A Succession of Isomers of Ruthenium Dihydride Complexes. Which One Is the Ketone Hydrogenation Catalyst?

Reaction of RuHCl(PPh<sub>3</sub>)<sub>2</sub>(diamine) (<b>1a</b>, diamine = (<i>R</i>,<i>R</i>)-1,2-diaminocyclohexane, (<i>R</i>,<i>R</i>)-dach; <b>1b</b>, diamine = ethylenediamine, en) with KO<sup>t</sup>Bu in benzene quickly generates solutions of the amido−amine complexes RuH(PPh<sub>3</sub>)<sub>2</sub>(NHC<sub>6</sub>H<sub>10</sub>NH<sub>2</sub>), (<b>2a</b>‘), and RuH(PPh<sub>3</sub>)<sub>2</sub>(NHCH<sub>2</sub>CH<sub>2</sub>NH<sub>2</sub>), (<b>2b</b>‘), respectively. These solutions react with dihydrogen to first produce the <i>trans</i>-dihydrides (OC-6-22)-Ru(H)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>(diamine) (<i>t</i>,<i>c</i>-<b>3a</b>, <i>t</i>,<i>c</i>-<b>3b</b>). Cold solutions (−20 °C) containing <i>trans</i>-dihydride <i>t</i>,<i>c</i>-<b>3a</b> react with acetophenone under Ar to give (<i>S</i>)-1-phenylethanol (63% ee). Complexes <i>t</i>,<i>c</i>-<b>3</b> have lifetimes of less than 10 min at 20° and then isomerize to the <i>cis</i>-dihydride, <i>cis</i>-bisphosphine isomers (OC-6-32)-Ru(H)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>(diamine) (Δ/Λ-<i>c</i>,<i>c</i>-<b>3a</b>, <i>c</i>,<i>c</i>-<b>3b</b>). A solution containing mainly Δ/Λ-<i>c</i>,<i>c</i>-<b>3a</b> reacts with acetophenone under Ar to give (<i>S</i>)-1-phenylethanol in 20% ee, whereas it is an active precatalyst for its hydrogenation under 5 atm H<sub>2</sub> to give 1-phenylethanol with an ee of 50−60%. Complexes <i>c</i>,<i>c</i>-<b>3</b> isomerize to the <i>cis</i>-dihydride, <i>trans</i>-bisphosphine complexes (OC-6-13)-Ru(H)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>(diamine) (<i>c</i>,<i>t</i>-<b>3a</b>, <i>c</i>,<i>t</i>-<b>3b</b>) with half-lives of 40 min and 1 h, respectively. A mixture of Δ/Λ-<i>c</i>,<i>c</i>-<b>3a</b> and <i>c</i>,<i>t</i>-<b>3a</b> can also be obtained by reaction of <b>1a</b> with KBH(Bu<sup>sec</sup>)<sub>3</sub>. A solution of complex <i>c</i>,<i>t</i>-<b>3a</b> in benzene under Ar reacts very slowly with acetophenone. These results indicate that the <i>trans</i>-dihydrides <i>t</i>,<i>c</i>-<b>3a</b> or <i>t</i>,<i>c</i>-<b>3b</b> along with the corresponding amido−amine complexes <b>2a</b>‘ or <b>2b</b>‘ are the active hydrogenation catalysts in benzene, while the <i>cis</i>-dihydrides <i>c</i>,<i>c</i>-<b>3a</b> or <i>c</i>,<i>c</i>-<b>3b</b> serve as precatalysts. The complexes RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>((<i>R</i>,<i>R</i>)-dach) or <b>1a</b>, when activated by KO<sup>t</sup>Bu, are also sources of the active catalysts. A study of the kinetics of the hydrogenation of acetophenone in benzene catalyzed by <b>3a</b> indicates a rate law:  rate = <i>k</i>[<i>c</i>,<i>c</i>-<b>3a</b>]<sub>initial</sub>[H<sub>2</sub>] with <i>k </i>= 7.5 M<sup>-1</sup> s<sup>-1</sup>. The turnover-limiting step appears to be the reaction of <b>2a</b>‘ with dihydrogen as it is for RuH(NHCMe<sub>2</sub>CMe<sub>2</sub>NH<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub> (<b>2c</b>‘). The catalysts are more active in 2-propanol, even without added base, and the kinetic behavior is complicated. The basic <i>cis</i>-dihydride <i>c</i>,<i>t</i>-<b>3a</b> reacts with [NEt<sub>3</sub>H]BPh<sub>4</sub> to produce the dihydrogen complex (OC-14)-[Ru(η<sup>2</sup>-H<sub>2</sub>)(H)(PPh<sub>3</sub>)<sub>2</sub>((<i>R</i>,<i>R</i>)-dach)]BPh<sub>4</sub><b> (4)</b> and with diphenylphosphinic acid to give the complex RuH(O<sub>2</sub>PPh<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub>((<i>R</i>,<i>R</i>)-dach) (<b>5</b>). The structure of <b>5</b> models aspects of the transition state structure for the ketone hydrogenation step. Complex <b>2b</b>‘ decomposes rapidly under Ar to give dihydrides <b>3b</b> along with a dinuclear complex (PPh<sub>3</sub>)<sub>2</sub>HRu(μ-η<sup>2</sup>;η<sup>4</sup>-NHCHCHNH)RuH(PPh<sub>3</sub>)<sub>2</sub> (<b>6</b>) containing a rare, bridging 1,4-diazabutadiene group. The formation of an imine by β-hydride elimination from the amido−amine ligand of <b>2a</b>‘ under Ar might explain some loss of enantioselectivity of the catalyst. The structures of complexes <b>1a</b>, <b>5</b>, and <b>6</b> have been determined by single-crystal X-ray diffraction.