A succession of isomers of ruthenium dihydride complexes. Which one is the ketone hydrogenation catalyst?
Reaction of RuHCl(PPh(3))(2)(diamine) (1a, diamine = (R,R)-1,2-diaminocyclohexane, (R,R)-dach; 1b, diamine = ethylenediamine, en) with KO(t)Bu in benzene quickly generates solutions of the amido-amine complexes RuH(PPh(3))(2)(NHC(6)H(10)NH(2)), (2a'), and RuH(PPh(3))(2)(NHCH(2)CH(2)NH(2)), (2b'), respectively. These solutions react with dihydrogen to first produce the trans-dihydrides (OC-6-22)-Ru(H)(2)(PPh(3))(2)(diamine) (t,c-3a, t,c-3b). Cold solutions (-20 degrees 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 degrees and then isomerize to the cis-dihydride, cis-bisphosphine isomers (OC-6-32)-Ru(H)(2)(PPh(3))(2)(diamine) (Delta/Lambda-c,c-3a, c,c-3b). A solution containing mainly Delta/Lambda-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 H(2) 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)(PPh(3))(2)(diamine) (c,t-3a, c,t-3b) with half-lives of 40 min and 1 h, respectively. A mixture of Delta/Lambda-c,c-3a and c,t-3a can also be obtained by reaction of 1a with KBH(Bu(sec))(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 RuCl(2)(PPh(3))(2)((R,R)-dach) or 1a, when activated by KO(t)Bu, 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)[H(2)] 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(NHCMe(2)CMe(2)NH(2))(PPh(3))(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 [NEt(3)H]BPh(4) to produce the dihydrogen complex (OC-14)-[Ru(eta(2)-H(2))(H)(PPh(3))(2)((R,R)-dach)]BPh(4) (4) and with diphenylphosphinic acid to give the complex RuH(O(2)PPh(2))(PPh(3))(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 (PPh(3))(2)HRu(mu-eta(2);eta(4)-NHCHCHNH)RuH(PPh(3))(2) (6) containing a rare, bridging 1,4-diazabutadiene group. The formation of an imine by beta-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.
Abbel
Robert
R
Davenport Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
Abdur-Rashid
Kamaluddin
K
Faatz
Michael
M
Hadzovic
Alen
A
Lough
Alan J
AJ
Morris
Robert H
RH