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

Abstract

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.

Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)

“…Such isomerisation in dihydride phosphine complexes is known. [44] Given that we do not observe D in our system, and that 3 is relatively long-lived, the overall rate-determining step must be involved with the dehydrogenation of the second cyclopentylphosphine. Gas chromatographic analysis of the reaction mixture over the same timescale shows the nbd !…”

“…A similar trans to cis isomerisation has been previously computed. [44] Reductive elimination of H 2 from cis-I3' is found to be a two-step process, involving the initial formation of a dihydrogen complex before H 2 dissociation to form 10'. The latter H 2 loss step has the higher-lying transition state (TS(cis-I3'-10'), E = + + 5.2 kcal mol À1 ) and this is indicated in Figure 11.…”

Ruthenium Complexes Carrying Hydride, Dihydrogen, and Phosphine Ligands: Reversible Hydrogen Release

“…The 1 H NMR chemical shift at about d = À5 ppm is assigned to 3, in agreement with a trans arrangement of two hydride ligands, as found in other trans-dihydride ruthenium complexes. [10,11] The equilibrium was studied as a function of the temperature in the range 293-313 K to prevent thermal decomposition. A vant Hoff plot for the conversion of 3 (see the Supporting Information) yielded an endothermic DH value of 6.1 AE 0.4 kJ mol À1 and a DS value of 27 AE 23 J K À1 mol À1 .…”

“…Isomerization between cis and trans dihydrides has been reported in a few cases in ruthenium systems. [10,11] DFT calculations will be performed to gather more information on the isomeric structures and on the exchange process, presumably through successive pseudo rotation steps.…”

Effect of Electronic Factor in Ru‐phosphine‐diamine Complexes on Selective Hydrogenation of CC and CO Bonds

“…To explain the excellent catalytic activity and selectivity, Noyori 4,5 proposed the metalligand bifunctional catalysis mechanism. Work by Morris et al further supported the mechanism [6][7][8][9][10][11] and indicated that the rate-limiting step is the heterolytic splitting of η 2 -H 2 in the catalytic recycling. 8 Bergens et al 12 studied the interaction between H 2 and rutheniumphosphine-diamine complex by means of NMR technology in situ and demonstrated that the coordination of H 2 with ruthenium-phosphine-diamine complex was easy and rapid even if the reaction temperature was decreased to -60 ℃.…”