A Succession of Isomers of Ruthenium Dihydride Complexes. Which One Is the Ketone Hydrogenation Catalyst?
2005-02-16T00:00:00Z (GMT) by
Reaction of RuHCl(PPh3)2(diamine) (1a, diamine = (R,R)-1,2-diaminocyclohexane, (R,R)-dach; 1b, diamine = ethylenediamine, en) with KOtBu in benzene quickly generates solutions of the amido−amine complexes RuH(PPh3)2(NHC6H10NH2), (2a‘), and RuH(PPh3)2(NHCH2CH2NH2), (2b‘), respectively. These solutions react with dihydrogen to first produce the trans-dihydrides (OC-6-22)-Ru(H)2(PPh3)2(diamine) (t,c-3a, t,c-3b). Cold solutions (−20 °C) containing trans-dihydride t,c-3a react with acetophenone under Ar to give (S)-1-phenylethanol (63% ee). Complexes t,c-3 have lifetimes of less than 10 min at 20° and then isomerize to the cis-dihydride, cis-bisphosphine isomers (OC-6-32)-Ru(H)2(PPh3)2(diamine) (Δ/Λ-c,c-3a, c,c-3b). A solution containing mainly Δ/Λ-c,c-3a reacts with acetophenone under Ar to give (S)-1-phenylethanol in 20% ee, whereas it is an active precatalyst for its hydrogenation under 5 atm H2 to give 1-phenylethanol with an ee of 50−60%. Complexes c,c-3 isomerize to the cis-dihydride, trans-bisphosphine complexes (OC-6-13)-Ru(H)2(PPh3)2(diamine) (c,t-3a, c,t-3b) with half-lives of 40 min and 1 h, respectively. A mixture of Δ/Λ-c,c-3a and c,t-3a can also be obtained by reaction of 1a with KBH(Busec)3. A solution of complex c,t-3a in benzene under Ar reacts very slowly with acetophenone. These results indicate that the trans-dihydrides t,c-3a or t,c-3b along with the corresponding amido−amine complexes 2a‘ or 2b‘ are the active hydrogenation catalysts in benzene, while the cis-dihydrides c,c-3a or c,c-3b serve as precatalysts. The complexes RuCl2(PPh3)2((R,R)-dach) or 1a, when activated by KOtBu, are also sources of the active catalysts. A study of the kinetics of the hydrogenation of acetophenone in benzene catalyzed by 3a indicates a rate law: rate = k[c,c-3a]initial[H2] with k = 7.5 M-1 s-1. The turnover-limiting step appears to be the reaction of 2a‘ with dihydrogen as it is for RuH(NHCMe2CMe2NH2)(PPh3)2 (2c‘). The catalysts are more active in 2-propanol, even without added base, and the kinetic behavior is complicated. The basic cis-dihydride c,t-3a reacts with [NEt3H]BPh4 to produce the dihydrogen complex (OC-14)-[Ru(η2-H2)(H)(PPh3)2((R,R)-dach)]BPh4 (4) and with diphenylphosphinic acid to give the complex RuH(O2PPh2)(PPh3)2((R,R)-dach) (5). The structure of 5 models aspects of the transition state structure for the ketone hydrogenation step. Complex 2b‘ decomposes rapidly under Ar to give dihydrides 3b along with a dinuclear complex (PPh3)2HRu(μ-η2;η4-NHCHCHNH)RuH(PPh3)2 (6) containing a rare, bridging 1,4-diazabutadiene group. The formation of an imine by β-hydride elimination from the amido−amine ligand of 2a‘ under Ar might explain some loss of enantioselectivity of the catalyst. The structures of complexes 1a, 5, and 6 have been determined by single-crystal X-ray diffraction.