B(C 6 F 5 ) 3 -catalyzed transfer dehydrogenation of pyrrolidines Joseph Gillions 1 , Dr Ana Alvarez-Montoya 2 , Laura Winfrey 1 , Dr Alexander P. Pulis 1 1 University of Leicester, UK, 2 Biosynth, UK Pyrroles are important biological scaffolds. Typically, they are synthesised from pre-functionalised pyrroles, or ring forming steps. 1 Current methods of dehydrogenating pyrrolidines to form pyrroles have received poor uptake, due to limited scope, which almost always require an electron-withdrawing group (EWG) on the pyrrolidine ring. 2 As pyrrolidines and pyrroles have orthogonal reactivity, and pyrrolidines are readily available and easily installed, the ability to carry a pyrrolidine through a synthesis before dehydrogenating to form the desired pyrrole would be a useful and complementary method. Our group explores the use of B(C 6 F 5 ) 3 , an extraordinary Lewis acid capable of abstracting hydrides from the α-position of amines. 3 We have discovered that pyrrolidines without EWGs can be dehydrogenated via B(C 6 F 5 ) 3 -catalyzed transfer dehydrogenation, using commercially available 2-methallyltrimethylsilane as a hydrogen acceptor. The method is easily applied without the need for a glove box or strictly anhydrous conditions: Commercially available B(C 6 F 5 ) 3 is supplied as the water adduct H 2 O.B(C 6 F 5 ) 3 , which can be weighed on the open benchtop before commercially available Et3SiH is used to form active B(C 6 F 5 ) 3 in situ . Furthermore, the reaction is performed using standard glassware, and solvents were used as received from commercial suppliers. The method shows good functional group tolerance, and include for example boronic esters, halides, ethers, and trifluoromethyls. Mechanistic studies have also been performed to delineate the catalytic cycle and have shown the critical role of the alkene. We have also observed, isolated and characterised several reaction intermediates. References 1. Singh, N.; Singh, S.; Kohli, S.; Singh, A; Asiki, H.; Rathee, G.; Chandra, R.; Anderson, E. A. Org. Chem. Front. , 2021 , 8 , 5550-5573. 2. a) Bonnaud, B.; Bigg, D.; Synthesis , 1994 , 1994 , 465-467. b) Nishimoto, S.; Nakahashi, H.; Toyota, M.; Tet. Lett. , 2020 , 61 , 152599. 3. Basak, S.; Winfrey, L.; Kustiana, B. A.; Melen, R. L.; Morrill, L. C.; Pulis, A. P. Chem. Soc. Rev., 2021 , 50 , 3720-3737.
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