of the quinoline backbone ligand does not presumably allow the O -function to decoordinate so that the use of two equivalents of L8 results in a complete inhibition of the catalytic activity (entry 10, Table 2) by the formation of a stable non labile square planar Pd complex with a saturated coordination sphere.

More importantly, a remarkable efficiency and selectivity towards the desired arylated product 3 was achieved using L5, 2-hydroxymethylpyridine (entry 4, Table 2). Indeed, 84% yield and 94% selectivity towards the formation of the expected methoxy lactol 3 was successfully attained with L5, whereas the other N,O-pyridine ligands (L4, L6 and L7) afforded up to 60% of arylated product 3 (entries 2-3 and 6-9, Table 2) and approximately up to 20% of 4. A careful look at the experimental results and the structural features of these N,O-mixed ligands suggests that ligands able to form six-membered metal chelates may induce an increase in the reaction rate and that the participation of a free hydroxyl group in the catalytic cycle is not mandatory. We postulate that with shorter or longer tailored alkoxy substituents, the resulting metal-ligand chelates should be less stable and more prone to decomposition resulting in lower yields of arylated products.

Surprisingly, L5 was the only ligand for which 2 equivalents (respect to Pd) proved to be more efficient than a ratio L/Pd = 1 (entries 4-5, Table 2).

A detailed overview on the literature survey concerning L5, 2-hydroxymethylpyridine, [41][42][43][44] and its coordination chemistry towards palladium and platinum metal precursors, has revealed the ability of this pyridine ligand to bind the metal center either as a mixed L,L-or L,X-type ligand (Figure 2), depending on the reaction conditions. However, only in the presence of a strong base (i.e. KOH or NaH) the hydroxyl function is deprotonated and 2-pyridinemethanolate acts as a bidentate ligand towards a palladium center through a dative bond with the pyridine nitrogen atom, and a covalent bond with the alkoxy function (Figure 2a). 41,45 We speculate that in the absence of a sufficiently strong base (pKa of K 2 CO 3 = 10), L5 (pKa of ROH = 16-20) should remain protonated and establish two dative bonds with the metal: a stronger one owing the π-acceptor ability of the N sp 2 atom, and a labile one with the lone pair of the O atom (Figure 2b). Another hypothesis on the coordination behavior of L5 concerns the formation of transition metal complex containing two L5 ligands where the square planar palladium complex is stabilized by two strongly coordinated pyridine moieties and two labile hydroxyl functions (Scheme 5) which may be able to dissociate from the metal center to the approach of the substrates.

Scheme 5. Postulated Pd(II) and Pd(0) complexes with L5.

In order to gain insight on the factors governing the efficiency and selectivity of the studied catalytic systems, coordination studies with the Pd(TFA) 2 metal precursor and 2hydroxymethylpyridine, L5, as the selected ligand owning its remarkable catalytic performance, are currently ongoing.

Optimisation studies

With the above presented notable results in hand, we looked for the optimal reaction conditions.

Typically, catalytic runs were conducted using the reaction conditions depicted in Table 2 unless specified.

Our optimisation studies revealed that in the absence of a palladium precursor, no Heck adducts were detected (entry 1, Table 3). [a] Reaction conditions: diol 1 (1 mmol), diazonium 2 (2 mmol), K2CO3 (0,5 mmol), [Pd] (5 mol%), L5 (2 equiv./Pd), 12 mL MeOH, 60 °C, 1 h. [b] Determined by GC analysis using n-dodecane as internal standard.

[c] Pd(TFA)2 1 mol%.

In the presence of L5, the best palladium precursor was found to be Pd(TFA) 2 (entry 6, Table 3). Replacement of Pd(TFA) 2 by several other classical palladium Pd(0) and neutral or cationic Pd(II) precursors had a detrimental effect on the chemical yield values, although they remained significantly good (entries 3-5, Table 3). The selectivity performance was substantially the same for all the metal precursors studied. Even so, the use of K 2 PdCl 4 proved ineffective (entry 2, Table 3).

It is worth pointing out that decreasing the catalyst loading to 1% led to a lower yield: 24% of product 3 exclusively (entry 7, Table 3).

We next proceeded to explore the already known accelerating effect of several type of organic and inorganic bases in the Heck-Matsuda reaction (Table 4). Control experiments have revealed that the use of a base appears essential since only 13% yield was obtained without any base (entry 1, Table 4).

Table 4 illustrates our results on screening different organic and inorganic bases. K 2 CO 3 allowed compound 3 with the best yield and selectivity. Replacing K 2 CO 3 for ZnCO 3 , Cs 2 CO 3 , CaCO 3 or AcONa was detrimental for the process affording lower chemical yields (entries 2-4 and 6, Table 4). Instead, 2,6-di-tert-butyl-4-methylpyridine (DTBMP) allowed high yields but also higher amounts of 4 (>10%). On the other hand, sodium citrate, an organic base bearing three carboxylate functions, afforded a catalytic performance equivalent to K 2 CO 3 , with only a marginally lower catalytic selectivity (entry 8, Table 4). Due to its efficiency, selectivity, low cost, and absence of metal counterion, K 2 CO 3 represented an interesting base allowing better results than the other classical candidates studied. Owing its high polarity, its capability to reduce Pd(II) to Pd(0) in the reaction media, and its green properties when compared to other organic solvents, 46,47 MeOH was considered the solvent of choice for the Heck-Matsuda reaction studied. The highest catalytic activity was observed at 0.08 M (entry 4, Table 2) reaction concentration. Lower concentrations did not have a strong impact on the reaction effectiveness and/or selectivity (0.07 M, 76% yield). However, at higher concentrations, a detrimental effect on the catalytic efficiency (68% yield at 0.1 M and 35% yield at 0.4 M) was observed, presumably due to the limited solubility of the diazonium salts in the reaction solvent.

Kinetics studies

One question that still needed to be addressed in order to conclude our optimization studies was the effect of the reaction time and the temperature on the efficiency of the studied process. The Pd(TFA) 2 catalyst was already shown to deposit inactive metallic Pd at high temperatures and/or long reaction times in protic polar solvents. We have therefore deeply investigated our catalytic system by monitoring the reaction at different temperatures. Once more, catalytic runs were conducted using the reaction conditions shown in Table 2, in the presence of L5, but at different temperatures as specified.

As depicted in Figure 3, temperature plays an important role in the kinetics of the reaction. As expected, decreasing the temperature results in a decrease of the reaction rate since longer reaction times are needed to achieve quantitative yields of arylated product 3.

For all conducted experiments, product 4 was detected (along with compound 3) but represented less than 5% of the total yield and decreased to 0% as the amount of product 3 approached quantitative yields, suggesting an evolution of 4 to 3, presumably by MeOH electrophilic addition to the double bond under acidic (HBF 4 ) conditions.

Our set of results have shown that decreasing the reaction temperature from 60 °C to 40 °C led to reaction times x2 longer, and rates x5 slower were recorded when performing the same reaction at room temperature (25 °C). Thus, high yields for product 3 were afforded in only 1 h at 60 °C, in 4 h at 40 °C and in 48 h at 25 °C. More interestingly, we evaluated the effect of our selected ligand L5 on the kinetics of the studied reaction by monitoring, in a similar manner, the evolution of the coupling reaction but in the absence of any ligand. In the absence of L5, only 4% (25 °C), 8% (40 °C) and 18% (60 °C) of 3 was obtained indicating that the reaction proceeded with very slow kinetics without any organic ligand. These latter results confirmed the usefulness of 2-hydroxymethylpyridine, L5, for the arylation of acyclic allylic alcohols with arenediazonium salts. To assess the synthetic potential of the newly disclosed catalytic system, its efficiency was evaluated in the arylation of diol 1 (Scheme 2) towards different arenediazonium salts containing electron releasing or electron withdrawing groups (Table 5).

Scope of the reaction

Table 5. Scope of the Heck-Matsuda arylation of diol 1 using L5. As already shown, yields were greatly enhanced when using diazonium salts bearing a chloro function (entries 1-2, Table 5). Remarkably, submission of diol 1 to coupling with electron rich diazonium salts in the presence of L5 (entries 3-4, 7-8, Table 5) gave also good to excellent results. As expected, the reaction was less improved by the use of L5 in the presence of highly electron poor diazonium salts bearing a nitro group (entries 5-6, Table 5), presumably due to their enhanced electrophilic character and, thus, greater reactivity towards a nucleophilic palladium.

We next studied the effect of L5 on the Heck-Matsuda arylation of other allylic alcohols towards diazonium 2. The results (Table 6) have shown that the protocol adopted for the arylation of diol 1 proved compatible with other substrates since the presence of L5 provoked up to 50% improvement of the initial yields obtained without ligand for the arylation of pent-3-en-2-ol (entries 1-2, Table 6).

To the best of our knowledge, our results represent the first example of the Heck-Matsuda arylation of cyclohex-2-en-1-ol with yields up to 82% (entries 3-4, Table 6). Table 6. Scope of the Heck-Matsuda arylation reaction using L5.

Conclusions

In summary, the association of Pd(TFA) 2 / 2-hydroxymethylpyridine / K 2 CO 3 has been shown to afford excellent catalytic activities for the arylation of structurally diverse allylic alcohols with arenediazonium salts to yield the expected arylated derivatives. From a practical point of view, this system offers several advantages because it involves simple starting materials, widely available pyridine-based ligands and tolerates the presence of a wide range of diazonium salts. Furthermore, our contribution concerns a stable "one-pot" ready-to-use metal-ligand catalyst precursor which does not need to operate under inert atmosphere and does not need laborious experimental procedures. This simple one pot catalytic system includes common pyridine-based ligands capable of playing the role of a potential chiral vector which provide great possibilities for an asymmetric version of this reaction, an almost unexplored area of research to date. Further research in this area is now in progress.

Figure 1 .

 1 Figure 1. Pyridine type ligands studied in this work: pyridine, L1; 2,2′-bipyridyl, L2; 1,10phenanthroline, L3; 2-hydroxyethylpyridine, L4; 2-hydroxymethylpyridine, L5; 2hydroxylpyridine, L6; 2-benzyloxymethylpyridine, L7 and 8-hydroxylquinoline, L8.

Figure 2 .

 2 Figure 2. Mixed L,X-or L,L-type ligand.

Figure 3 .

 3 Figure 3. Representation of the kinetics up to 1 h at 25 °C, 40 °C and 60 °C, for the formation of arylated product 3 in the presence of L5.

  Reaction conditions: diol 1 (1 mmol), diazonium (2 mmol), K2CO3 (0,5 mmol), Pd(TFA)2 (5 mol%), 12 mL MeOH, 60 °C, 1 h. [b] Determined by GC analysis. [c] Determined by 1 H NMR analysis using either 1,2,4-trimethylbenzene or 1,3,5-trimethylbenzene as internal standard.

  Reaction conditions: olefin (1 mmol), diazonium 2 (2 mmol), K2CO3 (0,5 mmol), Pd(TFA)2 (5 mol%), 12 mL MeOH, 60 °C, 1 h. [b] Determined by 1 H NMR analysis using either 1,2,4-trimethylbenzene or 1,3,5-trimethylbenzene as internal standard.

  Reaction conditions: diol 1 (1 mmol), diazonium 2 (2 mmol), K2CO3 (0.5 mmol), Pd(TFA)2 (5 mol%), ligand (1 or 2 equiv./Pd), 12 mL MeOH, 60 °C, 1 h. [b] Determined by GC analysis using n-dodecane as internal standard.

	[a]
		1)	44
	4	L5 (2)	84
	5	L5 (1)	61
	6	L6 (2)	44
	7	L6 (1)	48
	8	L7 (2)	50
	9	L7 (1)	46
	10	L8 (2)	0
	11	L8 (1)	24

Table 3 .

 3 Efficiency of several palladium precursors in the arylation of diol 1.[a]

	Entry	[Pd]	Yield	[b] [%] product 3
	1	-		0
	2	K 2 PdCl 4		0
	3	Pd(dba) 2		59
	4	[Pd(CH 3 CN) 4 ](BF 4 ) 2		64
	5	Pd(OAc) 2		64

Table 4 .

 4 Efficiency of several bases in the arylation of diol 1.[a]

	Entry	Base	Yield	[b] [%] product 3
	1	-		13
	2	CaCO 3		44
	3	Cs 2 CO 3		49
	4	ZnCO 3		65
	5	K 2 CO 3		84
	6	AcONa		55
	7	DTBMP		74
	8	Sodium citrate		80
	[a] Reaction conditions: diol 1 (1 mmol), diazonium 2 (2 mmol), base (0,5 mmol), Pd(TFA)2 (5 mol%), L5 (2
	equiv./Pd), 12 mL MeOH, 60 °C, 1 h. [b] Determined by GC analysis using n-dodecane as internal standard.

Acknowledgements

We are grateful to The "Centre National de la Recherche Scientifique" (CNRS), the University of Nantes and the "Institut Universitaire de France" (IUF) for financial support. The authors gratefully acknowledge Pavel S. Postnikov for fruitful discussions. Isabelle Louvet (CEISAM, University of Nantes), Denis Loquet (CEISAM, University of Nantes) and Julie Hémez (CEISAM, University of Nantes) are acknowledged for GC and GC-MS analyses.