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

摘要

Reaction of RuHCI(PPh_3)_2(diamine)(1a,diamine =(R,R)-1,2-diaminocyclohexane,(R,R)-dach;1b,diamine = ethylenediamine,en)with KO~tBu in benzene quickly generates solutions of the amido-amine complexes RuH(PPh_3)_2(NHC_6H_10NH_2),(2a'),and RuH(PPh_3)_2(NHCH_2CH_2NH_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 deg 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 20deg 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 c/s-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 RuCI_2(PPh_3)_2((R,R)-dach)or 1a,when activated by KO~tBu,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_2CMe_2NH_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_3H]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_2PPh_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)_2HRu(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.