Alena N Kulakova

Alexey N Bilyachenko
email: bilyachenko@ineos.ac.ru

Alexander A Korlyukov

Lidia S Shul'pina

Xavier Bantreil

Frédéric Lamaty
email: frederic.lamaty@umontpellier.fr

Elena S Shubina

Mikhail M Levitsky

Nikolay S Ikonnikov

Georgiy B Shul'pin
email: shulpin@chph.ras.ru

A new "

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The tremendous popularity of self-assembly reactions aimed at the creation of different types of polyhedral metallacomplexes is explicable by the great efficacy of the underlying methodology. 1 A diverse family of the resultant compounds has been studied widely due to potential material, 2 biological applications 3 and numerous catalytic tasks. 4 Intriguing opportunities for isolation of cage metallacomplexes could be provided by using non-trivial ligands such as a silicon-based sesquioxane (branched compound of general formula RSiO 1.5 ). Using silsesquioxanes has allowed a family of metalladerivatives (including several examples of heterometallic ones) to be obtained. 5 In turn, some scholars have reported a "heteroligand" approach 6 to metallacomplexes of this type, whereby organic ligands (e.g., tris(2-(dimethylamino)ethyl)amine, 6a cyclooctenyl, 6b 2,2′-bipyridine 6c,d ) cooperate with the silsesquioxane ligands in the assembling of the cage product. This novel (L 2 ) type of cage can be especially prospective for their catalytic studies because both ligands (silsesquioxane and organic) can promote the stability of catalytic species during the reaction. Here, we present our recent observations regarding: self-assembly of Cu(II),Na-silsesquioxane in the presence of a widely used 2,2′-bipyridine; determination of the unusual molecular topology of the product; testing of the product as a precatalyst for homogeneous reactions of organic synthesis. Target compound 1 was isolated (in 24% yield) through an interaction between in situ-generated Cu,Na-silsesquioxane species and 2,2′-bipyridine (Scheme 1) by varying the ratio between reactants and the nature of solvents. Carrying out the reaction/crystallization in THF favored formation of an earlier, unknown type of cage metallasilsesquioxane molecular topology (Fig. 1, top).

X-ray diffraction indicated that 1 was a mixed metal (Cu 5 Na) complex, which crystallized in the P2 1 /c space group. This type of nuclearity has not been observed for reported metallasilsesquioxanes. 7 All copper(II) sites (Fig. S1 †) possessed a similar distorted square pyramidal pentacoordinated geometry even though three of them were heteroligated (O from silsesquioxane and N from 2,2′-bipyridine). The presence of 2,2′-bipyridine ligands provoked specific packing in the crystal, with two of the three 2,2′-bipyridines participating in π-π stacking interactions of aromatic systems with phenyl groups (Fig. S2 †). Cu-N bond lengths ranged from 1.996 to 2.214 Å, and Cu-O bond lengths from 1.919 to 2.782 Å. The longest (2.523 Å and 2.782 Å) Cu-O contacts implied the coordination of copper centers to the oxygen atom of encapsulated water (Fig. S3 †). Such water "capture", despite some known examples reported for other types of complexes, 8 is not a widespread feature among the encapsulation abilities of cage silsesquioxanes. 9

The silsesquioxane skeleton of 1 comprised 12 silicon centers to form one cyclic ligand (Fig. 1, bottom). The topology of the ligand was unprecedented. First, it was important to differentiate this Si 12 -based ligand from the classical one, of "globular" geometry, observed many times for Cu(II)-silsesquioxanes. 10 Detailed structural comparison of two of these ligands is given in the ESI (Fig. S4 and Table S1 †). Here, 11 positive charges from five Cu(II) ions and one Na(I) ion in 1 could not compensate for the 12 negative charges expected from the dodecasilsesquioxane skeleton. Indeed, charge balance became possible due to the presence of hydrogen, which protonated one of the oxygen atoms of the silsesquioxane framework. It elicited a very specific final composition of the ligand: Ph 12 Si 12 (O) 12 (OH)(O -) 11 . The hydroxyl group and encapsulated water molecules formed a system of hydrogen bonds (Table 2, ESI †) within the metallasilsesquioxane cage that has rarely (if ever) been observed in compounds of this type. In view of its mono-hydroxyl functionality, only OHbearing silsesquioxanes belonging to the cubane structural family 11 and based-metallacomplexes 11b,12 have been reported. Logically, these compounds are quite different from 1 in terms of composition and structure.

Being interested in the application of 1 in catalysis, we undertook the first evaluation in homogeneous oxidative reactions. First, oxidations of hydrocarbons with donors of an oxygen atom (e.g., hydrogen peroxide or peroxy acids) constitute an important field of contemporary catalytic chemistry. Complex 1 exhibited high activity in the cyclohexane oxidation with hydrogen peroxide in the absence of any added acid. Thus, the reaction afforded after 2 h at 50 °C oxygenated in 20% yield and turnover number (TON) = 190 (Fig. 2). A comparison of chromatograms obtained before and after addition of PPh 3 (for this method developed by one of our research team, see ref. 13) to the sample indicated that the main product of the reaction, cyclohexyl hydroperoxide, was formed in high concentration because the alcohol/ketone ratio rose dramatically after the reduction, whereas this ratio was ∼1 before the reduction. Indeed, cyclohexyl hydroperoxide decomposed in the chromatograph (in the injector and columns) to generate cyclohexanol and cyclohexanone in approximately equal amounts. Unlike many oxidations under different conditions, this reaction gave a much lower product concentration before PPh 3 than after reduction. This finding can be explained by the assumption that cyclohexyl hydroperoxide was not completely reduced with PPh 3 and was not directly detected by our chromatograph. Regioselectivity parameters in the oxidation of n-heptane catalyzed by complex 1 [C(1) : C (2) : C(3) : C(4) = 1.0 : 5.5 : 5.0 : 4.5] and the oxidation of cis-1,2dimethylcyclohexane without retention of the configuration (trans/cis = 0.9 taking into account the mutual position of the two methyl groups) indicated that this reaction occurred with the participation of free hydroxyl radicals. 13b Complex 1 was also evaluated in the oxidative amidation of benzylic alcohol in the presence of alkylammonium chlorides, calcium carbonate and TBHP (Scheme 2). This reaction allows to circumvent the classical coupling of acids and amines to furnish the corresponding amides (which are present in many organic molecules, including pharmaceutical drugs), but which require at least a stoichiometric amount of coupling agent, which could be toxic and generate a stoichio- metric quantity of undesired waste. 14 It was originally developed with copper(II) oxide, 15 but the use of cage silsesquioxanes allowed reduction of catalyst loading thanks to their good solubility in organic solvents. 16 With compound 1, catalyst loading could be reduced to 0.4 mol% of copper and allowed us to obtain good-to-excellent yields of desired amides. The reaction of the hydrochloride salts of α-methylbenzylamine and cyclohexylamine furnished the corresponding amides in 83% and 88% yields, respectively. N-Butylbenzamide and N,Ndibenzylbenzamide could also be isolated in good yields of 70% and 68%, respectively. TON and turnover frequency (TOF) were calculated, giving values up to 220 and 9.2 h -1 (see ESI † for details on TON and TOF), respectively. If the value of TOF was similar to the one calculated in our seminal report because of the prolonged reaction times with use of 1 compared with CuO (24 h vs. 4 h), TON was increased fivefold, thereby confirming the efficiency of complex 1 in this reaction. Complex 1 was soluble under the reaction conditions, and seemed to be involved as a homogeneous catalyst. Consequently, because of the difficulty of recovery, recycling experiments were not considered at this stage of the project.

Conclusions

We presented a brand-new type of bimetallic (Cu,Na) oxacluster in a heteroligand ( phenylsilsesquioxane and 2,2′-bipyridine) surrounding to form a uniquely shaped product Ph 12 Si 12 O 12 (OH)(O -) 11 Cu 5 Na(bipy) 3 (H 2 O). Catalytic tests of this complex showed its high activity in oxidations. Further investigation of this prospective "ligand plus ligand" approach to self-assembly of complicated cage metallastructures and their evaluation in catalysis is currently underway in our research team.

Fig. 1

 1 Fig. 1 Top: Molecular structure of 1. Solvating ligands (THF) and encapsulated species (water) have been removed for clarity. Bottom. Structure of the silsesquioxane ligand in 1.

This journal is © The Royal Society of Chemistry 2018

Acknowledgements

This work was supported by the RUDN University Program "5-100", the Russian Foundation for Basic Research (17-03-00993, 16-03-00254, 16-53-150008, 16-29-05180, 16-29-05180), French Embassy in Moscow, and University of Montpellier, CNRS (Programme de Recherche Conjoint).

Conflicts of interest

There are no conflicts to declare.


R.-B Gerst

A Blazhev

N Warr

J N Wilson

M Lebois

N Jovancević

D Thisse

R Canavan

M Rudigier

D Étasse

E Adamska

P Adsley

A Algora

M Babo

K Belvedere

J Benito

G Benzoni

A Boso

S Bottoni

M Bunce

R Chakma

N Cieplicka-Oryńczak

S Courtin

M L Cortés

P Davies

C Delafosse

M Fallot

B Fornal

L Frail

D Gjestvang

A Gottardo

V Guadilla

G Häfner

K Hauschild

M Heine

C Henrich

I Homm

F Ibrahim

Ł W Iskra

P Ivanov

S Jazwari

A Korgul

P Koseoglou

T Kröll

T Kurtukian-Nieto

L Le Meur

S Leoni

J Ljungvall

A Lopez-Martens

R Lozeva

I Matea

K Miernik

J Nemer

S Oberstedt

W Paulsen

M Piersa

Y Popovitch

C Porzio

L Qi

D Ralet

P Regan

D Reygadas-Tello

K Rezynkina

V Sánchez-Tembleque

C Schmitt

P.-A Söderström

C Sürder

G Tocabens

V Vedia

D Verney

B Wasilewska

J Wiederhold

M Yavachova

F Zeiser

S Ziliani

Prompt and delayed γ-spectroscopy of the neutron-rich 94 Kr and observation of a new isomer

    Prompt and delayed γ spectroscopy of neutron-rich 94 Kr and observation of a new isomer

I. INTRODUCTION

The study of nuclear properties far away from the valley of stability to investigate their dependence on the number of neutrons and protons is crucial to our understanding of the atomic a Corresponding author: rgerst@ikp.uni-koeln.de nucleus and the nuclear forces. One important feature is the evolution of nuclear shape along the isotopic chains, which is determined by the interplay between the macroscopic and microscopic properties. In some mass regions, this interaction between collective and single-particle degrees of freedom can lead to a very sudden growth of collectivity. One way of interpreting this drastic behavior is within the context of the coexistence of two distinct configurations at low excitation energies, one spherical and one deformed, where the latter suddenly becomes energetically more favorable [1]. Experimentally, this usually manifests itself in even-even nuclei in a sudden drop of the E(2 + 1 ) energy and the gradual lowering of an excited 0 + state [2,3], which constitutes the bandhead of the competing configuration. The region of A∼100 exhibits one of the most abrupt shape transitions of this type. Experimentally, it was established that the isotopic chains of Sr, Zr, and Mo exhibit a very sudden onset of deformation going from N = 58 to N = 60 [4][5][6][7][8][9]. Mass measurements of krypton Z = 36 and rubidium Z = 37 isotopes were performed [10], labeling Z = 37 as the boundary of this region of deformation. Additionally, recent γ-spectroscopy identified 97 Rb as the lower corner stone of this phenomenon [11]. In agreement with this, earlier experimental results had established a rather gradual development of collectivity in krypton isotopes with a slow decrease in E(2 + 1 ) up to N = 60 [12,13] and no evidence for low-lying intruder states. However, more recent studies revealed a slightly different behavior for N≥60 [14,15]. In 96 Kr, an unexpectedly low value of the ratio E(4 + 1 )/E(2 + 1 ) was determined experimentally [14]. In 98,100 Kr, a significant drop of E(2 + 1 ) was observed and for 98 Kr a low-lying (0 + 2 , 2 + 2 ) state was identified [15]. The discovery of a low-lying (0 + 2 , 2 + 2 ) state in 98 Kr, as evidence for a coexisting deformed configuration, also encourages the search for excited bands in the Kr isotopes with N < 60 to further investigate the presence of deformed coexisting configurations in this region. To examine this development of shape coexistence in Kr isotopes, different theoretical approaches have been used [13,16]. Most recently, Gogny-D1M calculations were performed by Nomura et al. [17] for the even krypton isotopes. For 88-92 Kr, this reveals a defined γ-softness that develops into a γ-soft oblate minimum for 94 Kr. For the isotopes with N > 58, the energy surfaces show a pronounced prolate-oblate shape coexistence further cementing the experimental results. Hence, the changes between 94 Kr and 96 Kr are crucial for the understanding of the shape evolution in the krypton isotopes specifically and in this region generally. In this paper, we report on the observation of a new 32(3) ns isomer at 3444 keV in 94 Kr together with several new yrast and non-yrast γ-transitions extending the previously known level scheme. Prior to this work, the only published γ-transitions in 94 Kr were 665.5 keV, 853.2 keV, and 1001.3 keV, measured via Coulomb excitation at the REX-ISOLDE facility by Albers et al. [12] and following spontaneous fission of 248 Cm with the EUROGAM 2 array by Rz ąca-Urban et al. [18]. On the basis of angular correlation analysis, a spin and parity of 2 + and of 4 + were assigned to the levels at 665.5 keV and 1518.7 keV, respectively, with the remaining transition of 1001.3 keV populating the 4 + level [18].

II. EXPERIMENT

The measurement, presented in this paper, was performed in 2018 as part of the ν-Ball campaign at the ALTO facility at the IPN Orsay in France, using the 238 U(n,f) fast-neutron induced fission reaction [19,20]. Here, 94 Kr was populated much more strongly with a yield of 0.8 % as opposed to 0.04 % for fission of 248 Cm [21]. The directional fast-neutron beam was produced with the neutron source LICORNE [22], which uses the p( 7 Li, 7 Be)n reaction in inverse kinematics. The 7 Li beam was provided by the 15 MV Tandem Van de Graaff accelerator in a pulsed mode with a 2 ns pulse width and a repetition period of 400 ns. The 7 Li beam entered the hydrogen gas cell of LICORNE through a 2.8 µm tantalum foil. During the experiment a 16 MeV 7 Li beam was used together with a 3.5 cm H 2 gas cell with a pressure of 1.2 atm to maximize the neutron flux. The neutrons then impinged on an 81 g 238 U sample with a composition of 99.8 % 238 U and 0.2 % 235 U produced by JRC-Karlsruhe. The fissile material was arranged in five thin disks over a distance of 8 cm to allow low-energy γ-rays to escape from the target. No Doppler correction was required as the fission fragments were stopped in the material. Therefore, the energy resolution of the detectors only depended on their intrinsic resolution giving the spectrometer an excellent resolving power. The 238 U sample was placed at a distance of 4 cm to LICORNE to maximize the fission rate. 238 U has a fission threshold of ∼ 1.2 MeV and in comparison to 235 U a very small fission cross section of 0.5 barn at this energy [23]. During the experiment an average primary 7 Li beam flux of 2 • 10 10 /s was achieved, which translated into a secondary neutron beam of 2 • 10 6 /s and a rate of 10 4 fission events/s. Additionally, a neutron detector was placed behind the setup to measure the neutron energy (∼1.7 MeV) via time-of-flight, but was not relevant to this analysis. The 238 U sample was situated at the center of the ν-Ball array, a novel hybrid High Purity Germanium (HPGe)-LaBr 3 γ-spectrometer that was designed to combine the excellent energy and timing resolution of the two detector types, respectively [20]. A schematic view of the LICORNE chamber and the detector setup can be seen in Fig. 1. During the fission campaign, the array consisted of 24 HPGe Clover detectors, 10 coaxial Phase I HPGe detectors, all with BGO Compton shielding, and 20 LaBr 3 detectors without Compton shielding. The Phase I detectors were placed in a ring at a backward angle of 133.5 • , and the Clover detectors were placed in two rings around 90 • . The LaBr 3 detectors were mounted in two rings around 40 • . Because of the presence of the directional neutron beam, no γ-detector was placed at extreme forward angles (< 20 • ) in order to avoid neutron-induced damages to the detectors. Calibration measurements were performed using 152 Eu and 60 Co standard γ-sources. The combined resolution of all HPGe channels was 2.35 keV at 1 MeV, with resolutions of single detectors ranging from 1.7 keV for Clover detectors to 2.9 keV for Phase I detectors. A photopeak efficiency of 6.7 % at 1.3 MeV for the HPGe detectors was simulated with GEANT4 before the campaign [20]. The measured values were in excellent agreement for energies above 344 keV. For lower energies the efficiency was slightly lower than expected due to absorption in the aluminum gas cell of LICORNE [20]. The measurement was performed in a triggerless mode and the FASTER data acquisition system was used for processing [START_REF] Etasse | Fast acquisition system for nuclear research[END_REF]. In total, approximately 216 hours of data were taken. In the following, only results from the analysis of the HPGe detectors will be presented.

III. DATA ANALYSIS AND RESULTS

The 400 ns pulsed beam makes it possible to investigate prompt and delayed γ-transitions separately and in coincidence. For the HPGe detectors, the prompt peak in the timing distribution has a FWHM of ∼ 23 ns, which includes the intrinsic time resolution of the detectors, the beam pulse width, and the time-of-flight differences of the neutrons over the target length. For γ-rays emitted outside of the prompt time region, the energy spectra can be cleaned significantly by setting a timing condition of T ≥ 35 ns after the beam pulse, with the higher limit set dependent on the half-life range being investigated. A timing gate of T ≤ 35 ns after beam pulse was used to specifically look for prompt γ-rays. Additionally, the background could be reduced significantly by optimizing multiplicity conditions for the number of detected γ-rays in the full and in the delayed time windows. In this case, events were built with the total γ-multiplicity of ≥ 5 and ≤ 17 with a minimum of two HPGe detectors fired to select fission and to suppress low-multiplicity events such as inelastic scattering. An additional constraint on the delayed multiplicity was used after the number of delayed transitions was determined. The prompt background-subtracted energy spectrum produced by a gate on the 4 + 1 to 2 + 1 853.8 keV transition is presented in the upper panel of Fig. 2. The previously published transitions, 665.5 keV and 1001.3 keV, are the strongest peaks in the spectrum along with several lines from the fission partner isotopes 142-144 Ba. Three additional lines at 834.9 keV, 852.9 keV and 1267.1 keV are present. With an additional gate on the 1001.8 keV transition (Fig. 2 of the delayed time window. The other two peaks remain visible, as can be seen in the lower panel of Fig. 2, which proves them to be in coincidence with the 1001.8 keV and 4 + 1 to 2 + 1 transitions. The peak at 852.9 keV is very close in energy to the 4 + 1 to 2 + 1 transition of 853.8 keV. But since this transition is visible when gating on the 4 + 1 to 2 + 1 transition in any double gate combination of yrast states, it is assumed that such a transition at 852.9 keV exists. Due to their coincidence relation to the previously established yrast states, we place the 834.9 keV and 852.9 keV transitions above the three previously known transitions, see Fig. 5. The ordering and tentative spin assignment as (8 + ) and (10 + ) will be clarified by the analysis of delayed γ-transitions in the subsequent discussion.

Additionally, three transitions, 665.5 keV, 551.2 keV, and 695.3 keV, were observed in coincidence with each other. Even though 665.5 keV matches the known 2 + 1 to 0 + 1 transition energy, for fission data a second gate on a known transition would normally be necessary to reliably assign these transitions to 94 Kr. However, in 94 Kr a transition of approximately 550 keV was observed in coincidence with the 665.5 keV transition [START_REF] Moschner | [END_REF] in the data taken during the 2015 SEASTAR campaign at RIKEN [26]. Therefore, the transitions 551.2 keV and 695.3 keV are placed in the level scheme as seen in Fig. 5. They are not seen in coincidence with any other prompt or delayed transition. For the analysis of the delayed γ-rays, besides the total multiplicity of ≥5 and ≤17, the additional constraint on the delayed multiplicity was set, respective to the expected number of delayed transitions in the different isotopes. In Fig. 3, the background-subtracted energy spectrum gated on the 4 + 1 to 2 + 1 in a time window of 35 -180 ns after beam pulse with a delayed multiplicity of ≥ 2 and ≤ 6 can be seen. The delayed time window was optimized after the half-life of the new isomer had been determined. Next to three coincident peaks at 665.6 keV, 1001.8 keV and 1267.1 keV previously visible in the gated prompt spectrum, three additional peaks at 186.8 keV, 470.7 keV, and 736.0 keV show up. 1 to 0 + 1 transitions, with the exception of 834.9 keV all other delayed transitions appear in coincidence (see Fig. 4 c) ). With this information, the level scheme was expanded as shown in Fig. 5. Concerning the 834.9 keV line, visible both in the prompt energy spectra and the double gated delayed spectrum Fig. 4 a), we assume that a non-observed 88 keV transition connects the isomeric 3444 keV level with the (8 + ) level at 3356 keV. With our setup, a transition of this energy is very unlikely to be observed due to the low efficiency at that energy. The high Z and thickness of the target material attenuates low energy γ-rays strongly. To determine the half-life of the 3444 keV isomer, the data were sorted into a γγT-cube to obtain the timing distribution of the intensities of the delayed coincidences of 94 Kr. As test cases, the isotopes 134 Te and 95 Sr with well-known isomers with half-lives of 164.1(9) ns and 21.9(5) ns [27,28], respectively, were used for every step to verify the analysis method. For these cases, we obtain half-lives of 166(6) ns and 21.0(5) ns, in excellent agreement with the literature values. In Fig. 6, the time distribution used for the determination of the isomer half-life is shown. The timing spectra for the 665. defining and limiting the delayed multiplicity. The half-life was obtained by simultaneously fitting the time-dependent summed background spectrum of the coincidences (see black histogram in Fig. 6) together with the non-subtracted time spectrum of the five coincidence pairs (see blue histogram in Fig. 6) using the maximum likelihood method in the same manner as in Ref. [29]. By varying the energy ranges of the the systematic uncertainty could be estimated. Together, this yields a half-life of 30(5) ns. Figure 7 shows the background-subtracted energy spectrum in the prompt time window produced by double gating on the delayed transition pairs 665. up the coincident prompt spectra. The strongest peaks in the resulting spectrum are the lowest transitions in the partner fission isotopes 142-144 Ba (as also seen in Fig. 2), 511 keV and two new transitions at 756.6 keV and 1245.5 keV. Accordingly, these two transitions can be placed on top of the isomer in the level scheme. Since they are not seen in coincidence to each other, we place them in parallel to each other, as seen in Fig. 5. Since these new transitions are feeding the isomeric state, the time difference between them and the delayed transitions depopulating the isomer can be used for a second determination of the isomeric half-life. In Fig. 8, the prompt-delayed γγ∆T-spectrum for the isomer is shown. To maximize statistics, the time differences between both prompt transitions, 756.6 keV and 1245.5 keV, feeding the isomer, and the transitions 665.5 keV, 853.8 keV, 1001.8 keV and 1267.1 keV, below the isomer, were added up. As above, the half-life was obtained by simultaneous fitting of the time-dependent summed background spectrum and the non-subtracted ∆T-spectrum of the prompt-delayed pairs. This yields a half-life of 33(4) ns in accordance with the half-life obtained from Fig. 6. Due to the independence of the two results, we can take the weighted average which results in T 1/2 = 32(3) ns. The measurement of this short-lived isomer demonstrates the capabilities of ν-Ball in this half-life range, complementary to in-flight facilities where this time range is not always accessible.

IV. DISCUSSION

The new level scheme of 94 Kr is shown in Fig. 5. The excitation energies are indicated next to each level. The widths of the arrows are proportional to the efficiency-corrected intensities obtained for the transitions. The two new transitions 834.9 keV and 852.9 keV, observed in the prompt time window (see Fig. 2) to be coincident with the previously published yrast transitions [18], were assigned to be a feeding the (6 + 1 ) and (8 + 1 ) states. The ordering was deduced from the prompt coincidence analysis (see Fig. 2) and the delayed coincidence analysis (see Fig 4). The 834.9 keV transition is also observed in the delayed time window and therefore has to be below the isomeric state at 3444 keV. The state at 4209 keV, depopulated by the 852.9 keV transition in the prompt time window, needs to have a spin of J 4209 > 8, with the most likely candidate being (10 + 1 ). The level at 2786 keV decays to the 4 + 1 level at 1519 keV, while a transition to the 6 + 1 at 2521 keV was not observed. Thus, the most probable spin-parity assignments, producing the lowest possible multipole radiation, for the 2786 keV state are (5,6 + ). The higher level at 3257 keV, right below the isomeric state, decays to the 6 + 1 at 2521 keV and to the (5,6 + ) level at 2786 keV, not to the 4 + 1 or 2 + 1 , which suggests a spin J 3257 > 6. On top of the level, there is the 186.8 keV transition which also depopulates the isomer. Calculating the reduced transition strength based on a single-branch with the Transnucular code [START_REF] Saed-Samii | Computer code Transnucular, private communication[END_REF], a transition multipolarity ≥3 can be excluded. For a single branch E2 transition with this energy, this yields 2.8(3) W.u. while for E1, M1, M2 the results are either much too low or much too high to be reasonably compatible with physical expectations. The likely E2 character of this transition suggests 9 +/-and 7 +/-states as the most probable assignments for the 3444 keV isomer and the 3257 keV state, respectively. In fact, in the neighboring isotone 96 Sr, there is a well known 40 ns isomeric 9 -state depopulated via E2 and E1 transitions [32,33]. This has been well described by shell-model calculations suggesting a dominant neutron (g 7/2 , h 11/2 ) 9 -configuration [33]. This resembles the situation observed in our 94 Kr level scheme supporting the tentative spin assignments of (9 -), (7 -), and (8 + ) for the corresponding states. Additionally in 96 Sr, there is a prompt cascade 10 + → 8 + → 6 + bypassing the isomer [32,34] in further support of our tentative spin assignment for the 4209 keV state to be a (10 + ). Furthermore, very recently in the lower N = 58 isotone 92 Se, a long-lived 15.7µs isomeric state at 3072 keV was measured with the HPGe-spectrometer EURICA [35]. The isomer has been observed to decay only via a 67 keV E2 transition with a transition strength of around 1 W.u.. The authors also assigned this as a tentative (9 -) decaying into to a (7 -) state assuming single-particle character Figure 10. Comparison of partial level schemes of 92 Se, 94 Kr, and 96 Sr. Data from this work and Ref. [18,33,35,36].

we can see the corresponding Nilsson diagram for neutrons near the Fermi level of 94 Kr, which is very similar to the one for 92 Se. At an oblate deformation of β ∼ -0.22, the same two-quasiparticle states exist as in 92 Se (see red area in Fig. 9). Thus, the decay from (9 -) to (7 -) in 94 Kr can be interpreted as a transition of a neutron from the 7/2 + [404] to the 3/2 + [402] level. The orbitals involved are labeled in Fig. 9. From the microscopic point of view, the Nilsson 9 -2qp-configuration corresponds to the shell-model (g 7/2 , h 11/2 ) 9 -one. But, while interacting shell-model calculations are in spherical basis and deformation parameters can only be calculated in a framework of shape invariants, the Nilsson model allows for an estimation of the deformation parameter β based on the observation of unique single-particle configurations. In 94 Kr, the oblate deformation of the isomeric state, as deduced from the assumed Nilsson configurations in Fig. 9, is nearly the same as the one of the ground state (β ∼ -0.26) as predicted by the same calculations [START_REF] Delaroche | [END_REF]. To estimate the reduced transition strengths for the isomeric decay branches, we assume pure E1 and E2 decay branches similar to 96 Sr. The branching ratio of I 88 ≈ 1 2 • I 186.8 is derived by analyzing the intensities of the 470.7 keV and 736.0 keV and the 834.9 keV transitions in Fig. 4. Due to the low statistics, intensity uncertainties of 50 % were assumed and propagated in the calculation. Thus, when considering a two-branch decay of the isomer with an 88 keV E1 transition and a 186.8 keV E2 transition, reduced transition strengths of B(E1) = 4 +3 -2 • 10 -6 W.u. and B(E2) = 1.8 +0.5 -0.5 W.u. are obtained. In order to discuss aspects of the nuclear structure in the N = 58 isotones under the assumption that the tentative spin and parity assignments are correct, partial level schemes of 92 Se, 94 Kr, and 96 Sr are shown in Fig. 10. While in 94 Kr and 96 Sr the excitation energies and relative position of the 9 -and 7 -levels are relatively constant, both the excitation energies and their separation slightly decrease in 92 Se. In all three isotopes, the reduced E2 transition strengths of about 1 W.u. show a single-particle character for the isomeric decay. In 94 Kr and 96 Sr, a competing E1 branch is observed. This is not the case in 92 Se, most probably due to the higher relative position of the 8 + state as suggested by the upsloping trend of the corresponding states in 96 Sr and 94 Kr (see Fig. 10). In Wu et al. [34], relative B(E2) values for the internal decay of some states in 96 Sr were derived based on the corresponding branching ratios and normalised to one of the transitions strengths. The relative B(E2) values from the yrast 6 + to the two 4 + states in 96 Sr indicated a mixing of the 4 + 1,2 states [34], while in 92 Se the calculated relative B(E2) suggest no mixing of the two 4 + states based on the branching ratio observed by Lizarazo et al. [35]. Similarly, we obtained relative E1 transition strengths for the 7 -→ 6 + 1,2 transitions in 94 Kr and 96 Sr. In 94 Kr, the B(E1;7 -→ 6 + 1 ) is 26(1) % of the B(E1;7 -→ 6 +

2 ), while in 96 Sr, it is only 5(1) %. This implies a much greater mixing between the two 6 + states in 94 Kr compared to 96 Sr. In 92 Se, only one (6 + ) state was observed though from the relatively constant excitation energies of the 6 + states in 94 Kr and 96 Sr, one would expect a similar structure.

V. CONCLUSIONS

In conclusion, this work presents the first observation of an isomeric state in 94 Kr, measured using the novel hybrid spectrometer ν-Ball with its unique combination of a pulsed beam and fast-neutron induced fission of 238 U. Reduced E1 and E2 transition strengths for the isomer decay-branches were extracted. The isomer is interpreted as being built on a two-quasiparticle neutron state consistent with Gogny CHFB calculations, presenting a very similar isomeric spin structure as in the neighboring isotones 92 Se and 96 Sr. Additionally, the level scheme was expanded significantly, allowing to discuss nuclear structure by looking into level systematics of N = 58 isotones and mixing of states by comparing relative transition strengths.

Figure 1 .

 1 Figure 1. Schematic drawing of the ν-Ball array, coupled to the LICORNE fast-neutron source. The p( 7 Li, 7 Be)n reaction produced a focused fast-neutron beam which induced fission in the 238 U material. The γ-rays stemming from the excited fission products were measured with ν-Ball, consisting of 10 Phase I HPGe (blue), 24 Clover HPGe (green), and 20 LaBr 3 (violet) detectors. The solid angle coverage was approximately 70%.

Figure 3 .Figure 4 .

 34 Figure 3. Background-subtracted energy spectrum coincident with the gate on the 4 + 1 to 2 + 1 853.8 keV transition in the delayed 35-180 ns time window. The efficiency-corrected intensities normalized to the strongest transition at 665.5 keV (green) are shown next to the peaks.

Figure 5 .

 5 Figure 5. Level scheme constructed for 94 Kr. Energies are given in keV. The widths of the transitions arrows correspond to their efficiency-corrected intensities. New transitions and levels are shown in red. Transitions denoted by asterisk are only observed in the prompt time window.

Figure 7 .

 7 Figure 7. Background-subtracted prompt energy spectrum generated by double gating on coincidences of delayed γ-transitions below the 3444 keV isomer in the 35 -180 ns time window.

  lower panel), the peak at 1267.1 keV disappears which suggests a parallel branch. This will be explored further in the analysis The efficiency-corrected intensities normalized to the strongest transition at 665.5 keV (green) are shown next to the peaks. Only events where at least three different HPGe detector hits were recorded are considered. Lower panel: The energy spectrum after a second gate on 1001.8 keV is applied.

	Counts / 2 keV	3000 2500 2000 1500 1000 500 3500	144 Ba	143 Ba	142 Ba	665.5(1) 100(9)	834.9(2) 19(4)	852.9(2) 19(4)	1001.8(1) 49(5)	1267.1(2) 27(4) Gate 853.8 keV
			200	400	600		800		1000	1200
		50				665.5	Gate 853.8 & 1001.8 keV
	Counts / 2 keV	40 30 20					834.9	852.9
		10
			200	400	600		800		1000	1200
											Energy [keV]
	Figure 2. Upper panel: Background-subtracted energy spectrum coincident to the gate on the 853.8 keV 4 + 1 to 2 + 1 transition in the prompt time window.

  Figure 6. Time spectrum and fitted decay curve used to determine half-life of the 3444 keV isomer. For this spectrum, only events that have a delayed multiplicity of ≥ 2 and ≤ 6 in the delayed time region (beyond the dashed line) were considered. The decay curve is fitted with an exponential decay function (red) together with a time dependent background (black). To maximize statistics the time distributions of different γ-ray coincidences below the isomer were added up to one decay curve (blue).

					T 1/2 = 30(5) ns
		10 3
	Counts / 5 ns	10 2
		10
		50	100	150	200	250	300
							Time [ns]
							5/853.8 keV, 665.5/1001.8 keV, 665.5/470.7 keV,
							853.8/470.7 keV, and 853.8/1001.8 keV prompt coincidences

of transitions, which are situated below the 3444 keV isomer, were added up to maximize statistics. The other combinations of transitions below the isomer were not used here due to low statistics. The second, smaller peak in the time distribution at 75 ns is a result of the artificial boundary introduced by

  Figure 8. Prompt-delayed ∆T spectrum of prompt transitions populating and delayed transitions depopulating the 3444 keV isomer. The decay curve is fitted with an exponential decay function (red) with a time dependent background (black). To gain statistics, several prompt-delayed transition combinations were added up to one decay curve (blue).

							T 1/2 = 33(4) ns
	Counts / 2 ns	10 2 10
		1
		-100	-50	0	50	100	150	200	250	300
									Time [ns]

5/853.8 keV, 665.5/1001.8 keV, 665.5/1267.1 keV, 665.5/470.7 keV, 853.8/470.7 keV, 853.8/1001.8 keV and 853.8/1267.1 keV below the 3444 keV isomer in the delayed time window 35 -180 ns, and adding

  Figure 9. Partial Nilsson diagram for neutrons. The Fermi surface for N = 58 is denoted with black dots. Adapted from Delaroche et al. [31], obtained with Gogny CHFB calculations.and identifying the two states as oblate two-quasiparticle neutron configurations 7 -=11/2 -[505] ⊗ 3/2 + [402] and 9 -=11/2 -[505] ⊗ 7/2 + [404][35]. The Nilsson orbitals were determined with Gogny CHFB calculations[START_REF] Delaroche | [END_REF]. In Fig.9,

				Energy [MeV]	2 3 1.5 2.5 3.5		(4 + ,3 + ) (6 + ,5 + ) (9 -) (7 -)	(6 + ,5) 4 + (6 + ) (8 + ) (9 -) (7 -)	4 + 4 + 8 + 9 -7 (-) 6 + 6 +
					1		(4 + )	2 +
					0.5	92 Se	0 + (2 + )	94 Kr	2 + 0 +	96 Sr	0 +
	E[MeV]	1 1 / 2 -[ 5 0 5 ] + [4 0 2 ] 3 /2	7 / 2 + [ 4 0 4 ]	3/2 + [41 1]

ACKNOWLEDGMENTS

We thank the accelerator staff of the ALTO facility at the IPN Orsay for providing a stable beam and continuous support. This work is supported by the DFG under Grant No. BL 1513/1-1 and by the BMBF under Grants No. 05P15RDFN1, 05P19RDFN1.


Damien Tardieu

Marine Desnoyers

Claire Laye

Damien Hazelard

Nicolas Kern

Philippe Compain

Stereoselective Synthesis of C,C -Glycosides from exo-Glycals Enabled by Iron-Mediated Hydrogen Atom Transfer

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ABSTRACT:

We describe herein a convenient strategy for the construction of C,C-glycoside building blocks via the intermediacy of tertiary pseudoanomeric radicals. Application of an ironmediated hydrogen atom transfer/Michael-Giese coupling enables the anomeric quaternization of readily available exo-glycals with good to complete stereocontrol in the pyranose and furanose series. Carefully optimized conditions allow the use of challenging trisubstituted derivatives prone to undergo further elaboration to stable neoglycoconjugates. Preliminary results for direct C-disaccharide synthesis are also discussed.

C-glycosides constitute a major class of glycomimetics in which the customary C-O acetal linkage in natural O-glycosides has been substituted for a C-C bond. Such modification confers C-glycosides enhanced stability towards chemical and enzymatic hydrolysis, 1 providing useful probes for studies involving carbohydrate-processing enzymes. 2 Within this family, C,C-glycosides -also referred to as C-ketosides 3 -are characterized by the presence of a stereodefined quaternary pseudoanomeric center bearing two exocyclic C-substituents. Aside from their structural relationship with ulosonic acid derivatives (e.g. Nacetylneuraminic acid), these scaffolds are attractive platforms for the diversification of (one or two) aglycon moieties in stable carbon-linked neoglycoconjugates, as demonstrated by Schmidt et al. in the context of galactosyltransferase inhibition (Figure 1). 4,5 C,C-glycosides also stand as key intermediates en route to anomeric spiro-carbohydrates. Those notably include synthetic spiro-nucleoside analogs displaying antiviral, antimetabolic and antibacterial activities, [START_REF] Soto | For recent reviews see[END_REF] spiro-lactones, 7,[START_REF] Sridhar | For alternative approaches to spirolactone C,C-glycosides see[END_REF] lactams and benzodiazepines as GABA receptor ligands, 9 as well as syringolide, secosyrin and sphydrofuran natural products. [START_REF]For a review on total syntheses of 1,7-dioxaspiro[END_REF] Beyond the latter, ertugliflozin 11 (Steglatro TM ), a selective SGLT2 inhibitor, was recently approved by the FDA for the treatment of type II diabetes. The prevalence of 2,2-disubstituted tetrahydrofurans and tetrahydropyrans in complex molecules including thyrsiferols, 12 maitotoxin 13 and pectenotoxins 14 further establishes C,C-glycoside motifs as valuable synthons. In sharp contrast with the broad palette of methods targeting C-glycosides, 15 few approaches allow the stereoselective construction of the quaternary pseudoanomeric carbon embedded in C,C-glycosides. 16 This has been mostly accomplished by means of electrophilic activation 17,18,19 or reductive samariation 20 of appropriate sugar donors (Scheme 1). Albeit such reactions can proceed in good yields with moderate to high stereoselectivities, they display substantial limitations in terms of coupling partners (respectively allylsilanes or aldehydes). 21 Scheme 1. Established methods for C,C-glycoside synthesis and proposed strategy using HAT catalysis.

Aiming to establish a convenient route to novel original C,C-glycoside derivatives, we foresaw that a radical-based manifold may provide a useful tool for stereochemical control. Indeed, in the common D-gluco, D-galacto and Dmanno series, reactions of pseudoanomeric radicals often display high -selectivity due to their innate preference for an axial conformation. 22,[START_REF] Togo | For a review of radical C-glycosylation methods see[END_REF] Furthermore, very small energetic changes in the conformation of glycopyranosyl radicals have been predicted when replacing an anomeric hydrogen with a methyl substituent, 24 hinting a predictable stereochemical outcome for the addition of sought-after tertiary pseudoanomeric radicals. However, these reactive intermediates are hardly accessible given the low availability/stability of C-substituted anomeric bromides, sulfides or selenides precursors, 25 notwithstanding Suárez's alternative approach involving intramolecular 1,n-hydrogen abstraction which employs specific substrates. 26,[START_REF] León | For examples involving radical allylation and Norrish type II formal acetyl transfer see[END_REF] When considering alternative manifolds, catalytically generated transition metal hydrides have emerged as outstanding tools for the hydrofunctionalization of electron-rich alkenes with Marknovnikov-like selectivity. 28 Baran et al. have developed an iron-mediated Michael-Giese hydroalkylation of olefins that covers a broad substrate scope and whose mechanism was recently deciphered. 29,30 Under such conditions, exo-glycals could thus be viewed as precursors to C,C-glycosides. Although these C-glycosylidene derivatives have been historically designated as "poor acceptors, even when reacted with electrophilic radicals", 31,[START_REF] Staderini | Photoinduced hydrophosphonylation of two exo-glycals was reported using a large excess of dimethyl H-phosphonate: (a) Dondoni[END_REF] two examples of radical azidoselenation 33 and trifluoromethylation 34 were recently described. Here, we report the extension of the radical chemistry of exo-glycals to C,C-glycoside synthesis by means of iron-catalyzed hydrogen atom transfer (HAT).

To begin our studies, exo-glycal 1a was conveniently prepared from the parent -gluconolactone following Guey-rard's protocol relying on a modified Julia-Kocienski olefination. 35 Initial forays using methyl vinyl ketone (MVK) and PhSiH3 (4 equiv. each) in EtOH (0.1 M) with Fe(acac)3 as catalyst and Na2HPO4 as buffer under air at 60 °C36 led to the conversion of 1a into the desired C,C-glycoside 2a resulting from axial radical addition (see the Electronic Supporting Information for nOeSY data), albeit competitive, nonselective decomposition complicated its isolation (Table 1). These reactions revealed an interesting trend depending on catalyst loading, which was set to 10 mol% since extensive byproduct formation occurred above this mark (entries 1-3). [START_REF]Baran's studies (ref. 29), a benzyl-protected glucal (endo-alkene) was found to be a competent substrate using 30 mol% Fe(dibm)3 along with slow addition of PhSiH3 and methyl vinyl ketone (12 equiv. each). In our study, treating 1a under these conditions led to messy reaction mixtures where the formal alkene hydrogenation product arising from ((2[END_REF] Lowering the excess of PhSiH3 was detrimental to the reaction efficiency (entry 4). The most significant improvement was observed through the survey of higher dilution conditions: cleaner reaction mixtures were obtained at the cost of longer reaction times, with C = 0.03 M (EtOH) as an optimum (75% of 2a isolated after 3 h, entry 5); to the best of our knowledge, such trend is infrequent in the area of HAT-initiated C-C bond formation. 38 In addition, running the reaction in de-gassed EtOH under argon further improved its overall efficiency, in line with several related iron-catalyzed protocols (entry 6). [START_REF] Saladrigas | For recent examples of Fe(acac)3/silane-initiated HAT processes performed under an argon atmosphere[END_REF] Under these conditions, the excess of MVK could be reduced to 2 equiv. (entry 7). Attempts to limit the solvolysis rate of PhSiH3 using dichloroethane/ethanol mixtures [START_REF]The formation of the active hydrogen atom transfer agent PhSi(OEt)H2 is expected to be accompanied by the generation of less active PhSi(OEt)2H and inactive PhSi[END_REF] led to reduced yields, as did the use of (iPrO)PhSiH2 in dichloroethane 41 (entries 8-9). The reactivity of alkyl-substituted exo-glycal 1b [START_REF]Diastereoisomers (E)-1b and (Z)-1b were only separated partially by chromatography. Separate control reactions showed that (E)-1b and (Z)-1b reacted at similar rates and afforded very close yields of 2b. Optimization was thus carried using a mixture of both compounds (ca. 1:1 ratio)[END_REF] was next assessed using the previously optimized conditions, which engendered the formation of 2b in 31% yield after 24 h, but without any notable side reaction (65% of 1a recovered for entry 10). Such reactivity discrepancy between 1a and 1b was somewhat unexpected on the basis of recently reported mechanistic studies. 30 Indeed, related Fe-catalyzed hydrofunctionalizations have been shown to proceed via irreversible HAT to the olefin via a low barrier (due to the extremely weak BDE of Fe(III)-H bonds), establishing HAT to the olefin as a fast step in the catalytic cycle. 30 Oppositely, the generation of the active iron hydride active species is generally assumed to be turnover-limiting, 29,30 but clearly proceeded at a sufficient rate to promote the conversion of 1a (in view of entry 7). Comparatively, since 1b arguably stands among the most hindered alkenes studied to date, steric factors may dramatically impact the kinetics of the all-important HAT step. 43 An extensive survey of all reaction parameters was therefore undertaken. The isolated yield in 2b reached 44% when a second portion of phenylsilane was added after 16 h (entry 11), likely countervailing its nonproductive solvolysis to PhSi(OEt)3 over time. [START_REF]Experimental protocols involving slow addition of the silane were examined but systematically led to reduced yields[END_REF] Increasing the loading in Fe(acac)3 to 100 mol% also increased the reaction efficiency (entry 12), after which adjusting the concentration to 0.05 M led to an optimum (75% of 2b isolated, entry 13). Further attempts involving higher concentrations (relative to substrate) or substoichiometric loadings in Fe typically resulted in lower isolated yields (entries 14-15). Notably, reactions run with 1b in 1,2-DCE/EtOH afforded no product at all (compare entry 16 with entry 8), whereas employing (iPrO)PhSiH2 in DCE induced clean but low conversion onto 2b (compare entry 17 with entry 9). Overall, the observed trends suggest that, under moderate dilution conditions (C = 0.05 M) the radical derived from 1b is less prone to undergo nonproductive pathways, but is harder to generate via HAT due to the heavy steric demand around the olefin, thus requiring such forcing conditions.

Scheme 2. Scope of HAT-enabled C,C-glycoside synthesis.

a Reaction time: 1-3 h. b Reaction time: 20-24 h. c Diastereoisomers were not separated (d.r. determined by 1 H NMR analysis).

d Reaction run at 25 °C. Asterisks denote stereocenters at which partial control was observed (major isomer drawn).

Having established two sets of conditions for converting terminal and substituted exo-glycals to C,C-glycosides (denoted "A" and "B"), the scope of the reaction was examined (Scheme 2). First, using conditions "A" the HAT-initiated coupling of 1a with methyl or benzyl acrylate and acrylonitrile afforded the corresponding adducts 2ab-ad in good to excellent yields. -Methylene--butyrolactone also behaved as a competent acceptor whose coupling led to 2ae, albeit no stereoselectivity was observed in the reduction of the resultant electrophilic radical. Of note, attempted reactions with five-and six-membered cyclic enones, vinylsulfones or vinylphosphonates did not afford any coupling products. Pleasingly, maltose-derived exo-glycal 1c reacted smoothly with methyl acrylate to furnish 2c without cleavage of the O-glycosidic bond. Bifunctional glucoside 1d bearing an anomeric methoxy substituent at C-1 and an exo-glycal moiety at C-5 was found to decompose rapidly at 60 °C (and slowly upon storage at -20 °C), but running the reaction at 25 °C in the presence of acrylonitrile gave the expected product 2d in 71% yield with 82:18 d.r. (anti/syn). In contrast with the complete stereoselectivity observed so far in the D-hexopyranosyl series, such erosion may be attributed to an expected chairlike conformation induced by the -anomeric methoxy group. Indeed, such preference may hamper the adoption of the slightly distorted B2,5 boat conformation typical of glucosyl radicals (due to quasi-anomeric stabilization by the exocyclic C-O bond) 45 which overcompensates steric strain and enhances the stereoselectivity of radical coupling, according to Giese. 22a This set of conditions was also effective for the coupling of five-membered exo-glycals 1e-f bearing 2,3trans-dibenzyloxy and -disiloxy patterns, providing methyl acrylate adducts 2e-f. The predominant anti radical addition (ca. 4:1 d.r. in both cases) is directed by the neighbouring substituent. 22a Finally, bis-acetonide 1g bearing an allcis pattern could be added to methyl acrylate to furnish 2g with complete stereocontrol.

Aiming to prepare C,C-glycosidic scaffolds bearing practical handles for further functionalization, the reactivity of trisubstituted exo-glycals was assessed under conditions "B". Attempts to add the D-glucose-derived substrate 1h to ,-unsaturated esters were hampered by product transesterification with ethanol, but synthetically useful yields of coupling products 2ha and 2hb (54-58%) could be obtained using methyl vinyl ketone and acrylamide as acceptors. Again, the choice of acrylonitrile allowed to improve the efficiency of this challenging reaction, yielding 73% of 2hc on the mmol scale. The reliability of the protocol was eventually confirmed using derivatives 1i and 1j, whose couplings with acrylonitrile granted access to D-galacto (2i) and D-manno (2j) analogs in similar yields with complete diastereoselectivity.

Finally, we opted to design a sugar-derived acceptor whose radical coupling with an exo-glycal could directly generate a carbon-linked disaccharide. In this view, enone 3 was easily prepared from perbenzylated glucuronic acid and treated under conditions "A" in the presence of 1a (1 equiv each), which led to the expected diglycoside 4 in 33% yield (Table 2, entry 1). Importantly, formal conjugate reduction of the acceptor was observed, generating 5: although this process was unexpected under iron(III)initiated HAT catalysis, Larionov et al. have recently shown that electron-poor olefins can add to nickel(II)-ligated Schiff base acceptors. 46 Yields close to 50% could be reached using 2 equiv of 3 or through a double addition protocol (entries 2-3); conversely, slow addition of 3 by syringe pump over 3 h was unproductive (entry 4), suggesting the need for a sufficient concentration of acceptor to drive the radical coupling. Nevertheless, the reaction proceeded stereospecifically and its yield stands favorably when compared to the xanthate-based C-disaccharide synthesis 47 recently reported by Usui and Hirai, besides allowing the incorporation of an additional substituent in the anomeric position of the donor sugar. In conclusion, a novel manifold for the highly stereoselective construction of C,C-glycosides has been devised. Under carefully controlled reaction conditions, the challenging anomeric C-quaternization of exo-glycals was achieved through a Baran-type iron-mediated HAT/Michael-Giese coupling sequence. Further investigations on the generation of sterically demanding pseudoanomeric radicals and their applications to the synthesis of novel stable glycoconjugates are currently underway and will be reported in due course.

ASSOCIATED CONTENT Supporting Information

Complete experimental procedures, characterization data, and spectral data. This material is available free of charge via the Internet at http://pubs.acs.org.

ORCID

Nicolas Kern: 0000-0002-8825-1669 Philippe Compain: 0000-0002-6877-0859

Notes

The authors declare no competing financial interest.

Stereoselective

  Synthesis of C,C-Glycosides from Exo-Glycals Enabled by Iron-Mediated Hydrogen Atom Transfer Damien Tardieu, Marine Desnoyers, Claire Laye, Damien Hazelard, Nicolas Kern,* and Philippe Compain* Laboratoire d'Innovation Moléculaire et Applications (LIMA), Université de Strasbourg / Université de Haute-Alsace / CNRS (UMR 7042), Equipe de Synthèse Organique et Molécules Bioactives (SYBIO), ECPM, 25 rue Becquerel, 67087 Strasbourg, France Supporting Information Placeholder

Figure 1 .

 1 Figure 1. Prevalence of C,C-glycoside scaffolds in bioactive synthetic and natural products.

  1a was observed after 3 h in all cases. a Isolated yield based on 1a. b Isolated yield based on 3. c Second portion of 3 added after 1.5 h.

Table 1 . Optimization of the reaction for di-and tri- substituted exo-glycals.

 1

	entry compd	[Fe] (mol%)	PhSiH3 (equiv)	MVK (equiv)	C (M)	atm	yield (%) a
	1	1a	5	4	4	0.1	air	46
	2	"	10	4	4	0.1	air	61
	3	"	20	4	4	0.1	air	21
	4	"	10	2.5	4	0.1	air	44
	5	"	10	4	4	0.03 air	75
	6	"	10	4	4	0.03 Ar	86
	7	"	10	4	2	0.03 Ar 85 (A)
	8	"	10	4	2	0.03 b Ar	50
	9	"	10	4 c	2	0.03 Ar	47
	10	1b	10	4	4	0.03 Ar	31 d
	11	"	10	4+4 e	4	0.03 Ar	44 d
	12	"	100	4+4 e	4	0.03 Ar	54 d
	13	"	100	4+4 e	4	0.05 Ar	75 d (B)
	14	"	100	4+4 e	4	0.1	Ar	47 d
	15	"	50	4+4 e	4	0.05 Ar	46 d
	16	"	100	4+4 e	4	0.05 b Ar	NR d
	17	"	100	4 c	4	0.05 Ar	29 d

a Isolated yields. b Reaction run in 9:1 (CH2Cl)2/EtOH. c (iPrO)PhSiH2 used in (CH2Cl)2 instead of PhSiH3 in EtOH. d Reaction quenched after 24 h. e Second portion of PhSiH3 added after 16 h. NR = no reaction (<5% conversion).

Table 2 . Synthesis of carbon-linked diglycoside 4.

 2

	entry deviation from conditions A	yield of 4 (%) a

ACKNOWLEDGMENTS

The authors are grateful to financial support from the CNRS (UMR 7042), the University of Strasbourg, and the International Centre for Frontier Research in Chemistry (icFRC). D. T. and M. D. thank the French Department of Research for a doctoral fellowship.

AUTHOR INFORMATION

Corresponding Authors * E-mail: nicolas.kern@unistra.fr; philippe.compain@unistra.fr


O Šofranko
email: sofranko@saske.sk

R Leriche

A P Morales

T Cren

S Sasaki

L Cario

P Szabo

P Samuely

T Samuely

Periodic Surface Modulation

Keywords: scanning tunneling microscopy, misfit layer compound, Lanthanum niobium selenides, electronic properties

HAL is

Introduction

The layered transition metal dichalcogenides (TMD), TX2, where T is the early transition metal and X is S, Se or Te, are remarkable compounds due to their fascinating physical properties. They can accommodate various physical phenomena, such as the charge density waves (CDWs) and superconductivity [1][2][3]. Though TMDs with 1T structure could be superconducting only by doping or external pressure [4,5],TMD materials with 2H structure, e.g. 2H-NbSe 2 , exhibit intrinsic superconductivity [6,7]. Their two-dimensional structure is an important feature giving rise to e.g. Ising or topological superconductivity [8,9].

Transition-metal dichalcogenides (TMDs) are the promising candidates for tuning their properties by inducing strain. They can withstand up to 10 % of in-plane strain before breaking, thus providing enough space for engineering new strain induced properties [10]. Charge density wave (CDW) is one of the emergent states occurring in TMDs [10] often accompanied by other, possibly competing phases.

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In the case of 2H-NbSe 2 superconductivity (T c = 7.2 K) meets triangular (3Q) charge density wave phase (T CDW = 33 K) [2]. Exact mechanisms driving the choice of particular CDW wavevector remains unanswered even though various mechanisms have been proposed [11][12][13][14][15][16].

Small strain-induced changes in the electronic band structure and phonon dispersion can lead to dramatic change in CDW wavevector and geometry [17]. Single crystal attached to the silica plate at room temperature can be stretched while cooling down to 4 K because of mismatch between their thermal expansion coefficients. Shang Gao et al. induced strain in 2H-NbSe 2 via this method and observed change of CDW period. In addition to the well-known CDW ordering of ~ 3a 0 period (below 33 K), where a 0 is the triangular NbSe 2 lattice constant, they observed two additional CDW phases, unidirectional "stripe" (1Q) CDW ordering with 4a 0 period and a triangular (3Q) ordering with 2a 0 period. The wavevectors of all observed CDWs were found to be oriented along Γ-M direction. Presence of various CDW phases in different regions of sample was attributed to inhomogeneous strain [17].

Misfit compounds are based on alternating hexagonal TMDs and square MX lattice, where M can be for example Sn, Pb or La. They have peculiar structure arising from stacking layers with different lattice types. Lattice mismatch results in variety of physical properties misfit compounds can have. There are misfit materials in which we can observe superconductivity and CDWs. The single-crystalline misfit compound (LaSe) 1.14 (NbSe 2 ) has been successfully synthesized in the 90s [18]. It is possible to grow very good superconductive (T c = 1.2K) single crystals with RRR close to 6. It is expected that vertically stacked TMDs are strongly affected by strain [19,20].

Here we report the observation of surface modulation on (LaSe) 1.14 (NbSe 2 )with period ~ 2a 0 , commensurate with the crystal lattice. This observation correlates to the strain induced change of CDW order parameter in pure NbSe 2 .

Preparation of experiment

Scanning tunneling microscopy (STM) experiments were carried out by means of the Specs JT STM system operated in ultra-high vacuum (10 -10 mbar). Atomically sharp Au STM tips were formed ex situ by electrochemical etching followed by Ar + sputtering and annealing in situ.

(LaSe) 1.14 (NbSe 2 ) samples have layered structure with weak iono-covalent bonds between layers, which enables easy cleaving procedure. The sample was glued to a molybdenum sample holder, conductively connected by silver paste. We glued a thin metal stripe to the top layer of the sample. By removing the metal stripe in situ prior to the STM experiment, we exfoliated the upper layers, thus

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achieving an atomically flat surface without contaminations. Moreover, during the cleavage, the sample was placed on the liquid nitrogen shield of the refrigerator. This way we were able to obtain larger atomically flat terraces compared to the cleaving procedure at room temperature.

Surface topography was acquired in the constant current mode with the set point current I set = 50 pA. Bias voltage V bias = 100 mV was applied to the sample while the tip was grounded. Both the STM head and sample were kept at 1.2 K.

Results

Although it is not clear in advance which of the two planes will terminate the surface of the (LaSe) 1.14 (NbSe 2 ) monocrystal after cleavage, STM topography (Fig. 1 left) reveals a triangular lattice.

LaSe layer has square crystal structure; therefore, we assume that the surface is terminated by the NbSe 2 layer. In addition to the triangular lattice of individually resolved Nb atoms characteristic for the NbSe 2 surface, the topography reveals a plethora of elevated or depressed areas. To facilitate the analysis of these intricate patterns, we employed the two-dimensional Fourier transform (FT). The evident six atomic Bragg peaks of the FT, marked by the green circles in figure 1b, correspond to the triangular NbSe 2 lattice with period a 0 . Notably, another distinct feature appears halfway along two of the three Bragg vectors, i.e.

along the Γ-M directions. This surface modulation with period ~ 2a 0 , marked by white circles in Fig. 1b, is strongly reminiscent of the triangular (3Q) CDW ordering with 2a 0 period observed by Gao et al. on

NbSe 2 under strain [17].

Fig. 1: a) STM topography image (32 x 32 nm 2 ), color corresponds to the height according to the colorbar on the left. b) Fourier transform of the topography. Bragg peaks in green circles have lattice constant a 0 , areas representing period ~ 2a 0 are marked by white circles.

To better illustrate this surface modulation, we extracted the relevant FT peaks, namely the Bragg peaks and the peaks corresponding to the 2a 0 modulation in one direction (Fig. 2a). Fig. 2b, a zoom of the inverse FT of Fig. 2a, reveals this one particular surface modulation in real space, where every second

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No.X . atomic row, i.e. 2a 0 period, is elevated. Still, while Gao et al. observed this modulation along all thee Bragg vectors, in our case one pair of the FT peaks is missing. Possibly, the peaks in this direction are suppressed by the adjacent focused sharp peaks. Several of these sharp peaks, as well as a long-range modulation represented by a bright spot around Γ point, are also present. These probably represent the Moiré pattern induced by the misfit structure of the two distinct layers. However, this tentative explanation requires further analysis, which will be discussed elsewhere. Also, even though it is certain, that the misfit structure of the studied compound induces strain on the NbSe 2 layer, whether this strain is sufficient to alter the conventional CDW with 3a 0 period, or if other effects are in play, remains for further study.

Conclusions

In conclusion, by analyzing the FT of the STM topography of (LaSe) 1.14 (NbSe 2 ), we have observed a short-range modulation with a period ~ 2a 0 . This is reminiscent of the CDW in NbSe 2 under strain. [17] Indeed, the LaSe layer in our studied compound induces strain on the NbSe 2 layer due to incommensurability of their respective 2D crystal lattices. However, whether this strain is big enough to induce such change in the periodicity of the CDW, or a different unheeded effect is in play, remains for further investigations. A theoretical model of the (LaSe) 1.14 (NbSe 2 ) system that could shed more light on the unresolved issues is currently being developed.

Acknowledgement

This work was supported by the EU ERDF (European regional development fund) grant No.

Fig. 2 :

 2 Fig. 2: a) Relevant parts of Fourier transform, i.e. main Braggs peaks and one pair of peaks with 2a 0 period. b) Zoom of topography reconstructed from the FT showing only atomic lattice and the short range modulation with period ~ 2a 0 .

  26220220186, by the Slovak Research and Development Agency under Grant No. APVV-16-0372 and FR-2017-0015, by Slovak Scientific Grant Agency under contract VEGA 2/0149/16 and VEGA 1/0743/19, COST action CA16218 Nanocohybri, and by the U.S. Steel Košice, s.r.o.

	Vol.XXX (20XX)	CSMAG`19	No.X

. SK-


Marcin Drop

Xavier Bantreil

Katarzyna Grychowska

Gilbert Umuhire Mahoro

Evelina Colacino

Maciej Pawłowski

Jean Martinez

Gilles Subra

Pawel Zajdel

Frédéric Lamaty
email: frederic.lamaty@umontpellier.fr

Continuous flow ring-closing metathesis, an environmentally-friendly route to 2,5-dihydro-1Hpyrrole-3-carboxylates †

5-Dihydro-1H-pyrrole-3-carboxylates are important building blocks for the synthesis of high value pyrroles and pyrroloquinoline derivatives with interesting biological activities. The use of continuous flow allowed us to perform a key synthetic step, namely ruthenium-catalyzed ring-closing metathesis, with a residence time of 1 min at 120 °C. Dimethyl carbonate, a green solvent, was demonstrated for the first time to be an excellent solvent for this reaction in continuous flow. The continuous flow conditions proved to be general and the scale-up of this reaction was not only possible, but also highly efficient. Conversion of 10 grams of diene was realized in 37 minutes under continuous flow, yielding the desired heterocycle in 91% yield. Fig. 1 Pharmaceutical drugs synthesized using olefin metathesis.

Introduction

Olefin metathesis is increasingly used at the industrial level for the synthesis of API (Active Pharmaceutical Ingredients). 1 For example, ring-closing metathesis (RCM) was applied in the synthesis of ciluprevir (BILN-2061), a pseudopeptide inhibitor of HCV NS3 protease (Fig. 1). 2 Vaniprevir (MH-7009), a ciluprevir analog whose synthesis also requires a RCM step, is now being evaluated in phase III clinical trials. 3 The six-membered ring oseltamivir phosphate, also known as Tamiflu, and used in the treatment of influenza, could also be synthesized thanks to a RCM process. 4 The seven-membered heterocyclic ring in SB-462795, a cathepsin K inhibitor for the treatment of osteoporosis, was also obtained using RCM as a key step. [START_REF] Wang | [END_REF] In this context, besides our general interest in olefin metathesis, specifically using greener solvents such as poly(ethylene)glycol, glycerol and water, 6 we recently demonstrated that 2,5-dihydro-1H-pyrrole-3-carboxylates could be obtained via ring-closing metathesis (RCM) using ethyl acetate as a solvent and 1 mol% of NO 2 -Grela 7 as a catalyst (Fig. 2). These compounds were then converted efficiently into the corresponding pyrroles, which are highly versatile building blocks for biologically active compounds (Scheme 1). 8 We also demonstrated that the 2,5-dihydro-1H-pyrrole-3-carboxylate 2a was a highly significant intermediate in the synthesis of pyrroloquinolines, 9 which behave as 5-HT 6 receptor neutral antagonists. These compounds, displaying pro-cognitive properties, might be useful in the treatment of the cognitive decline associated with Alzheimer's disease and cognitive disorders caused by genetic abnormalities. 9a,10 Ring-closing metathesis on substrates similar to 1a, bearing an electron-withdrawing group on the olefin, is usually more difficult than RCM of benchmark substrates such as diallyltosyl amine. 11 An improvement of the method consisted in performing the reaction under microwave activation, which allowed reducing the reaction time to 5 min. 9c,11b,g,h However, scalability of the microwave conditions still represents a huge challenge for this method. One efficient way to circumvent this issue would be to use continuous flow chemistry. Indeed, high temperature microwave synthesis could be easily translated to continuous flow synthesis, since both techniques allow efficient heating/cooling and generation of a superheated solvent. 12 In order to obtain the reaction conditions that could be applicable on even a larger scale, we decided to focus on the RCM of 1a using continuous flow chemistry. Continuous flow is increasingly prevalent in organic chemistry, 13 providing several advantages over batch synthesis. 14 As small quantities of reagents are in contact with each other in microfluidic systems, a perfect control of stoichiometry, 15 important for the selectivity of reactions, as well as the control of exothermic reactions, 16 could be envisaged. In addition, versatile conditions, in terms of temperature, pressure and flow rates, could be applied to facilitate the optimisation process.

Literature reports dealing with ruthenium-catalyzed metathesis under continuous flow conditions remain scarce 17 and focus mostly on the development of heterogeneous catalysts. 18 Since, for the moment, most heterogeneous catalysts show limited recyclability, our approach was to focus on the development of environmentally friendly conditions using easily available homogeneous catalysts.

Results and discussion

Reactions were performed using a Uniqsis Flowsyn system, and reagents (substrates and catalysts) were dissolved together in a single reservoir bottle prior to pumping into the residence coil. To make sure that this methodology could be robust and practically scalable, reactions were performed in air using ACS grade solvents, without degassing or drying. Second generation commercially available catalysts, bearing N,N′-bis (2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) as the NHC ligand, from different families, were tested (Fig. 2). Benzylidene (G-II 19 ), boomerang-type (HG-II 20 and NO 2 -Grela 7 ), "Scorpio" 21 (M5 1 22 and M5 2 23) and robust indenylidene 24 (M2, 25 M2 0 , 26 ) catalysts were screened in the RCM of compound 1a, with an o-NO 2 phenyl ring, in dichloromethane as a solvent, at 90 °C, with a residence time of 30 min and 3 mol% of ruthenium catalyst (Table 1, entry 1). Pre-catalysts M5 1 , HG-II, M2 and M2 0 , which are more resistant to temperature than G-II, NO 2 -Grela, M5 2 and M3 1 , gave the best HPLC conversions (up to 98%). Using M2 as a catalyst, reducing the catalyst loading from 3 to 2 mol% was detrimental to the reaction. While decreasing the residence time to 20 min at 90 °C resulted in a lower conversion, a concomitant increase of temperature to 110 °C allowed us to obtain 94% conversion in only 5 min. 27 It is important to note that traces (up to 5%) of isomerization of 1a, resulting from the formation of a ruthenium hydride species, 6b were detected in the crude reaction mixtures; interestingly, the percentage of isomerization was below 1% when M5 2 , M2, M2 0 and M3 1 were used. 27 Although these conditions could be suitable for efficient RCM, in a continuous effort to find environmentally friendly conditions, 6a-e we decided to search for a greener solvent than dichloromethane. Previous studies in batch showed that ethyl acetate and dimethyl carbonate (DMC) could be suitable solvents for RCM. 28 In addition, these solvents were classified as non-toxic for the industrial synthesis of API. 29 Since it was an appropriate solvent for the RCM of 1a in batch, 8 ethyl acetate was first tested (Table 1, entry 2). Reactions using HG-II, M5 1 and M2 2 gave interesting HPLC conversions (>85%) in AcOEt, but the results were not better than those in dichloromethane. DMC was then evaluated in continuous flow RCM for the first time, to the best of our knowledge. To our delight, catalysts HG-II and M2 gave excellent results under the conditions tested. Catalyst M2, which is a highly stable indenylidene-type pre-catalyst, allowed reaching almost full conversion when reactions were performed at 110 °C for 5 min or at 120 °C for 1 min (Table 1, entries 6-8). However, HG-II gave a lower con- Scheme 1 Synthesis of high value molecules through metathesis. 9a version at 120 °C, probably because of a slightly lower thermal stability. Additionally, isomerisation of 1a was observed when HG-II was used (up to 5%). Finally, a third interesting set of conditions was found with catalyst M2 2 , which has a latent character that already proved to be useful in RCM and polymerisation reactions. 26,30 At 130 °C, catalyst M2 2 gave 92% conversion within only 5 min reaction (Table 1, entry 6).

M3 1 , M2 2
As stated above, reactions were run using one solution that was pumped into the residence coil. Since catalyst decomposition could occur rapidly when the catalyst is in the presence of an alkene substrate, through the formation of a ruthenium methylidene species, 31 reactions were also performed using two separate solutions, one containing the substrate 1a, and the other one containing the catalyst. Catalyst M2, which is temperature stable and gave the best results in DMC, and M3 1 , which activates faster and thus is more prone to decomposition, were tested. Then, the substrate and catalyst solutions were pumped at the same flow rate and mixed in a coil heated at 120 °C. After a residence time of 1 min, as for Table 1, entry 5, conversions reached 96% and 19% with M2 and M3 1 resp., similar to the ones obtained in the one pump experiments. This demonstrates that decomposition, if happening, does not distort the results obtained in Table 1 using a single reservoir bottle. Performed on a 1 mmol scale, conditions with M2 using two separate solutions gave, after 1 min residence time at 120 °C, filtration over silica, evaporation and precipitation in diethyl ether, the cyclized compound 2a in 83% yield (Scheme 2). Such a result permits to envision the use of an automated system, allowing the screening of substrates or catalysts.

In the next step, we investigated the utility of the developed method for the RCM of compounds 1b-g, bearing different substituents on the phenyl ring (Scheme 3). Two sets of experimental conditions (A: 110 °C for 5 min and B: 120 °C for 1 min) were compared using substrates 1a-d to select the one that would give the best results. Compounds 2a-d, bearing respectively a nitro group, a chlorine atom, a methoxy group or a hydrogen atom in ortho position of the phenyl ring, were obtained through RCM, after evaporation and precipitation in diethyl ether. Experimental conditions B resulted in a shorter reaction time and provided slightly better yields. Conditions B were thus selected and applied to substrates 1e-g, bearing a chlorine atom or a methoxy group either in meta or para position of the phenyl ring. Compounds 2e, 2f and 2g were isolated in 52%, 68% and 81% yields, respectively. In all cases, conversion was almost complete and neither degradation nor intermolecular unwanted side reactions that would have given dimer side-products was observed. Moderate yields were mainly due to incomplete precipitation during product isolation. 27 Continuous flow is an innovative technology since, once optimal conditions are found, they can be directly applied for reaction scale-up. The RCM of 10 g (23 mmol) of compound 1a was thus attempted, using 3 mol% of M2 catalyst in DMC, with a reaction coil heated at 120 °C and a flow rate to ensure a 1 min residence time. Gratifyingly, 10 g of substrate were pumped into the continuous flow system within 37 min and the cyclized dihydropyrrole 2a could be isolated in 91% yield, which is exactly the same yield as that obtained on a 1 mmol scale (Scheme 4). In comparison, RCM of 15 g of 1a in batch gave less satisfying results. 8 To confirm that the RCM reaction proceeded in the reaction coil and not in the reservoir bottle before pumping into the coil, the conversion was checked, in the reservoir solution, after 37 min. Less than 5% conversion was measured, demonstrating that the RCM process required heating at 120 °C in continuous flow. Since HG-II is one of the most used catalysts for API synthesis, it was also evaluated on a larger scale using conditions from Table 1, entry 4. Thus, on a 2 mmol scale with a residence time of 5 min at 110 °C in DMC, the cyclized compound 2a was isolated in 92% yield. The total reaction time was 16 min in this case. HG-II is thus a good candidate for large scale RCM, even though, on the same scale, the total reaction time would be longer than that with M2. In addition, isomeri-zation of 1a also occurred on this scale; the undesired isomer was separated from 2a upon precipitation, and then isolated and fully characterized. 27

Conclusions

We reported in this study a highly efficient continuous flow protocol allowing the ring-closing metathesis of dienes 1a-g to obtain 2,5-dihydro-1H-pyrroles 2a-g. We demonstrated that dimethyl carbonate, an environmentally friendly solvent, was an excellent solvent for the metathesis reaction in flow, which could be completed within 1 min at 120 °C. In addition, scaling-up the reaction using continuous flow, from 1 to 23 mmol, did not change the outcome of the ring-closing metathesis, and yielded the cyclized compound 2a in 91% yield after 37 min.

Fig. 2

 2 Fig. 2 Structure of the catalysts screened in this study.

Table 1

 1 Optimization of the RCM continuous flow conditions a Reaction conditions: 1a (0.125 mmol), catalyst (3 mol%), solvent (1 mL), T, residence time; HPLC conversions of 1a into 2a are given. See TableS1in ESI for the complete optimization. b 2 mol% catalyst were used.

					HPLC conv. (%)
	Entry	Solvent	T (°C)	Res. time (min)	G-II	NO 2 -Grela	HG-II	M5 1	M5 2	M2	M2 0	M3 1	M2 2
	1 2	CH 2 Cl 2 AcOEt	90 90	30 30	72 75	74 73	94 95	96 85	14 22	98(84) b 23	94 60	25 23	35 88
	3	DMC	90	30	74	84	94	82	10	98	50	21	75
	4	DMC	110	5	76	85	94	78	7	98	57	11	77
	5		120	1	71	73	85	69	5	96	46	19	53
	6		130	5									92

a Scheme 2 Continuous flow RCM of 1a using 2 pumps.

This journal is © The Royal Society of Chemistry 2017

This journal is © The Royal Society of Chemistry 2017 Green Chem.,2017, 19, 1647-1652 | 1651

Acknowledgements

We acknowledge the financial support from the University of Montpellier, CNRS, National Science Center, Poland (grant no. 2016/21/B/NZ7/01742), Institut Carnot Chimie Balard through ANR programme no. 11 CARN 0001-01, and the PHC Polonium programme (French Embassy in Poland/French Ministry of Foreign Affairs and International Development). M. D. thanks the Erasmus+ Programme for a fellowship. Umicore AG & Co. KG is gratefully acknowledged for the gift of catalysts M2, M2 0 , M3 1 , M5 1 and M5 2 . Materia, Inc. is gratefully acknowledged for the gift of the catalyst HG-II.

Notes and references