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-RhC2B9H9]

Formally 16-electron closo- and pseudocloso-(eta(3)-cyclooctenyl)rhodacarboranes of the general formula [3-{(1-3-eta(3))-C8H13}-1,2-R,R'-3,1,2-RhC2B9H9] (1 (closo), R, R' = mu-1',2'-CH2C6H4CH2; 2 (Pseudocloso), R = R' = PhCH2) 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-(oxylylene)-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-(PhCH2)(2)-3,1,2-RhC2B9H9] (17), the formation of which and further conversion into the salt [(S)-(PipPhos)(4)Rh](+)[7,8-(PhCH2)(2)-nido-7,8-C2B9H10](-) (18) was detected by time-dependent H-1 NMR spectra. Some conclusions regarding the catalysis mechanistic pathway, which is consistent with that generally accepted for the rhodacarborane-catalyzed alkene hydrogenation, are made.