Balasubramaniyam Ramasamy, M. Gangwar, P. Ghosh
Jul 17, 2017
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Influential Citations
21
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Journal
European Journal of Inorganic Chemistry
Abstract
The catalytic potential of a N‒heterocyclic carbene ligand derived from a chiral fused bicyclic ring scaffold, having a restricted rotation along the C‒N bond bearing the chiral auxiliary, have been explored in transition metal mediated asymmetric transfer hydrogenation reactions of ketones. In particular, the chiral oxazolidine fused N‒heterocyclic carbene precursors namely, {(3S)-3-R-6-methyl-7-phenyl-2,3-dihydroimidazo[5,1-b]oxazol-6-ium iodide (R= sec-butyl (1f), i-butyl (2f), i-propyl (3f)} were synthesized from commercially available optically pure amino acids in a multi-step sequence avoiding tedious chiral resolution protocol. The reaction of the chiral imidazolium iodide salts (1‒3)f with Ag2O yielded the corresponding silver complexes (1‒3)g, which when treated with {(COD)MCl}2 (M = Rh, Ir) gave the rhodium(I) (1‒3)h and the iridium(I) (1‒3)i complexes. The rhodium(I) (1‒3)h and the iridium(I) (1‒3)i complexes conveniently carried out the asymmetric transfer hydrogenation of acetophenones for a wide variety of substrates ranging from the electron rich ones namely, 4‒methyl acetophenone, 3,4 dimethyl acetophenone, 4‒t-butyl acetophenone, 4‒methylthio acetophenone to the electron deficient ones namely, 4‒bromo acetophenone, 4‒chloro acetophenone, 4‒fluoro acetophenone, 4‒nitro acetophenone and 3‒fluoro acetophenone in moderate to good yields (ca. 18‒95 %) but low enantioselectivities (ca. 4‒25 %).