New rhodacarborane-phosphoramidite catalyst system for enantioselective hydrogenation of functionalized olefins and molecular structure of the chiral catalyst precursor [3,3-{(S)-PipPhos}_2-3-H-1,2-(o -xylylene)- closo -3,1,2-RhC_2B_9H_9]

摘要

Formally 16-electron closo- and pseudocloso-(a~3-cyclooctenyl) rhodacarboranes of the general formula [3-{(1-3-η~3)-C _8H_(13)}-1,2-R,R-3,1,2-RhC_2B_9H _9] (1 (closo), R, R = μ-1′,2′-CH_2C _6H_4CH_2; 2 (pseudocloso), R = R = PhCH 2) coupled in situ with the chiral phosphoramidite (S)-PipPhos (3) were found to catalyze an asymmetric hydrogenation of functionalized olefins (enamides) with enantioselectivities as high as 97-99.7% and with 92-100% conversions. The key catalyst precursor [3,3-{(S)-PipPhos}2-3-H-1,2- (o-xylylene)-closo-3,1,2-RhC2B9H9] (16), independently prepaped by the stoichometric reaction of 1 with 3 in benzene, was found to show of enantioselectivities and conversions upon the hydrogenation of prochiral enamides at the same levels as those observed for the relevant precursor formed in situ from 1 and 3. The structure of 16 has been established on the basis of analytical and multinuclear NMR data as well as a single-crystal X-ray diffraction study. In contrast to complex 1, complex 2 reacts with 3 to afford the unstable hydrido-rhodium species [3,3-{(S)-PipPhos} 2-3-H-1,2-(PhCH_2)_2-3,1,2-RhC_2B _9H_9] (17), the formation of which and further conversion into the salt [(S)-(PipPhos)_4Rh]~+[7,8-(PhCH _2)_2-nido-7,8-C_2B_9H 10]- (18) was detected by time-dependent 1H NMR spectra. Some conclusions regarding the catalysis mechanistic pathway, which is consistent with that generally accepted for the rhodacarborane-catalyzed alkene hydrogenation, are made.