Ludovic Charpentier
email: ludovic.charpentier@promes.cnrs.fr

Marianne Balat-Pichelin
email: marianne.balat@promes.cnrs.fr

Diletta Sciti
email: diletta.sciti@istec.cnr.it

Laura Silvestroni

HIGH TEMPERATURE OXIDATION OF CARBIDES UHTC IN AIR

Keywords: UHTC, high temperature, oxidation, solar plants

Ultra-High Temperature Ceramics having melting points above 3500 K and high thermal conductivities are envisaged to elaborate future receivers of concentration solar power plants. The high pressure and solar temperature reactor (Réacteur Hautes Pression et Température Solaire, REHPTS) at the focus of the Odeillo 5 kW solar furnace was used to investigate the oxidation of three carbides (ZrC/MoSi 2 , ZrC/TaSi 2 , HfC/MoSi 2 ) that could be candidate. The concentration of the additives (TaSi 2 or MoSi 2 ) was 20 v.% in each composition. Each sample was oxidized in static air (P = 87 kPa) during 20 minutes at 1800, 2000 and 2200 K. Experiments were filmed using a video camera and the gaseous phase was analyzed by mass spectrometry. Various characterizations have shown that the natures of the carbide and additive would affect the composition of the oxide layer and therefore the hightemperature behavior of the sample.

INTRODUCTION

The efficiency of a concentration solar power plant strongly relies on the high temperature behavior of the solar receiver. Silicon carbide (SiC) begins to be one ceramic material used to produce various geometries for solar absorbers [1,2], but the degradation of the material becomes too important above 1700 K because of the formation of bubbles and the production of gaseous SiO and CO which implies a severe mass loss of the material. Therefore a SiC receiver cannot be heated at a temperature so high that the air coolant can issue it above 1100 K. Consequently, an extra source of fossil energy (or biomass) has to be added after the receiver in order to end up the heating of pressurized air up to more than 1300 K and therefore insure a sufficient efficiency of the energy conversion to make the technology financially viable [3].

We have therefore studied the oxidation behavior of new ultra-high temperature ceramics (UHTC) having good mechanical properties even above 2300 K, in order to identify which would be the best candidates to elaborate new high temperature solar receivers. Among potential materials for such application, zirconium carbide (ZrC) presents a high melting point (3500 K) and interesting mechanical properties, especially its hardness around 27 GPa, making of it one of the hardest materials among UHTC [START_REF] Gasch | Handbook of Ceramic Composites[END_REF]. The main limitation for high temperature applications is the oxidation kinetics: zirconia (ZrO 2 ) is non protective and ZrC continuously oxidizes according to a linear kinetic [START_REF] Shimada | [END_REF]. Incorporation of silicon was found to enable the formation of mixed oxide layer such as zircon (ZrO 2 •SiO 2 ), which was shown to improve the resistance to oxidation [6]. Sarin et al. [7] also recently reported the thickness of the oxide layer formed on ZrB 2 /SiC composite materials during oxidation in air around 1500 K decreased when the amount of SiC in the initial material increased from 0 till 50 v.%.

We decided to focus on two carbides with disilicide additives as several routes were recently developed at ISTEC-CNR to elaborate such materials by pressureless sintering or hot pressing [8][9][10][11]. The first material we deeply investigated was ZrC with MoSi 2 additive [12]. We have decided to compare the behavior of ZrC/MoSi 2 with the behaviors of HfC/MoSi 2 or ZrC/TaSi 2 , changing either the carbide or the additive.

EXPERIMENTAL PROCEDURE

The reactor we used to perform high temperature oxidation is the REHPTS (Réacteur Hautes Température et Pression Solaire, High Temperature and Pressure Reactor), at the focus of the Odeillo 5 kW solar furnace, shown in figure 1. A flat mirror (heliostat) whose position is servo-controlled to the apparent movement of the sun reflects the incident solar flux to a concentrator with facetted mirrors. A shutter enables to control the fraction of the concentrated solar flux delivered to the sample placed inside the reactor and therefore its surface temperature. This set-up is placed so that the sample is 25 mm above the focus of the solar furnace, thus elevated temperatures on materials may be obtained at very fast rate (up to 100 K s -1 ) on a homogeneous 10 mm diameter area. Two mirrors enable a monochromatic (5 µm) optical pyrometer (Ircon, Modline Plus) to measure the surface temperature of the sample through a fluorine window. The pyrometer with all the parts present on the optical path was calibrated on a black body. The accuracy of the temperature measurements is going from 1400 ± 15 K to 2100 ± 22 K. A mass spectrometer (Pfeiffer Omnistar) enables in situ gas phase analysis.

Materials were elaborated by ISTEC-CNR in Faenza, Italy. ZrC/20 v.% MoSi 2 (final density: 95%, 6.2 g.cm -3 ) and HfC/20 v.% MoSi 2 (f. d.: 97%, 11.1 g.cm -3 ) were produced by pressureless sintering at 2220 K during 90 min. ZrC/20 v.% TaSi 2 (f. d.: 99%, 7.1 g.cm -3 ) was sintered by Hot Pressing under 30 MPa loading at 1970 K during 6 min. The samples were shaped into 25mm-diameter and 2mm-thickness discs cut using Electrical Discharge Machining (EDM). This technique consists in cutting the material thanks to rapid discharges between two electrodes, so that the material locally reaches its melting temperature. This might allow the formation of zirconia at the surface during cutting. Thermodynamic calculations were performed using the GEMINI [13] software in order to compare the stable products due to the oxidation of both ceramics in air.

The oxidations were performed in static air. Due to the altitude of the laboratory, the total atmospheric pressure is around 87 kPa and the oxygen partial pressure pO 2 is 17 kPa. The surface temperature of the samples was maintained at a constant plateau during 20 min. and a video camera was used to follow in situ the oxidation process. Mass spectrometry enables to identify the gaseous products. As explained further in the results from thermodynamic calculation, CO is expected to be the main gaseous product during oxidation, but its molar weight is the same as N 2 one (m/e = 28), so it is impossible to separate the contribution of CO from the one of preponderant N 2 . Therefore we mainly follow the signal corresponding to m/e = 44, corresponding both to CO 2 and gaseous SiO.

The samples were weighted before and after oxidation in order to access the mass variation (converted to a mass variation rate expressed in mg cm -2 min -1 ). Some surfaces have been analyzed after oxidation using XRD and SEM with EDS. We can observe in figure 5 that at 1800 K, the oxide layer that forms on ZrC materials looks adherent to the carbide, whereas the oxide layer that forms on HfC is breaking into several fragments. This can be explained by the thermal expansion: HfC has a higher thermal expansion coefficient than HfO 2 (6.6x10 -6 mm -1 K -1 vs. 5.6x10 -6 mm -1 K -1 ), so the oxide layer can break due to the expansion of the carbide. On the other side, ZrC has a lower coefficient than ZrO 2 (6.7x10 -6 mm -1 K -1 vs. 10 -5 mm -1 K -1 ), so the oxide dilates more than the carbide, which can explain why the oxide layer covers a larger area over the carbide.

In situ analyses

Video images

In figure 6, we can observe a bubble on the ZrC/20v. % MoSi 2 . This bubble may be due to the formation of silica that is liquid at 2000 K, and it can blow due to the production of gaseous CO or SiO, which are expected products from the thermodynamical calculations. There is still a fragile oxide that breaks on the HfC material, that has slightly moved from the support due to the release of gaseous products. ZrC/20 v.% TaSi 2 seems to be the less damaged. This observation is confirmed at 2200 K (figure 7): whereas ZrC/20v. % MoSi 2 and HfC/20v.% MoSi 2 support an important boiling, the oxide on ZrC/20 v.% TaSi 2 supports a less important deformation due to the presence of tantalum.

Mass spectrometry

Figure 8 is presenting the evolution of the CO 2 and SiO concentration determined using mass spectrometry. We can observe that at 1800 and 2000 K the HfC/ 20%v. MoSi 2 is the material that releases the highest amount of gaseous products during oxidation. This can be due to the fact its oxide layer breaks as seen in figure 5 and 6 and enables the gases to escape from the oxide/carbide interface. Nevertheless the production of gases decreases after 10 minutes, probably due to the formation of a protective oxide layer preventing the release of gaseous products. Equivalent amount of gases is detected during the oxidation of ZrC/MoSi 2 and ZrC/TaSi 2 materials at 1800 K and during the 600 first seconds of oxidation at 2000 K. During the second half of the oxidation at 2000 K, there are more gases released by ZrC/TaSi 2 than by ZrC/MoSi 2 . We can give a first explanation from the results of thermodynamic calculations (figures 2 and 4): additive is less stable under oxidizing atmospheres than MoSi 2 , and therefore more SiO is produced. A second explanation comes from the video observations: liquid oxide blows at the surface of ZrC/MoSi 2 and can therefore prevent gaseous products from being released. Presence of tantalum seems to limit the presence of big bubbles of liquid oxide, therefore gaseous products may faster escape through fissures and the bursting of smaller bubbles. Both hypotheses could also explain why ZrC/TaSi 2 releases more CO 2 and SiO at 2200 K than the two other materials. We observe that the higher is the temperature, the most important is the amount of gaseous products released on both ZrC materials, in agreement with the thermodynamic calculations. For HfC/MoSi 2 the production of gases increases from 1800 to 2000 K, then decreases from 2000 to 2200 K. The apparition of liquid oxide that blows and may fill the fissures in the oxide layer could explain why less gaseous products can be released at 2200 K than at 2000 K.

Mass variations

Table 1 shows the global mass variations for each material. The higher is the temperature, the faster is the oxide growth. The mass variation of the HfC/MoSi 2 is much lower (and negative at 1800 K) than the one of the ZrC materials, which can be explained by the important breaking and removal of the oxide layer on HfC during the experiments. The nature of the additive in the ZrC materials has hardly influence at 1800 K. At higher temperature, the mass variation is more important when TaSi 2 is used. Tantalum seems therefore to play a significant role in the oxide formation, post-experiment analyses should enable us to accede the composition of the oxide layers and to understand this influence.

Post-experimental characterizations

X-Ray Diffraction

Figure 9 is presenting the XRD patterns for samples of each composition before and after 20 minutes oxidation in air. The reference patterns we used for the indexation are: 19-1487 (cubic ZrC), 80-0544 (tetragonal MoSi 2 ), 65-8750 (cubic HfC), 83-0944 (monoclinic ZrO 2 ), 65-1142 (monoclinic HfO 2 ), 77-1759 (tetragonal HfSiO 4 ), 42-0060 (orthorhombic TaZr 2.75 O 8 ). Figure 9 (a) shows that the only crystalline phase which forms on ZrC/20 v.% MoSi 2 is monoclinic zirconia. In figure 9 (b), we cannot identify on the reference sample any pattern of the TaSi 2 additive, but we observe the main crystalline phase is not zirconia (whose we can nevertheless still identify its two main peaks), but orthorhombic tantalum-zirconium oxide TaZr 2.75 O 8 . This difference in the composition of the final oxide probably explains why ZrC/20 v.% TaSi 2 gains more mass (table 1) and its oxide layer supports less deformation (figures 6 and 7) than ZrC/20 v.% MoSi 2 above 1800 K.

We observe in figure 9 (c) that the main crystalline phase that has grown on HfC/20v.% MoSi 2 is monoclinic hafnia, but the presence of tetragonal hafnium-silicon oxide HfSiO 4 was also evidenced.

Cross-section SEM and EDS

Figures 10 to 12 present cross-section SEM images of the oxide layers (phases were identified using EDS) on ZrC/MoSi 2 , ZrC/TaSi 2 oxidized in air at 2000 K.

The total modified area on ZrC/MoSi 2 is 435µm-thick in all and can be divided into 5 different layers (figure 10 (a)). Layer 1 is 30 µm-thick silica, probably amorphous as we could not detect any silica phases through XRD (figure 9 (a)). Layer 2 is 150µm-thick and its composition is brain-like ZrO 2 (mainly monoclinic from XRD) surrounded by darker SiO 2 as shown on figure 10 (b). Layer 3 consists of 85 µm-thick porous and brittle ZrO 2 . Layer 4, 80-µm thick, involves dense ZrC grains surrounded by granulous Zr-C-O, MoSi 2 and Mo 5 Si 3 surrounded by thin SiO 2 layer. Finally, layer 5, 90 µm-thick, is composed with the same phases as in layer 4, plus some SiC. ZrO 2 forms from the oxidation of the carbide and our observations seem coherent with the mechanism proposed by Shimada [14]: an intermediary oxycarbide forms close to the interface with ZrC, then further completely oxidized into ZrO 2 . SiO 2 forms due to the oxidation of the additive, together with the production of Mo 5 Si 3 , then moves toward the top surface of the oxide. SiC grains in layer 5 show that some more complex reactions, involving both the carbide and additive may occur close to the interface. 11 (c)), TaZr 2.75 O 8 is still mainly found, but the secondary phase is SiC. Finally, the 240 µm-thick third layer is a complex mixture of (Zr,Ta)Si 2 (white), (Zr, Ta)-C-O (grey), and SiC (black) phases. Oxidation of this material looks more complex: both carbide and additive reacts with oxygen to form mixed oxide (the oxycarbide and disilicide phases observed close to the interfaces are probably intermediate compounds) and SiC, that has partially oxidized close to the surface. Some gaseous SiO may also be produced, which could explain the mass spectrometry measurement in figure 8 (b). According to Bhattacharya et al. [15] who computed a recent phase diagram for the ZrO 2 -Ta 2 O 5 system, zirconium -tantalum -oxygen solution melts from 2100 K, which could explain why no deformation is observed at 2000 K in figure 6 whereas silica formed on ZrC/MoSi 2 material may melt and blow due to gaseous release.

Only one 100µm-thick layer was remaining at the surface of HfC/MoSi 2 (figure 12 (a)). Its composition involves HfO 2 , SiO 2 , Mo 5 Si 3 and HfSiO 4 (figure 12 (b)). This latest compound melts incongruently at 2020 K [16]. So the presence of this mixed oxide may explain why little boiling is observed at 2000 K on the surface of HfC/MoSi 2 compared with ZrC/MoSi 2 . We can notice an important porosity through the remaining oxide, favouring the gaseous release.

The calculations could not predict the stability of HfSiO 4 and TaZr 2.75 O 8 because the thermodynamic data of both mixed oxides are not well-known and therefore they could not be included in the calculations. Moreover, the calculations considered only the thermodynamic equilibrium and did not take into consideration kinetic control or the energy required to create interfaces, that is the reason why the solid compounds observed are slightly different than the ones predicted by the thermodynamic calculations.

CONCLUSIONS

Oxidation of ZrC/20 v. % MoSi 2 occurs with important boiling above 2000 K due to the production of molten silica and gaseous products (SiO, CO). Changing either the carbide (HfC/20 v.% MoSi 2 ) or the additive (ZrC/20 v.% TaSi 2 ) appears to reduce the deformation of the materials at 2000 K through the production of mixed oxides (HfSiO 4 or TaZr 2.75 O 8 ) with higher melting points than silica. Nevertheless, HfC produces a very fragile oxide that breaks during oxidation, producing solid particles and therefore HfC does not seem accurate for the conception of solar receivers at temperature lower than 2000 K. One drawback of tantalum incorporation is that liquid TaZr 2.75 O 8 may flow out of the sample at 2200 K [16]. These removals have to be avoided as they would favour continuous oxidation. In our future works, we will develop tests on ZrC/SiC materials to check the influence of another additive as these materials under development seem promising [17,18]. Another solution to get high temperature resistive materials could be to use two additive (ZrC/MoSi 2 /TaSi 2 or ZrC/SiC/TaSi 2 ), with a smaller amount of tantalum as recommended by Opila et al. [16].

FIGURE 1 .

 1 FIGURE 1. View of the REHPTS reactor at the focus of the 5kW solar furnace.

Figures 2 FIGURE 2 .

 22 Figures 2 to 4 present the amount in moles of solid phases and the molar fraction of main gaseous products (the gaseous phase being completed with N 2 , not plotted on the graph) according to the temperature at the thermodynamic equilibrium with the following initial conditions: -8 moles of carbide (ZrC or HfC) -2 moles of additive (MoSi 2 or TaSi 2 ) -10 moles of air (8 moles N 2 , 2 moles O 2 ) -Total pressure: 10 5 Pa

FIGURE 3 .FIGURE 4 .

 34 FIGURE 3. Amount of solid phases (a) and molar fraction of main gaseous products (b) at thermodynamic equilibrium according to temperature, calculated using GEMINI software. Initial conditions: 8 moles HfC, 2 moles MoSi 2 , 8 moles N 2 , 2 moles O 2 , P = 10 5 Pa, pO 2 = 2.10 4 Pa.

Figures 5

 5 Figures 5 to 7 present video captions of the various materials after 15 minutes of oxidation tests in air at 1800, 2000 and 2200 K respectively.

FIGURE 5 .FIGURE 6 .FIGURE 7 .

 567 FIGURE 5. Video captions of the three UHTC after 15 min. oxidation tests in air at 1800 K

FIGURE 8 .

 8 FIGURE 8. Measurement of the concentration at m/e = 44 (SiO and CO 2 produced) at a) 1800 K b) 2000 K c) 2200 K during the oxidation of the UHTC materials.

FIGURE 9 .

 9 FIGURE 9. XRD patterns of materials, reference and oxidized during 20 min. in air at 2000K. a) ZrC/20 v.% MoSi 2 b) ZrC/20 v.% TaSi 2 c) HfC/20 v.% MoSi 2

FIGURE 10 .

 10 FIGURE 10. Cross-section SEM images of a ZrC/20 v.% MoSi 2 sample after 20 min. oxidation in air at 2000 K. a) Global cross-section, b) zoom on modified layer 2, c) zoom on modified layer 4, d) zoom on modified layer 5. The modified area on ZrC/TaSi 2 (figure 11 (a)) has nearly the same total thickness (430 µm) as for ZrC/MoSi 2 and can be divided into only three layers. The first layer (figure 11 (b)), 110-µm thick, mainly consists of TaZr 2.75 O 8 , with some inclusions of Si-C-O phases. In the 80µm-thick second layer (figure11 (c)), TaZr 2.75 O 8 is still mainly found, but the secondary phase is SiC. Finally, the 240 µm-thick third layer is a complex mixture of (Zr,Ta)Si 2 (white), (Zr, Ta)-C-O (grey), and SiC (black) phases. Oxidation of this material looks more complex: both carbide and additive reacts with oxygen to form mixed oxide (the oxycarbide and disilicide phases observed close to the interfaces are probably intermediate compounds) and SiC, that has partially oxidized close to the surface. Some gaseous SiO may also be produced, which could explain the mass spectrometry measurement in figure8 (b). According to Bhattacharya et al.[15] who computed a recent phase diagram for the ZrO 2 -Ta 2 O 5 system, zirconium -tantalum -oxygen solution melts from 2100 K, which could explain why no deformation is observed at 2000 K in figure6whereas silica formed on ZrC/MoSi 2 material may melt and blow due to gaseous release.Only one 100µm-thick layer was remaining at the surface of HfC/MoSi 2 (figure12 (a)). Its composition involves HfO 2 , SiO 2 , Mo 5 Si 3 and HfSiO 4 (figure12 (b)). This latest compound melts incongruently at 2020 K[16]. So the presence of this mixed oxide may explain why little boiling is observed at 2000 K on the surface of HfC/MoSi 2 compared with ZrC/MoSi 2 . We can notice an important porosity through the remaining oxide, favouring the gaseous release.The calculations could not predict the stability of HfSiO 4 and TaZr 2.75 O 8 because the thermodynamic data of both mixed oxides are not well-known and therefore they could not be included in the calculations. Moreover, the calculations considered only the thermodynamic equilibrium and did not take into consideration kinetic control or the energy required to create interfaces, that is the reason why the solid compounds observed are slightly different than the ones predicted by the thermodynamic calculations.

FIGURE 11 .

 11 FIGURE 11. Cross-section SEM images of a ZrC/20 v.% TaSi 2 sample after 20 min. oxidation in air at 2000 K. a) Global cross-section, b) zoom on modified layer 1, c) zoom on modified layer 2, d) zoom on modified layer 3.

FIGURE 12 .

 12 FIGURE 12. Cross-section SEM images of a HfC/20 v.% MoSi 2 sample after 20 min. oxidation in air at 2000 K. a) Global cross-section, b) zoom on modified layer 1

TABLE 1 .

 1 Average mass variation rates for each sample according to the temperature Temperature

(K) ZrC/MoSi 2 (mg.cm -2 .min -1 ) HfC/MoSi 2 (mg.cm -2 .min -1 ) ZrC/TaSi 2 (mg.cm -2 .min -1 )

	1800	1.28	-0.20	1.20
	2000	1.72	0.61	2.37
	2200	2.23	1.42	3.86


Abderrahman El Bouakher

Sébastien Arnaud Martel

Sébastien Comesse

α-Halogenoacetamides: Versatile and Efficient Tools for the Synthesis of Complex aza-Heterocycles

α-Halogenoacetamides: versatile and efficient tools for the synthesis of complex aza-heterocycles

Introduction:

α-Halogenoacetamides are classical substrates that have long been employed by organic chemists. Even if, at first glance, they seem to be simple, they have proved over the years to be powerful intermediates for the synthesis of molecules of interest. The presence of the halogen atom in α-position compared to the amide function allows a wide range of potential functionalizations at this position and thus prompted their use as versatile building blocks. Furthermore, the use of α-halogenoacetamides as formal or real 1,3-dipole precursors led to the development of efficient domino 1-6 or concerted strategies for the synthesis of various polysubstituted N-heterocyclic molecules. Such approaches have recently attracted increasing interest since the development of aza-oxyallyl cation chemistry. The variety of structures reachable from these simple and tunable building blocks makes them good substrates for Diversity-Oriented Synthesis (DOS).

7-10

The amide function is probably the main group in biologically relevant compounds, and therefore, many efforts have been devoted to their formation. 11 α-Halogenoacetamides A (X = Cl, Br, I) play an important role 12 due to their ability to behave both as a nucleophile (at the nitrogen atom) and as an electrophile (at the carbon bearing the halogen) (Figure 1). One of the most interesting features of α-halogenoacetamides is related to the high electrophilicity of the carbon bearing the halogen. Comparably to αhalogenoketones or α-halogenoesters, but to a slightly lower extent, α-halogenoacetamides are among the most reactive halogenated electrophiles. This enhanced reactivity of a carbon atom bearing a halogen in alpha position of a carbonyl (higher than for benzyl or allyl groups) has been investigated in several studies and various explanations have been proposed. 13 One of the most commonly admitted involves the contribution of the π* orbital of the carbonyl during the approach of the nucleophile. As the LUMO orbital is a combination of the σ* C-X with the π* orbital of the carbonyl, during the approach the nucleophile is also stabilized by overlap with the π* (Figure 2). 13d The nucleophilic substitution is also considered to be stabilized by the pseudoenolate transition state that divides the negative charge of the nucleophile between the carbon and the oxygen. In addition, contrary to the widely studied α-haloesters and αhaloketones, 14 α-haloamides offer the opportunity to behave as a nucleophile in basic media by deprotonation of the amide NH to give amidate B and in some favorable conditions lead to the spontaneous formation of a dipole C (Figure 1).

Figure 3. General access and different uses of α-halogenoacetamides

The review will present the reactivity of α-halogenoacetamides 1 in various types of reactions in the following order (in this paper all αhalogenoacetamides will be numbered 1):

- The case of azaoxyallyl cations generated from an α-halogenated Weinreb amide type was the object of a recent review 15 but will be considered in the present paper. Indeed, it is interesting to have an overview of the dramatic change in the behavior of N-alkyl vs Nalkoxy α-halogenoacetamides in order to better understand the particular reactivity of the latter such as the lower pKa of the Weinreb amide function compared to alkylamides. More precisely, these species can generate, in specific conditions, azaoxyallyl cations allowing the use of [3+m]-cycloaddition reactions for the synthesis of a wide variety of aza-heterocycles (Scheme 1).

Scheme 1. Formation of the azaoxyallyl cation

N-alkyl 1 and N-alkoxy α-halogenoacetamides 1' are easily accessible from commercially available 2-halogenoacetyl bromide or 2-halogenoacetic acid derivatives in reaction with a primary amine. Alternatively, they can also be obtained in a few stapes starting from substituted acetic acid derivatives (Scheme 2). Piperazine-2,5-dione scaffolds have been prevalent in the scientific literature due to their significant interest in biology and drug discovery. [16][17][18][19] In recent years, the interest of these skeletons was highlighted in transition metal free reactions, used to promote carbon-carbon coupling between aryl and aryl halides by electron transfer processes.

20-27

Dimerization reactions, if not desired, are the main side reaction of α-halogenoacetamides. To limit these reactions, the reaction conditions have to be adjusted or highly reactive partners need to be involved. However, dimerization reactions were initially used successfully for the synthesis of piperazine-2,5-diones. The dimerization was first applied on simple α-chloroacetamides 1 in the early 1980s by Okawara et al. using phase transfer catalysis Duolite A-109 as catalyst in 50% of NaOH/CH 2 Cl 2 (Scheme 1). 28 The synthesis of 1,4-disubstituted piperazine-2,5-diones 2 in a twophase medium (CH 2 Cl 2 /alkaline solution) in the presence of triethylbenzylammonium chloride (TEBA) as the phase transfer catalyst was reported by O'Reilly and co-workers (Scheme 3).

29

Compounds 2 were also obtained in good yields by Cho and coworkers starting from α-chloroacetamides 1 in acetonitrile in presence of NaOH as the base this time. 30 It should also be mentioned that Hazra and co-workers 31 reported piperazine-2,5dione derivatives 2 as antileishmanial agents and developed another alternative synthesis employing NaH in dry DMSO leading to the desired aza-heterocyclic systems in yields up to 95%.

Scheme 1. Formation of Piperazine-2,5-diones 2 using phase transfer catalysis

The dimerization promoted by NaH in toluene was extended to 2bromopropanilides 1 on enantiopure and racemic mixtures leading to cis/trans ratios ranging from 8.5/1 to 1/2 due to partial racemization (Scheme 4). 32 However, in a more polar solvent such as THF, the racemization is enhanced leading to cis/trans ratios ranging from 2/1 to 4/1.

Please do not adjust margins

Please do not adjust margins Interestingly, it was also demonstrated that α-bromoacetamides 1 were efficient precursors of piperazine-2,6-diones. In this particular case, they were reacted with α-aminoesters 5 in basic conditions leading to piperazine-2,6-diones 6 in good yields (Scheme 5).

33,34

The first step of the mechanism is the nucleophilic addition of the amino group of 5 onto the carbon bearing the bromine atom of 1 followed by the formation of the imide by addition of the amide function onto the methyl ester. The synthesis of lactams starting from α-halogenoacetamides employing a domino process has been mostly described with α-or β-dicarbonyl compounds (such as malonate derivatives) as reaction partners. Among these strategies, the electrochemical reaction of diethyl bromomalonate 7 with α-bromoacetamides 1 allowed the formation of various skeletons depending on the reaction conditions as demonstrated by the work of Moracci and co-workers (Scheme 6) 35 The electrogenerated diethyl malonate anion derivatives A and B were used to gain access to β-lactams 8 or spirobis--lactam 9 depending on the substrates 1 employed. In the case of N-aryl α-bromoacetamides 1, the β-lactams 8 were obtained in yields ranging from 29 to 69% via the formation of carbanion A.

When the reaction was run starting from N-benzyl αbromoacetamide 1, the spirobis--lactam 9 was isolated with 70% yield. In the same paper, the authors demonstrated that the products 11 (R 1 = Bn) and 9 could be obtained in 40% and 10% yield, respectively, by reaction between N-benzyl α-bromoacetamide 1 and diethyl malonate 10 in the presence of NaH in DMF. In order to explain both the formation of β-lactams 8 and spirobis-lactam 9, two mechanisms were proposed (Scheme 7). Electrochemical reduction of diethyl bromomalonate 7 led to the formation of carbanion B which can further react with another equivalent of diethyl bromomalonate 7 to furnish the corresponding anion A. Addition onto N-aryl α-bromoacetamides 1 was followed by the nitrogen deprotonation and its addition onto the carbon bearing the bromine atom to furnish the β-lactams 8. As for the spirobis--lactam 9, the authors stipulated the addition of anion B followed by the nitrogen addition onto one of the ester functions leading to the intermediate 11. Deprotonation of 11, followed by the addition of the corresponding anion onto another equivalent of N-benzyl α-bromoacetamide 1 and a second nitrogen cyclization led to 9.

Synthesis of erythrinan and homoerythrinan from α-iodo acetamides

A beautiful use of α-halogenoacetamides for the synthesis of erythrinan and homoerythrinan alkaloid cores was achieved by Tu and co-workers in 2006 (Scheme 11). 45 One of the key steps in this strategy was the formation of the aza-heterocyclic 5-membered ring system with the wisely selected N-substituted αiodoacetamides 1. Refluxing ketone 27 in the presence of dimethyl carbonate and an excess of NaH led to the corresponding sodium enolate 28. Addition of N-substituted α-iodoacetamides 1 led to the formation of the bicyclic system 29 in a two-step domino sequence with yields ranging from 84 to 87% depending on the aromatic ring on acetamides 1. It is important to note that the same strategy for the synthesis of 31 (R = Me) was developed at the same time by Padwa and co-workers. More recently, an attractive modified version of this transformation was reported by Nishida and co-workers for the enantioselective total synthesis of (-)-minovincine 34 (Scheme 12). 47 In their case, the enol ether 32 resulting from a highly enantio-and totally diastereoselective Diels-Alder reaction was used instead of an enolate as described above. In the presence of TBAF, the desired lactam 33 was obtained in 90% yield as a single diastereomer. The latter was converted in several steps into the desired indole

Please do not adjust margins Please do not adjust margins alkaloid, a backbone well-known for its therapeutic properties, (-)minovicine 34.

Scheme 12. N-Benzyl α-iodoacetamide in the total synthesis of (-)-minovincine

Synthesis of 2-imino-4-thiazolidin-4-ones by reaction with potassium isothiocyanate

Interestingly, potassium thiocyanate (KSCN) reacts readily in the absence of any added base in acetonitrile and at room temperature with chloroacetamides 1 to furnish the corresponding 2-imino-4thiazolidin-4-ones 35 (Scheme 13). [48][49][50][51][52][53][54][55][56][57][58][59][60][61] The good nucleophilicity of the sulfur atom and the high reactivity of the chloroacetamides 1 promote the initial nucleophilic substitution, followed by the addition of the amide function onto the C≡N bond generating only KCl as side product Scheme 13. Access to 2-imino-4-thiazolidin-4-ones from potassium isothiocyanate

Synthesis of oxazolidin-4-ones via a domino O-alkylation/aza-Michael cyclization

An access to oxazolidin-4-ones 37 was reported by Marchetti when employing ambident potassium enolates 36 (Scheme 14). 62 In the presence of a promoter, i.e. Ag + or Ag 2 O, a domino Oalkylation/aza-Michael cyclization took place (see the proposed mechanism) and oxazolidin-4-ones 37 were isolated in yields ranging from 80 to 89%. Oxazolidin-4-ones related to 46 were also obtained via a domino process involving a rearrangement and the subsequent migration of the carbonyl group as published by Martel, Comesse and coworkers. In the presence of water and using the uncommon KNaCO 3 as base, the Michael acceptors 45 underwent hydrolysis. Then, the hydrolyzed Michael acceptors A reacted with α-bromoacetamides 1 to give enol ethers B followed by an aza-Michael cyclization leading to the intermediates C.

69, 70

An alternative access to oxazolidin-2,4-diones 47 was published by Rossi and co-workers by treatment of α-haloacetamides 1 with tetraethylammonium hydrogen (TEAHC) in acetonitrile (Scheme 18). 71 The cyclic product 47 was obtained in average to good yields via the postulated intermediate B. The authors proposed that the first step of the domino process was a condensation reaction followed by an intramolecular S N 2 reaction with the formation of water and an ammonium salt as side products.

More recently, Saliu and co-workers (Scheme 18) 72

73

. The high reactivity of isocyanates strongly favors the reaction with αbromoacetamides 1, leading to the formation of intermediate A which at 60 °C evolved spontaneously to form the products 49 in very good yields.

Interestingly, a dramatic change in the regioselectivity of the process was observed when replacing sulfonylisocyanates by arylisocyanates as presented in Scheme 19. 74,75 In fact the latter afforded access to 2-imino-oxazolidin-4-diones 50, instead of hydrantoins 49, in good yields and mild conditions from αchloroacetamides 1. These reactions were performed in CH 3 CN in the presence of K 2 CO 3 as base.

Synthesis of imidazolidin-4-ones by addition to an imine

The annelation of a five-membered lactam ring fused to the (benzo)indole or carbazole nucleus was accomplished by the addition of 2α-halogenoacetamides 1 onto imines 51 (Scheme 20).

76-78

Scheme 20. Formation of imidazolidin-4-ones

An intramolecular addition of compound 55 onto an imine was published by Pearson and coll. (Scheme 21). 79 In their paper, they described an elegant synthesis of the imidazolidin-4-one 56 via the imine B formed in situ from the aldehyde A. The iminium ion C, resulting from the quaternization of the imine, was trapped by the nitrogen atom of the amide function allowing the formation of the tetracyclic molecule 56 in a limited number of steps.

Please do not adjust margins Please do not adjust margins . 90 After the deprotonation of the nitrogen atom with NaH, the domino sequence starts with an aza-Michael addition leading to the anionic intermediate A. This step is followed by a carbocyclization furnishing the desired -lactam 59 in yields ranging from 49 to 69% and excellent diastereoselectivity when E ≠ E'. This domino aza-Michael/nucleophilic substitution reaction was applied to the concise synthesis of the tricyclic core of (±)-martinelline 61 and was afterward successfully applied to the synthesis of the attractive tricyclic spirooxindole skeletons. 91 When

Michael acceptors 63 bearing an ethoxy group were engaged in the domino process, N-acyliminium ion precursors C were isolated in high yields (Scheme 24, R 4 = OEt). 92 In order to prove the interest of such precursors, efficient inter-and intramolecular αamidoalkylation reactions were performed in the presence of a catalytic amount of Tf 2 NH (5 mol%).

Formation of polycyclic lactams

The strategy was then extended to the synthesis of polysubstituted oxazolo-pyrrolidinones 66 (Scheme 25). [94][95][96] For that purpose, One of the specific features of these α-bromoamides is, as for αbromoesters or α-bromoketones, related to their ability to transform the electrophilic into a nucleophilic center by deprotonation of the carbon bearing the halogen in basic media. This reaction is further favored by the presence of a ketone in βposition as in the beautiful domino Darzens reaction/hemiaminal formation described by Kuramochi and co-workers in their development of the synthesis of natural products (Scheme 27).

98-100

For that purpose, the authors engaged ketoaldehydes 71 with αbromo-β-ketoacetamides 1 in the presence of Et 3 N and were rewarded with the creation of 3 bonds during the domino sequence. Indeed, the Darzens reaction was followed by an intramolecular addition of the nitrogen atom to lead directly to the desired bicyclic product 72. For some substrates they were unable to isolate the too unstable hemiaminal A or the Darzens reaction was not followed by the nitrogen cyclization. Finally, the domino Darzens reaction/hemiaminal formation led to the total synthesis of berkeleyamide D (73) and rubrobramide (74). Another example of a domino reaction including a Darzens reaction was proposed by Yu and co-workers for the diastereoselective synthesis of epoxide-fused benzoquinolizidine derivatives 76 (Scheme 28). 101 In fact, when α-chloro-β-ketoacetamides 75 possessing an internal Michael acceptor were treated with a base, i.e. CsOH.H 2 O, a domino aza-Michael addition/Darzens reaction took place leading to the formation of three bonds, three rings and three fully controlled stereogenic centers. It is important to point out that, in this case, the order in which the bonds are formed is reversed compared to the examples above, i.e. the N-C bond was created prior to the C-C bond since an intramolecular aza-Michael addition, assisted by the base, took place prior to the Darzens reaction leading to the tetracyclic system 76.

Scheme 28. α-Chloro-β-ketoacetamides in domino aza-Michael/Darzens reaction.

α-Halogenoacetamide in a catalyzed domino coupling/cyclization sequence

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Metal-catalyzed reactions

The versatility of α-haloamides as reagent presenting two different reactive sites is not limited to simple basic media, but can also be extended thanks to a coupling of the amide with an aromatic species, or a coupling involving the carbon bearing the halogen. The first example was reported by Bao and coll. 102 with the copper catalyzed coupling of an o-iodophenol 77 and various chloro or bromo amides 1. The nucleophilic substitution of the halogen by the phenate is described to occur first and to be followed by the intramolecular Cu(I)-catalyzed coupling with the iodoaryl to give benzoxazin-3-ones 78 (Scheme 29). The second example was reported by Yin and coll. 103 using palladium catalyzed intramolecular cyclization. The benzoxazin-3-ones 78 were also synthesized by Diz et al. 104 using an efficient heterogeneous nanocatalyst Si-Cu for Ullmann intramolecular coupling reactions (Scheme 29).

Scheme 29. Formation of benzoxazin-3-ones

Interestingly, another Cu(I)-catalyzed reaction from αhaloacetamides 1 mostly with acrylamides (EWG = amide function) or acrylic esters (EWG = ester function) 79 was developed by Nishikata and co-workers (Scheme 30). 105 The authors managed to fully control the N-vs O-cyclization of the amide function of αhaloacetamides 1 depending on the base, the source of copper (CuBr.Me 2 S vs CuI), the ligand used and the presence of an ammonium salt, leading selectively to -lactams 80 or to the corresponding iminolactones 81. Remarkably, it proved possible to totally control the course of the domino process by tuning the reaction conditions and the desired heterocyclic systems 80 or 81 were isolated in good to high yields.

Scheme 30. Copper-catalyzed intermolecular aminolactonization

Among the α-halooacetamides presented, the α,α-difluoro αhaloacetamides display a very specific reactivity related to their ability to stabilize the radical formed by homolytic cleavage of the C-X bond. 32). This reaction is a modified Heck type reaction of α-bromo acrylamides with 1,3-dienes catalyzed by PdCl 2 (PPh 3 ) 2 (10 mol%) using ZnCl 2 as cocatalyst, in presence of Na 2 CO 3 as base. The yields were mostly impacted by the substituents onto the diene and the nitrogen atom (R 1 = Ar or Cy).

Scheme 32. Formation of γ-lactams from α-bromoacryl Pd-catalyzed amides

A straightforward access to optically active γ-lactams 91 bearing up to three fully controlled contiguous stereogenic centers, constructed from α-halogenoamides 1 and chloroalkenes ) and opens the access to a large variety of polycyclic lactams 95 in a limited number of steps. The two bonds were created by an aminopalladation followed by a reductive elimination regenerating the catalyst. Note that this palladium catalysis did not require the use of an expensive and/or sensitive ligand or external oxidant.

Scheme 36. Palladium-catalyzed intramolecular aminoalkylation: access to polycyclic lactams

Use of N-alkoxy α-halogenoacetamides

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As stipulated in the introduction section of this review, there is a dramatic change in the reactivity of α-halogenoacetamides when shifting from N-alkyl to N-alkoxy derivatives. While the former can be powerful domino partners as highlighted in the previous section, the latter have mainly been used as aza-oxyallyl cation precursors. These cations were first reported by Kikugawa and co-workers in the presence of a base in mild conditions thanks to the lower pKa of N-alkoxy compared to N-alkyl amides. The chemistry of azaoxyallyls has been lately a topic of growing interest due to their high ability to promote [3+m] cycloaddition reactions with various types of partners (dipole, C=C, C=O, diene, …) and is the subject of a recent review. 15 The present review will summarize the main results and discuss the most recent' ones published since, and will focus on the differences in reactivity between N-alkyl and N-alkoxy αhalogenoacetamides.

First, aza-oxyallyl cations in [3+4]-cycloadditions

Based on the previously reported generation of aza-oxyallyl cations by Kikugawa, Jeffrey's group was able in 2011 114 to develop the first aza-[3+4] cycloaddition with cyclic dienes. After screening various solvents, bases and Lewis acid additives, the best reaction conditions were found to be, depending on the N-alkoxy αhalogenoacetamides employed, either trifluoroethanol (TFE) or hexafluoroisopropanol (HFIP) with triethylamine as a base. Then, various alkoxyamides were reacted with furan (96, Y = O) or cyclopentadiene (97, Y = CH 2 ) via the aza-oxyallyl cations A, and cycloadducts 98 were isolated in yields ranging from 52 to 86%. This result involving furan as cycloreactant is remarkable considering the poor reactivity of furans in cycloaddition reactions. It should be mentioned that no reaction occurred when N-benzyl αhalogenoacetamide 1 was employed and this result was assessed by computational studies comparing the influence of the substituent onto the nitrogen atom. No evidence of the formation of an azaoxyallyl cation could be found when the OBn was replaced by a simple Bn on the amide. Finally, it was also shown that, as expected for the formation of cationic species, no reaction occurred when starting from primary N-alkoxy bromoamide 1' (R 2 = R 3 = H, Scheme 37).

Scheme 37. First example of the generation together with some reactivity of azaoxyallyl cationic intermediates A

Shortly afterwards, Jeffrey et al. described an intramolecular version of this cycloaddition reaction. [115][116][117] Since these first papers demonstrating the high potential of N-alkoxy bromoamides as aza-oxyallyl cation precursors, many applications involving them have been reported. Recently, Zhao et al. 118 have revisited this chemistry for the development of a rapid access to multisubstituted benzodiazepine derivatives 100 using a formal [3+4] cycloaddition reaction between azaoxyallyl cations and anthranils 99 (Scheme 38). These attractive polycyclic systems were obtained under mild conditions usually in high yields via the proposed formation of the zwitterionic intermediate A.

Scheme 38. [3+4]-cycloadducts from anthranil derivatives

Aza-oxyallyl cations in [3+3]-cycloadditions and recent developments

These aza-oxyallyl cations also proved to be good partners in [3+3]cycloadditions, either concerted or not. Several examples involving aryl or alkyl nitrones were described leading to 103, 105 [119][120][121] or 104. 122 Interestingly, these formal dipoles are also effective with less reactive nitrones such as isoquinoline N-oxides leading to 102. 123 This reaction with isoquinoline N-oxides proved to be compatible with N-alkyl α-bromoamides. Depending on the substrate, these reactions are either conducted in fluorinated solvents 119, 121, 122 or in acetonitrile, 120, 122 with probably a direct relationship between the solvent used and the mechanism involved. In some cases, the dipole involved in the cycloaddition reaction with the aza-oxyallyl cations is formed in situ like the nitrile imine formed from the hydrazonoyl chloride 108 (Scheme 39).

124

Alkenylindoles bearing an electron-withdrawing group can also behave as a formal dipole in these basic conditions with N-alkoxy αbromoamides to form indole alkaloids with potential antitumor activity. This journal is © The Royal Society of Chemistry 20xx

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[3+3] annulation of aza-oxyallyl cations with nitrones

The fully diastereoselective synthesis of cyclopenta [b]piperazinones 121 was reported by Gandon, Leboeuf and co-workers by a reaction sequence between 2-furylcarbinols, anilines and N-alkoxy αhaloacetamides (Scheme 44). 132 The scope of the process is wide and allows access to polysubstituted cyclopenta [b] piperazinones 121 in good yields. The proposed mechanism, assessed by DFT computations, does not proceed via an aza-oxyallyl cation. Indeed, the authors were unable to compute the aza-oxyallyl cation previously envisioned. The formation of such zwitterionic species was considered unlikely compared to the corresponding α-lactam A.

The reaction was proposed to proceed by reaction between the deprotonated form of the aza-Piancatelli product C and the oxirane-2-imine B resulting from A.

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Please do not adjust margins An elegant and novel intermolecular approach to access both C3unsubstituted and C3-substituted oxindoles 149 employing an aryne annulation was developed by Prasanthi and co-workers (Scheme 51). 155 For that purpose, the aryne derivatives were generated in situ from substituted 2-(trimethylsilyl) aryl triflates 147 and were reacted with haloamides 1'. It is important to note that the reaction conditions were different from the one classically employed in this chemistry, i.e. the use of THF instead of a fluorinated solvent without a base. Moreover, contrary to the other strategies involving azaoxyallyl cations, primary N-alkoxy αbromoamides 1' (R 2 = R 3 = H) were efficient substrates but not disubstituted N-alkoxy bromoamides 1' (R 2 and R 3 ≠ H). Thus, the authors considered that the mechanism probably involved a concerted cycloaddition of aryne 148 with transient α-lactam B instead of the formation of an azaoxyallyl cation. They also proved that no reaction occurred when a N-benzyl amide was engaged, once again demonstrating the dramatic role of the N-alkoxy substituent.

Scheme 51. Aryne annulation via the formation of α-lactams B

Recently, another [3+2] cycloaddition reaction between azaoxyallyl cations and 1,2-benzisoxazoles 150 or 3-methyl-1,2-benzisoxazoles 152 has been apprehended (Scheme 52). 156, 157 This reaction provides a rapid access to functionalized oxazoline 151 in good yields. It was found that the reaction was sensitive to the substituent onto 1,2-benzoisoxazoles. When 3-methyl-1,2-benzoisoxazoles 152 were used, unstable tricyclic imidazolones 153 were obtained only in 13-16% yields. In their paper, the authors clearly demonstrated that N-alkoxy αbromoamides 1' reacted with sulfamate-derived cyclic imines 154 via a domino Aza-Mannich/S N 2 cyclization, since no reaction occurred in HFIP or when a disubstituted N-alkoxy bromoamide 2 and R 3 ≠ H) was used. Moreover, the reaction proceeded smoothly when primary N-alkoxy α-bromoamides (R2 = R3 = H) were engaged and N-benzyl amide 1 led to the desired product albeit in a lower yield. But more importantly, the reaction between the enantioenriched (R)-1' and unsubstituted sulfamate-derived cyclic imine 154 led to product 155 in 95% yield with only a minor loss in ee. All these experiments unambiguously prove that the reaction pathway is not via the formation of an azaoxyallyl cation but via a domino aza-Mannich/S N 2 cyclization as depicted in the proposed mechanism (Scheme 53).

Scheme 53. N-alkoxy α-bromoamides XX in a domino Aza-Mannich/SN2 cyclization

The cycloadditions with sulfur ylides proved to be very particular, allowing the authors to control the course of the reaction either toward the formation of a 5-membered ring or a 4-membered ring depending on the substituents and the base used (Scheme 54.

[3+2]-and [3+1]-cycloadditions with sulfur ylides). 159 In fact, the amidate formed intermediately can either substitute the sulfonium to form the 4-membered ring 159 or add to the carbonyl and eliminate Me 2 S to form 157. This versatility of the sulfur ylide leads

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Scheme 54. [3+2]-and [3+1]-cycloadditions with sulfur ylides

A very surprising reactivity of these N-alkoxy α-bromoamides was reported by Zhang and co-workers. 160 They observed a base-

Conclusion

Despite the wide scope of reactions involving αhalogenoacetamides in domino and/or cycloaddition reactions, the recent developments proceeding by photoredox activation 112 or involving a Cu promoted coupling step [107][108] open to several new applications. Anyway, these reactions remain at this time limited to α-difluoroacetamides and the extension to other αhalogenoacetamides will be one of the challenges to overcome. In addition, the use of α-bromoacetamides displaying a chiral center at the carbon atom bearing the bromine in their enantiopure form remain underexploited in these reactions. The reactivity of the species generated in basic conditions is closely related to the substituent onto the nitrogen atom. Hence, aza-oxyallyl cations can be generated in soft conditions from N-alkoxy αhalogenoacetamides and benefit from the low basicity of the amidate and the high nucleophilicity of the nitrogen from the α effect, allowing several types of cycloadditions. On the contrary, Nalkyl α-halogenoacetamides are mostly involved in domino processes usually in the presence of stronger bases. Undoubtedly, there is still an underexploited chemical territory where N-alkoxy α-halogenoacetamides can be used as powerful domino partners and N-alkyl α-halogenoacetamides in cycloaddition reactions. The opportunity to further modulate the reactivity of the nitrogen of the amidate by the presence of electron withdrawing groups lead to several possible new developments starting from αhalogenoacetamides.

  Figure 1.

Figure 2 .

 2 Figure 2.

13e- h Scheme 2 . 2 .

 h22 Scheme 2. Synthesis of N-alkyl and N-alkoxy α-halogenoacetamides

Scheme 4 . 2 , 5 -diones 3 and 4 from chiral α- bromoacetamides 1 2 . 2 .

 42522 Scheme 4. Formation of piperazine-2,5-diones 3 and 4 from chiral αbromoacetamides 1

Scheme 5 . 2 , 6 -diones 6 by reaction with α-aminoesters 5 3.

 5265 Scheme 5. Formation of piperazine-2,6-diones 6 by reaction with α-aminoesters 53. Reaction of α-halogenoacetamide with nucleophiles3.1. Synthesis of β-, -lactams and bis--lactams by reaction with malonate derivatives The synthesis of lactams starting from α-halogenoacetamides employing a domino process has been mostly described with α-or β-dicarbonyl compounds (such as malonate derivatives) as reaction partners. Among these strategies, the electrochemical reaction of diethyl bromomalonate 7 with α-bromoacetamides 1 allowed the formation of various skeletons depending on the reaction conditions as demonstrated by the work of Moracci and co-workers (Scheme 6)35 The electrogenerated diethyl malonate anion

Scheme 6 .

 6 Scheme 6. Access to β-lactams 8, -lactam 11 and spiro bis--lactam 9

46 Scheme 11 . Synthesis of the erythrinan and homoerythrinan cores 3 . 4 .

 461134 Scheme 11. Synthesis of the erythrinan and homoerythrinan cores

Scheme 14 . 66 Scheme15. 67 Scheme 16 .

 14666716 Scheme 14. Access to oxazolidin-4-ones via a domino O-alkylation/aza-Michael cyclization 3.7. Synthesis of oxazolidin-4-ones by addition to the C=O bond of an amide, a ketone or an aldehyde

Scheme 19 .

 19 Scheme 19. Formation of Hydrantoins and 2-(imino)oxazolidin-4-ones

93 Scheme 24 .

 9324 Scheme 24. Domino aza-Michael /nucleophilic substitution reaction

Scheme 25 .

 25 Scheme 25. Access to spiro-imides

Scheme 26 .

 26 Scheme 26. Divergent access to -lactams and 1,4-oxazepines

Scheme 27 .

 27 Scheme 27. α-Bromo-β-ketoacetamides in domino Darzens reaction/hemiaminal formation.

Scheme 52 .

 52 Scheme 52. [3+2] Cycloaddition with 1,2-benzisoxazoles

  158

  Scheme 55. [2+4] cycloadditions with aza-o-quinone methides aza-oQMs

. Scheme 7. Electrochemical mechanisms for the synthesis of β-lactams 8 and spiro bis-

  An independent synthesis of this spiroproduct 9, albeit in a lower 10% yield, was performed with diethyl malonate 10 in the presence of NaH in DMF to validate the passage via the anion B

	the spirooxindole derivatives 15 were obtained from N-alkyl α-
	bromoacetamides 1 following two pathways, one in two steps by
	reaction with dimethyl 2-(2-nitrophenyl)malonate 13 followed by a
	nitro-reduction/amino-ester cyclization and the second in one step
	by reaction with ethyl oxindole-carboxylate. Using a similar
	intermediate, Rammah et al. described the access to
	spirosuccinimide-γ-butyrolactone 12 via a sequence involving the
	saponification of the ester and the formation of the γ-butyrolactone
	by treatment with NBS or NIS to form the halogenated lactone
	displaying, in the case of the alkyne, an exocyclic double bond.	38
	More recently, gram scale access to succinimide derivative 11' (40 g
	batches) from α-chloroacetamide 1 was performed by Bhogle and
	co-workers as a key step for the preparation of an aldose reductase
	inhibitor, namely AS-3201 (16).
		-lactam 9
		In fact, the above strategy employing diethyl malonate proved to be
		an open route to spiro-compounds. Based on the above, Allous et
		al. published a modified and more comprehensive method for the
		synthesis of symmetrical (R	1 = R'	1 in one step) and unsymmetrical
		(R	1 ≠ R'	1 in two domino steps) spiroimides 9 (Scheme 8).	36 Due to
		the high biological interest of the spirooxindole ring system, the
		same domino strategy was successfully applied by Comesse, Daïch

39 Scheme 8. Access to spiro-imides by reaction with malonate derivatives

	It should be mentioned that other active methylene compounds
	10'' can be used instead of dialkyl malonates in order to reach other
	N-heterocyclic systems following the same domino pathway.	40-42
	When nitrile functions are present on the active methylenes 10'',
	the formation of enamino -lactams 19, from the imine
	intermediate 18, can be observed. An interesting domino reaction
	on these lines was published by Tverdokhlebov and co-workers with
	the construction of 2 cycles during the process leading to 22
	(Scheme 9).

43 Scheme 9. Use of various active methylene compounds

		isolated in yields ranging from 58 to 84%. Pesquet and Othman later
		employed the same strategy for the formation of
		spiroisoindolinones 26 engaging phthalimidines 25 (Scheme 10) as
		domino partner.
	3.2. Synthesis of spiro-imides derived from oxindole and
	isoindolinone derivatives
	Allous et al. also demonstrated the efficiency of such strategies for
	access to spiroxindoles employing ethyl N-methyloxindole-
	carboxylate 23 instead of malonate derivatives.	37 In refluxing
	acetonitrile in the presence of K 2 CO 3 , the tricyclic products 24 were

44 Scheme 10. Access to spirooxindole derivatives

Scheme 21. Access to imidazolidin-4-ones

		Scheme23. Addition on thioamides
		4.6. Addition to a C=C bond
		4.6.1. Formation of pyrrolidinones by a domino aza-Michael
		/nucleophilic substitution reaction
		Comesse and co-workers developed a new domino sequence
		involving the α-bromoacetamides 1 by reaction with various
		Michael acceptors 45 (Scheme 24)
	Scheme 22. Reactivity with isothiocyanates
	4.5. Synthesis of spirothiazolidinones by addition onto thioamides
	More surprisingly, α-bromoamides 1 can also formally perform
	addition onto the C=S bond of a thioamide as demonstrated with
	compound 57 (Scheme 23).	88, 89 This particular reactivity is related
	to the nucleophilicity of the sulfur atom of the thioamidate A
	formed in the presence of the base (NaH) and the lower pKa
	associated to the thioamidate. Indeed, the sulfur of the thioamidate
	can add to the α-bromoamide 1 to form the intermediate B that
	cyclizes to form the spirothiazolidinone 58.

4.4. Synthesis of 2-imino-thiazolidin-4-ones by addition onto isothiocyanates

Similarly to potassium thiocyanate (Scheme 13), α-haloacetamides 1 can react efficiently with isothiocyanates to form 2-imino-4thiazolidinones 53 but the sequence of the process is different.

80-87

Contrary to potassium thiocyanate, in this case the reaction will first involve the addition of the amide onto the isothiocyanate, and then, the sulfur atom substitutes the halogen to form the thiazolidine-4-one 53. It should be noted that a soft base such as a carbonate is mandatory to promote this reaction with good yields (Scheme 22).

  α-halogenoamides 1 and Michael acceptors 45 bearing a leaving group led efficiently, in the presence of NaH or K 2 CO 3 as the base, to bicyclic lactams 66. In this domino oxa-Michael/aza-Michael/intramolecular nucleophilic substitution sequence assessed by DFT, three bonds were created and the main difference with the previous work was the formation of the intermediate enol ether A prior to the aza-Michael addition. It is worth noting that in most of the cases when two different electron-withdrawing groups were tested, only one diastereomer was detected by

	1 H NMR on
	the crude mixture.

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Scheme 31. Copper-catalyzed aminodifluoroalkylation of alkenes and alkynes: access to difluoro--lactams Iyer

  Chen et al.

		106 published the synthesis of α,α-difluoro-γ-
	lactam derivatives 84 using a copper/amine catalyst via a tandem
	radical cyclization pathway from easily available alkenes 82 and N-
	aryl bromodifluoroacetamide 1 as starting materials. The reaction
	was initiated by a single electron transfer (SET) from the Cu(I)
	catalyst to compound 1 to produce radical A. Then the addition
	reaction of A to alkene 82 gave an alkyl radical intermediate B
	(Scheme 31).
	A similar reaction to prepare difluoro-γ-lactams 84 or 85 was
	described by Lv et al.	107, 108 starting from monosubstituted alkenes
	82 or mono or disubstituted alkynes 83, respectively (Scheme 31).
	The formation of the radical species A from α,α-difluoro α-
	bromoacetamides 1 was promoted by Cu(I) .

et al.

109

described the synthesis of γ-lactam derivatives 88 starting from 1,3-dienes 87 and α-bromoacryl amides 86 (Scheme

Formation of γ-lactams catalyzed by planar-chiral Cp'Ru complex

  Their strategy was based on an asymmetric auto-tandem allylic amidation/atomtransfer radical cyclization (ATRC) reaction, which proceeds in a highly regio-, diastereo-, and enantioselective manner, catalyzed solely by a planar-chiral Cp'Ru complex. The first step in this domino process was the enantioselective allylic amidation of αhaloacetamides 1 followed by a diastereoselective ATRC, both catalyzed by the same catalyst.

	Scheme 33.

89, was This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins developed by Kanbayashi et al. (Scheme 33). 110 5.2. Transition-Metal-free reaction The synthesis of difluoro-γ-lactams 84 from α,α-difluoro αiodoacetamides 1 was reported by Chen and co-workers in 2012 via a radical addition onto alkenes 82 (Scheme 34). 111 In the presence of sodium hydrosulfite, radical species A were generated. Regioselective addition onto alkenes 82 and subsequent trapping of the resulting radical with iodine led to -iodo amide C. S N 2 cyclization of the nitrogen atom and deprotonation of the intermediate ammonium salt formed led to the desired -lactam 84. Scheme 34.

Access to difluoro--lactams via a radical addition onto alkenes

  Ir(ppy) 3 . After addition onto alkene 92 and another oxidative step, the carbocation C was quickly trapped by iodide ion preventing side reactions. -Iodoacetamides D then underwent an intramolecular S N 2 reaction leading to difluoro-γ-lactams 93 while regenerating the iodide ion.

	5.3. Photoredox/iodide-catalyzed reaction
	Zhu and co-workers lately demonstrated that the α,α-difluoro α-
	bromoacetamides 1 are also powerful tools for a cascade
	photoredox/iodide catalysis order to access 93
	in good yields (Scheme 35).	112 In order to prevent the formation of
	side products from competitive reaction pathways when
	employing radical chemistry, the authors applied a mild and dual

catalytic protocol allowing the synthesis of various difluoro-γlactams 93 in good to excellent yields. The proposed mechanism involved two catalytic cycles, the first one leading to the radical intermediate A via a single-electron-transfer (SET) radical process thanks to a photocatalyst, fac-Scheme 35. Access to difluoro--lactams via a cascade photoredox/iodide catalysis 5.4. Metal-catalyzed intramolecular cyclization: access to polycyclic γ-lactams A Pd-catalyzed intramolecular aminoalkylation starting from α-halo acetamides 94 bearing an alkene moiety was published recently by Yang and co-workers (Scheme 36). 113 This intramolecular Pdcatalyzed coupling is highly complementary to the previously presented Cu(I)-catalyzed aminoalkylations (see Schemes 30 and 31

Scheme 39. Examples of [3+3]-cycloadducts obtained from aza-oxyallyl cations

  Since the publication of the review by Xuan and co-workers on azaoxyallyl cations,15 some new examples have been developed and are listed below.

	Recently, Xuan and co-workers reported a [3+3]-cycloaddition
	reaction with azomethine imines A generated in situ (Scheme
	40).	126 Under Ag(I) catalysis, the hydrazide 254 undergoes in
	acetonitrile a cyclization to form the intermediate azomethine
	imine A. The isoquinoline-fused triazine derivatives 111 were
	obtained in good to excellent yields. The same team developed a
	metal-free version of this cycloaddition reaction in the presence of
	iodine or bromine (I 2 or Br 2 ) via the in situ generated C,N-cyclic
	azomethine imines. This one-pot process afforded a series of
	biologically important isoquinoline-fused triazines 112 in good to
	excellent yields, which contain valuable alkenyl C-X bonds (X=Br, I)
	(Scheme 40).

127 Scheme

40. Base-mediated [3 + 3] cycloaddition of C,N-cyclic azomethine imines

  This reaction promoted by Cs 2 CO 3 as the base in HFIP at room temperature proceeded in good 65-89 % yields (Scheme 21).

			Scheme 42. [3+3] formal cycloaddition of aza-oxyallyl cations with azides
			Cycloaddition with nitrones 117 was recently considered as a formal
			[3+3] annulation reaction (Scheme 43).	130, 131 The best reactions
			were conducted in CH 3 CN in the presence of Na 2 CO 3 and the
			targeted 1,2,4 oxadiazinan-5-ones 118 were obtained with yields up
			to 99%. The authors proposed the formation of the zwitterion
			intermediate A resulting from the nucleophilic attack of the oxygen
			of the nitrones 117 onto an aza-oxyallyl cation generated in situ
			from N-benzyloxy α-bromoamides 1'. The second step of the
			domino process was the intramolecular nucleophilic attack of the
			nitrogen atom of the amide function leading to the final products
			118.
	1,2,4-Oxadiazin-5-ones 114 were obtained by Xing, Sun and coll.	128
	via a [3+3]-cycloaddition of in situ generated nitrile oxides formed
	in the basic conditions required for the formation of aza-oxyallyl
	cations and their reaction with hydroximoyl chlorides 113. Scheme 21. [3+3] annulation of aza-oxyallyl cations with nitrile oxides
	A [3+3]-cycloaddition reaction between azides 115 and once again
	in situ formed aza-oxyallyl cations in HFIP was performed by Xu et
	al. (Scheme 42).	129 In this study, the authors obtained 1,2,3,4-
	tetrazines 116 in good yields under mild conditions.

[3+2] formal cycloaddition of aza-oxyallyl cations

	Scheme 44. Synthesis of cyclopenta[b]piperazinones
			Scheme 46. [3+2]-Cycloadditions with imines
	6.3. Aza-oxyallyl cations in [3+2]-cycloadditions and recent
	developments		As previously mentioned, alkenes and alkynes were used as
	The first examples of [3+2] cycloadditions were given by the teams of Jeffrey, 117, 133 Wu 134 and later Liao 135 on indole derivatives close to 123. Recently, Zhang et al. 136 published the total synthesis of (+/-	cycloaddition partners by several authors. Recently, a [3+2]-cycloaddition reaction implicating styrene derivatives 137 was developed by Huang and coworkers. 148 The multi-substituted
	)-minfiensine 126 from the N-protected indole 125 via an efficient	pyrrolidinones 138 were consequently obtained in moderate to
	[3+2]-cycloaddition between the trichloro acetamide 1' and the	good yields (Scheme 47). Xu and coworkers also demonstrated the
	indole derivative 124 (Scheme 45). In their paper the authors	high efficiency of such a process when starting from substituted
	demonstrated that the protection of indole was crucial to obtain	chalcones 139 and the α-bromo N-alkoxyamide 1' in the presence
	(+/-)-minfiensine 126. While this type of cycloaddition has been extended to alkynes, 137 most of the efforts mainly focused on the	of Na 2 CO 3 in HFIP at room temperature (Scheme 47). this strategy, polysubstituted γ-lactams 140 were obtained in good 149 Employing
	cycloaddition with dipolarophiles bearing a heteroatom in the double (C=O, C=N, C=S) or the triple (C≡N) bond, to form 132 138-142	yields.
	by reaction with an aldehyde or a ketone, 131 143-144 by reaction with
	an isothiocyanate or 129 and 130	145 by reaction with a nitrile. These
	reactions are mainly performed in fluorinated solvents such as HFIP.
	Erden and coworkers	146 also contributed to this field by developing
	a [3+2]-cycloaddition from N-benzyloxy-2-chloroamides 1' in the
	presence of NEt 3 or Na 2 CO 3 , onto N-arylimines 133 (Scheme 46).
	The desired imidazolidin-4-ones 135 were isolated in moderate to
	good yields. The same year, Ji and Sun reported an analog [3+2]-
	cycloaddition reaction of arylimines generated in situ from
	hexahydro-1,3,5-triazines.	147

Scheme 45.

Scheme 47. [3+2]-

Cycloaddition with alkenes and alkynes

  CN instead of HFIP in which no reaction occurred and to performing the reaction in the presence of Na 2 CO 3 as a base. In these conditions, the hydantoins 142 were isolated in good to excellent yields. The proposed mechanism involved a domino process via the intermediate A resulting from the nucleophilic addition of the nitrogen atom of isocyanates 141 in α compared to the amide.

	Scheme 49. [3+2] Annulation of thiocarbonyls
	A beautiful formal [3+2]-cycloaddition reaction was published by
	Xuan et al.	154 The starting materials engaged in the process were
	cyclopropenones (Y = O) or cyclopropenethiones (Y= S) 145
	together with in-situ-formed azaoxyallyl cations (Scheme 50). Under
	mild conditions the spirocyclic oxazole (Y = O) or thiazole (Y= S) 146
	derivatives were isolated in good yields. The authors proposed the
	formation of the zwitterionic intermediate A followed by an
	intramolecular nucleophilic addition of the nitrogen atom.
		In complement to the previously described reaction with
		isothiocyanates,	143, 150, 151 a reaction between N-benzyloxy α-
		bromoamides 1' and isocyanates 141 has been reported by Wang
		and coworkers (Scheme 48).	152 This procedure allows the rapid
		assembly of hydantoins 142 in good to excellent yields from simple
		and readily available starting materials. The solvent screening led to
		the use of CH 3 Scheme 48. Domino reaction with isocyanates
		These annulation reactions were extended beyond isothiocyanates
		by Saha, Das and co-workers to thioketones and thioamides with a
		large diversity of substituents on the thioketones or the thioamides
		with good to excellent yields.

153

However, the cycloaddition proved to be poorly diastereoselective (Scheme 49). This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins Scheme 50. [3+2] Cycloaddition with cyclopropenones and cyclopropenethiones

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

Acknowledgements

We thank the "Fédération de Chimie": FR CNRS 3038 (INC3M), the Région Normandie, the URCOM and IMMM laboratories for their financial support. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins

Conflicts of interest

There are no conflicts to declare.


Ludovic Charpentier
email: ludovic.charpentier@promes.cnrs.fr

Marianne Balat-Pichelin
email: marianne.balat@promes.cnrs.fr

Very high temperature oxidation of SiC in Ar-O 2 -H 2 O atmosphere

Keywords: SiC, oxidation, high temperature, water steam

The oxidation of SiC follows two regimes, according to temperature and partial pressure of oxidizing species. The first one, occurring at low temperatures and relative high partial pressures is the passive one, with the production of solid silica (SiO 2 ). The second one at higher temperatures and lower partial pressures is the active one, with the production of gaseous SiO. The presence of water steam may favour the volatility of the SiO 2 layer through the production of gaseous hydroxides such as Si(OH) 4 . We studied the behaviour of some SiC samples heated using a solar furnace in a 'wet' gas flow around 2150 K. The experiments were filmed through a video camera and in parallel, some thermodynamic calculations were performed to check the domains of stability of SiO and Si(OH) 4 gaseous products. It appeared that SiO was preponderant above 2000 K.

INTRODUCTION

Silicon carbide (SiC) is a ceramic that can be used for some high-temperature applications (solar receiver, nuclear cladding, combustion engine…). The oxidation of SiC follows two regimes, according to temperature and partial pressure of oxidizing species (O 2 , H 2 O…). The first one, occurring at low temperatures and relative high partial pressures is the passive one, with the production of solid silica (SiO 2 ). The second one at higher temperatures and lower partial pressures is the active one, with the production of gaseous SiO. As for example, Kim et al. [1] reported that at 1570 and 1670 K, the passive oxidation mode occurred when the partial pressure of H 2 O (pH 2 O) was above 500 Pa (using H 2 as carrier gas with total pressure P of 10 5 Pa), whereas active oxidation mode was encountered for pH 2 O < 100 Pa.

Nevertheless, Opila [2,3] discovered that water steam played a different role than dry O 2 on the oxidation behaviour of SiC. Performing thermogravimetric analyses (TGA) between 1470 and 1870 K, she discovered that adding water steam (pH 2 O = 10 4 Pa) to dry oxygen (at P = 10 5 Pa) increased the growth rate of silica [2]. She explained that H 2 O could favour the transportation of impurities from the protective tube (containing Na, Al) and the contamination could enhance the diffusion through the silica layer and therefore its growth rate. In wetter atmosphere (a gas mixture of 50% H 2 O -50% O 2 ), it was reported that part of the formed silica could volatilise between 1470 and 1670 K [3]. Mass spectrometry enables to identify the Si(OH) 4 species as the primary vapour species produced during this volatilisation [4].

TGA presents nevertheless two limitations to study the reaction of ceramics in wet atmosphere: -Its heating rate is slow, around 15 K min -1 . The absorption of concentrated solar flux, the nuclear accidents, the explosion inside combustion chambers provoke much faster heating rates (around 100 K s -1 ). During TGA analyses, some reactions may occur during transition from ambient to the desired temperature plateau, and therefore affect the final result compared to which could be expected in a real application. -Its temperature limitation: it is hardly possible to reach higher temperatures than 2000 K.

Therefore we have performed investigations at the 5 kW Odeillo solar furnace, using the REHPTS facility (Réacteur Hautes Température et Pression Solaire, High Temperature and Pressure Solar Reactor) that is described in another presentation [START_REF] Charpentier | this conference. 6. Thermodata[END_REF] in order to perform high temperature oxidation and to study the influence of water steam on the high temperature corrosion of SiC at 2150 K. Thermodynamic calculations performed using GEMINI [6] software enabled to predict the domain of stability of gaseous SiO and Si(OH) 4 .

EXPERIMENTAL PROCEDURES

The REHPTS set-up previously described [START_REF] Charpentier | this conference. 6. Thermodata[END_REF] was equipped with a controlled evaporator mixer (CEM from Bronkhorst [START_REF] High-Tech | [END_REF]) to study the experimental influence of water steam on the high-temperature oxidation of SiC. The SiC samples are heated inside the REHPTS using concentrated solar flux and a 'wet' gas flow for a total pressure of P = 10 5 Pa. The carrier gas is a mixture of 80% mol. Ar and 20% mol. O 2 , close to air composition about oxygen content and H 2 O is added to it using the CEM device. H 2 O content is 30 or 50 % mol. The oxidation experiments are carried out during 5 min. and are filmed through a video camera. Temperature is measured using an optical pyrometer at 5µm and the error for SiC sample is ± 25 K at 2200 K. Samples of α-SiC from Boostec (Fr) were weighed before and after the experiment so that the mass variations were reported into a percentage (ratio of the mass variation toward the initial mass).

We also computed thermodynamic calculations using GEMINI [6] software. The data required (∆H f °(298 K), S°(298 K), c p (T) ) for most of expected chemical species were found in the JANAF tables [8], excepted for Si(OH) 4 . Several authors have determined thermodynamic data for this compound [START_REF] Krikorian | Thermodynamics of the Silica-Steam system[END_REF][10][11][12][13][14]. The most recent data are in the work from Plyasunov [14]. We used the same standard enthalpy and entropy than he did, i.e.:

∆H f °(298 K) = -1340.68 kJ mol -1 S°(298 K) = 347.78 J K -1 mol -1 Nevertheless we had to change the formula he used to calculate c p (T). Plyasunov fitted his experimental measurements by a 5-degree polynom: c p /R = a 0 + a 1 T + a 2 T 2 + a 3 T 3 + a 4 T 4 + a 5 T 5 (1) with the following values : a 0 = 2.87914, a 1 = 5.89126x10 -2 , a 2 = -9.47715x10 -5 , a 3 = 7.84564x10 -8 , a 4 = -3.15382x10 -11 , a 5 = 4.89073x10 -15 .

This formula was determined for a temperature range between 250 and 2000 K. The matter is that GEMINI computes values of c p using the following formula:

c p = a + b T + c T 2 + d/T 2
(2) Therefore we had to calculate values of a , b, c and d coefficients on various temperature intervals in order to be the closest possible to the model that Plyasunov proposed. Table 1 resumes the values of coefficients we used and figure 1 compares the result of our interpolation with Plyasunov formula. The deviation is less than 1% between the two calculations.

TABLE 1.

Values of the coefficients used to calculate the c p for Si(OH) 4 on various temperature ranges Temperature range (K) a (J K -1 mol -1 ) b (J K -2 mol -1 ) c (J K -3 mol -1 ) d (J K mol - ∆H f °(298 K), S°(298 K), and the values of the coefficients we interpolated were incorporated to the existing thermodynamic database in order to add the possibility of Si(OH) 4 production. Unfortunately, these data are only valid up to 2000 K, we cannot check if Si(OH) 4 is stable around 2150 K as in our experimental conditions (no thermodynamic data are available at so high temperature range as far as we know).

RESULTS AND DISCUSSION

Thermodynamic calculations

In an atmosphere with 50% mol. H 2 O Figure 2 presents the results of the thermodynamic calculations that were performed with the following initial conditions:

-

It can be noticed on figure 2 (a) that a slight amount of silica (it stays lower than 0.4 % mol. at 2200 K) is volatising above 2000 K. Figure 2 (c) is a semi-logarithmic graph of the partial pressures of the gases SiO and Si(OH) 4 that could result from this volatilisation, the ratio between the two partial pressures is also plotted. This last

)

Plyasunov model [14] Our interpolation figure show that Si(OH) 4 is the main product of volatilisation at temperatures lower than 1750 K (where pSi(OH) 4 /pSiO > 10), whereas SiO is preponderant at 2000 K (pSi(OH) 4 /pSiO ≈ 0.1). We could not plot pSi(OH) 4 above 2000 K, but from the regular decrease of the ratio between the two partial pressures from 1600 to 2000 K, we may conclude that SiO is much more preponderant than Si(OH) 4 above 2000 K. So the volatilisation of silica into Si(OH) 4 is possible up to 1750 K, which is coherent with the observations of Opila and co-workers [4,[START_REF] Charpentier | this conference. 6. Thermodata[END_REF] who noticed the experimental production of Si(OH) 4 up to 1670 K. Above 2000 K, and particularly during our experiments at 2150 K, SiO is the expected gaseous product of the volatilisation of silica.

In an atmosphere with 30% mol. H 2 O

Figure 3 presents the results of thermodynamic calculations on the oxidation of 1 mol α-SiC, from 1600 to 2200 K, with initial composition of the atmosphere: 5.6 mol Ar, 1.4 mol O 2 , 3 mol H 2 O. We observe two main difference with figure 2: -SiO 2 volatilises less (figure 3 (a) ). -More CO 2 and less CO are stable (figure 3 (b)). -Production of Si(OH) 4 is less significant (figure 3 (c)). The production of SiO stays nearly the same in figures 2 and 3 on the investigated temperature range.

Experiments

Figure 4 shows the video captions of various experiments after 3 min. at 2150 K in wet atmosphere. Experiments 1 and 2 were performed in an atmosphere containing 30% mol. H 2 O, 3 and 4 in an atmosphere containing 50% mol. H 2 O. We notice for these two latest cases the presence of water droplets around the observation window. In all of the experiments we reported an important production of gases (CO, CO 2 and a smaller amount of SiO according to the thermodynamic calculations), with the presence of smokes and an important formation of bubbles of molten silica. The sample 3 and 4 are even leaning due to an important bubble formation on the back side of the sample. These results are different from the predictions of themodynamic calculations. According to figures 2 (a) and 3 (a), a wetter atmosphere should favour the volatilisation of silica. The explanation could be that increasing the content in H 2 O would favour a more important production of silica, and a thicker silica layer would limit the diffusion of gaseous species, reducing the volatilisation kinetics.

CONCLUSIONS

Thermodynamic calculations enable us to determine than in a wet atmosphere containing 30% mol. or 50% mol. H 2 O, silica could volatilise into Si(OH) 4 and SiO, Si(OH) 4 being preponderant at temperatures lower than 1750 K,

FIGURE 2 .

 2 FIGURE 2. Thermodynamic calculations performed from 1600 to 2200 K. Initial conditions: 1 mol α-SiC, 4 mol Ar, 1 mol O 2 , 5 mol H 2 O. P = 10 5 Pa. a) Number of moles of silica produced ; b) partial pressures of CO and CO 2 ; c) partial pressures of Si(OH) 4 and SiO and their ratio (semi-logarithmic scale).

FIGURE 3 .

 3 FIGURE 3. Thermodynamic calculations performed from 1600 to 2200 K. Initial conditions: 1 mol α-SiC, 5.6 mol Ar, 1.4 mol O 2 , 3 mol H 2 O. P = 10 5 Pa. a) Number of moles of silica produced ; b) partial pressures of CO and CO 2 ; c) partial pressures of Si(OH) 4 and SiO and their ratio (semi-logarithmic scale).

FIGURE 4 .

 4 FIGURE 4. Video captions after 3 min. of α-SiC oxidation at 2150 K and in Ar/O 2 mixture + 30 % mol. H 2 O (1, 2) or 50% mol. H 2 O (3,4).

Table 2

 2 presents the mass variations of the samples during the experiments. We observe the samples lose less weight (sample 4 is even gaining weight) in an atmosphere containing 50% mol. H 2 O than in an atmosphere containing 30% mol. H 2 O.

TABLE 2 .

 2 Mass variations of the samples treated at 2150 K in an atmosphere Ar/O 2 + H 2 O

N° of the experiment H 2 O content (% mol.) Mass variation (%)

	1	30	-0.54
	2	30	-0.80
	3	50	-0.01
	4	50	+ 0.10

SiO being the main product at temperatures higher than 2000 K. Experiments performed at 2150 K reveal an important production of gases, in agreement with the expected production of CO, CO 2 , and a smaller amount of SiO from the thermodynamic calculations. The mass losses are nevertheless less significant in an atmosphere with 50% mol. H 2 O than in an atmosphere with 30% mol. H 2 O, although thermodynamic calculations predict a more important volatilisation. This might be due to the kinetics: a wetter atmosphere could favour the production of a thicker silica layer that would control the diffusion of gaseous species.