Étude mécanistique computationnelle des réactions d’amination catalysées par des dimères de rhodium

Catalytic amination reactions are a powerful tool in organic synthesis. They aim to introduce nitrogen atom to alkane, alkene or thioether moieties, giving rise to amine products that have various medical and industrial applications. The Lebel group has developed catalytic amination reactions in the presence of rhodium dimers using N-sulfonyloxycarbamates as nitrene precursors. In the presence of a base, N-sulfonyloxycarbamates presumably afforded rhodium nitrenes which underwent C-H insertions, C=C additions or reactions with the sulfur atom of thioethers resulting in acyclic and cyclic carbamates, aziridines and sulfilimines respectively. In addition, good diastereoselectivities were observed in the presence of a chiral N-sulfonyloxycarbamate reagent and a chiral rhodium dimer for all three reactions. In this dissertation, we are interested in the mechanistic aspects of these amination reactions. Given the absence of experimental proofs of in-situ generated rhodium nitrene species, playing the role of the amination agent, nor of its precomplex, the rhodium nitrenoid, the different amination reactions mechanisms remain uncertain. Our approach is based on the scan of the potential energy surfaces of different mechanistic paths, for each of the amination reactions, well established on the experimental level, by resorting to the Functional Theory of Density (DFT).  The Ernzerhof research group is expert on the development of exchange-correlation functionals, therefore relevant strict criteria have been considered when choosing and validating the theoretical model used during the mechanistic studies. The correlation exchange functional developed by Perdew-Burke-Ernzerhof (PBE) was established as the best to study reactions involving rhodium dimers where the electronic correlation is strong. We studied the formation and reactivity of rhodium nitrene species considering their two lower energy spin states. Singlet rhodium nitrenes appeared to be the most reactive intermediates for the C-H amination reaction. In addition, singlet rhodium nitrenes were shown responsible for the formation of secondary products such as carbonyls and primary carbamates derived from the corresponding N-mesyloxycarbamates. In sharp contrast, in the aziridination reaction, both singlet and triplet rhodium nitrene species acted as aminating agents in a process involving an intersystem spin crossover. To further rationalize the asymmetric induction of catalytic aziridination reactions, we have undertaken the calculation of the diastereoselectivity ratios in the presence of the chiral catalyst Rh2[(S)-nttl]4. An exhaustive study was performed and it revealed that the asymmetric induction was due to a reactive conformation of rhodium nitrene species in which the ligand adopts C4 symmetry. Up to now, no mechanistic study involving DFT calculations have been reported in the literature for the amination of thioethers, no matter what catalytic system is used. To study catalytic sulfimidation reactions, we calculated the different mechanistic paths of rhodium catalyzed thioanisole imidation with and without DMAP and bis(DMAP)CH2Cl2 additives. The study showed a 'classical' insertion mechanism of rhodium nitrene species into the thioether in absence of bis(DMAP)CH2Cl2. In the presence of the latter, the mechanism diverged to a thioether insertion/salt (bis(DMAP)CH2Cl-OMs) elimination reaction where the rhodium nitrenoid complex was, henceforth, the imidation reagent.

Keyphrases: N-Mésyloxycarbamates, DFT, PBE, MECP, Sulfimidation, Bis(DMAP)CH2Cl2, Rhodium Dimers, N-Mesyloxycarbamates, Chiral Rhodium Dimer, Rhodium Nitrene Species, Nitrenoid Complex, C-H Bond Amination, Catalytic Aziridination, Multi-States Reactivity, Diastereoselective Aziridination