New Rhodacarborane−Phosphoramidite Catalyst System for Enantioselective Hydrogenation of Functionalized Olefins and Molecular Structure of the Chiral Catalyst Precursor [3,3-{(<i>S</i>)-PipPhos}<sub>2</sub>-3-H-1,2-(<i>o</i>-xylylene)-<i>closo</i>-3,1,2-RhC<sub>2</sub>B<sub>9</sub>H<sub>9</sub>]

Formally 16-electron <i>closo</i>- and <i>pseudocloso</i>-(η<sup>3</sup>-cyclooctenyl)rhodacarboranes of the general formula [3-{(1−3-η<sup>3</sup>)-C<sub>8</sub>H<sub>13</sub>}-1,2-R,R′-3,1,2-RhC<sub>2</sub>B<sub>9</sub>H<sub>9</sub>] (<b>1</b> (closo), R, R′ = μ-1′,2′-CH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>; <b>2</b> (pseudocloso), R = R′ = PhCH<sub>2</sub>) coupled in situ with the chiral phosphoramidite (<i>S</i>)-PipPhos (<b>3</b>) 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-{(<i>S</i>)-PipPhos}<sub>2</sub>-3-H-1,2-(<i>o</i>-xylylene)-<i>closo</i>-3,1,2-RhC<sub>2</sub>B<sub>9</sub>H<sub>9</sub>] (<b>16</b>), independently prepaped by the stoichometric reaction of <b>1</b> with <b>3</b> 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 <b>1</b> and <b>3</b>. The structure of <b>16</b> 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 <b>1</b>, complex <b>2</b> reacts with <b>3</b> to afford the unstable hydrido−rhodium species [3,3-{(<i>S</i>)-PipPhos}<sub>2</sub>-3-H-1,2-(PhCH<sub>2</sub>)<sub>2</sub>-3,1,2-RhC<sub>2</sub>B<sub>9</sub>H<sub>9</sub>] (<b>17</b>), the formation of which and further conversion into the salt [(<i>S</i>)-(PipPhos)<sub>4</sub>Rh]<sup>+</sup>[7,8-(PhCH<sub>2</sub>)<sub>2</sub>-<i>nido</i>-7,8-C<sub>2</sub>B<sub>9</sub>H<sub>10</sub>]<sup>−</sup> (<b>18</b>) was detected by time-dependent <sup>1</sup>H NMR spectra. Some conclusions regarding the catalysis mechanistic pathway, which is consistent with that generally accepted for the rhodacarborane-catalyzed alkene hydrogenation, are made.