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-(η³-cyclooctenyl)rhodacarboranes of the general formula [3-{(1−3-η³)-C₈H₁₃}-1,2-R,R′-3,1,2-RhC₂B₉H₉] (1 (closo), R, R′ = μ-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-(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-(PhCH₂)₂-3,1,2-RhC₂B₉H₉] (17), the formation of which and further conversion into the salt [(S)-(PipPhos)₄Rh]⁺[7,8-(PhCH₂)₂-nido-7,8-C₂B₉H₁₀]⁻ (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.