Mechanism of the Hydrogenation of Ketones Catalyzed by trans-Dihydrido(diamine)ruthenium(II) Complexes

Abstract

The complexes trans-RuH(Cl)(tmen)(R-binap) (1) and (OC-6-43)-RuH(Cl)(tmen)(PPh(3))(2) (2) are prepared by the reaction of the diamine NH(2)CMe(2)CMe(2)NH(2) (tmen) with RuH(Cl)(PPh(3))(R-binap) and RuH(Cl)(PPh(3))(3), respectively. Reaction of KHB(sec)Bu(3) with 1 yields trans-Ru(H)(2)(R-binap)(tmen) (5) while reaction of KHB(sec)Bu(3) or KO(t)Bu with 2 under Ar yields the new hydridoamido complex RuH(PPh(3))(2)(NH(2)CMe(2)CMe(2)NH) (4). Complex 4 has a distorted trigonal bipyramidal geometry with the amido nitrogen in the equatorial plane. Loss of H(2) from 5 results in the related complex RuH(R-binap)(NH(2)CMe(2)CMe(2)NH) (3). Reaction of H(2) with 4 yields the trans-dihydride (OC-6-22)-Ru(H)(2)(PPh(3))(2)(tmen)(6). Calculations support the assignment of the structures. The hydrogenation of acetophenone is catalyzed by 5 or 4 in benzene or 2-propanol without the need for added base. For 5 in benzene at 293 K over the ranges of concentrations [5] = 10(-)(4) to 10(-)(3) M, [ketone] = 0.1 to 0.5 M, and of pressures of H(2) = 8 to 23 atm, the rate law is rate = k[5][H(2)] with k = 3.3 M(-1) s(1), DeltaH++ = 8.5 +/- 0.5 kcal mol(-1), DeltaS++ = -28 +/- 2 cal mol(-1) K(-1). For 4 in benzene at 293 K over the ranges of concentrations [4] = 10(-4) to 10(-3) M, [ketone] 0.1 to 0.7 M, and of pressures of H(2) = 1 to 6 atm, the preliminary rate law is rate = k[4][H(2)] with k = 1.1 x 10(2) M(-1) s(-1), DeltaH++ = 7.6 +/- 0.3 kcal mol(-1), DeltaS++ = -23 +/- 1 cal mol(-1) K(-1). Both theory and experiment suggest that the intramolecular heterolytic splitting of dihydrogen across the polar Ru=N bond of the amido complexes 3 and 4 is the turn-over limiting step. A transition state structure and reaction energy profile is calculated. The transfer of H(delta+)/H(delta-) to the ketone from the RuH and NH groups of 5 in a Noyori metal-ligand bifunctional mechanism is a fast process and it sets the chirality as (R)-1-phenylethanol (62-68% ee) in the hydrogenation of acetophenone. The rate of hydrogenation of acetophenone catalyzed by 5 is slower and the ee of the product is low (14% S) when 2-propanol is used as the solvent, but both the rate and ee (up to 55% R) increase when excess KO(t)Bu is added. The formation of ruthenium alkoxide complexes in 2-propanol might explain these observations. Alkoxide complexes [RuP(2)]H(OR)(tmen), [RuP(2)] = Ru(R-binap) or Ru(PPh(3))(2), R= (i) Pr, CHPhMe, (t)Bu, are observed by reacting the alcohols (i)PrOH, phenylethanol, and (t)BuOH with the dihydrides 5 and 6, respectively, under Ar. In the absence of H(2), the amido complexes 3 and 4 react with acetophenone to give the ketone adducts [RuP(2)]H(O=CPhMe)(NH(2)CMe(2)CMe(2)NH) in equilibrium with the enolate complexes trans- [RuP(2)](H)(OCPh=CH(2))(tmen) and eventually the decomposition products [RuP(2)]H(eta(5)-CH(2)CPhCHCPhO), with the binap complex characterized crystallographically. In general, proton transfer from the weakly acidic molecules dihydrogen, alcohol, or acetophenone to the amido nitrogen of complexes 3 and 4 is favored in two ...

Computational Studies Explain the Importance of Two Different Substituents on the Chelating Bis(amido) Ligand for Transfer Hydrogenation by Bifunctional Cp*Rh(III) Catalysts

“…Scheme 4 shows that the two C-O bond distances in A3 Bs/H are consistent with a description as a rhodium  1 -formate hydrogen-bonded to NH 2 . Similar coordination was found in related ruthenium complexes 11,33 and in the product of hydrogenation of CO 2 by an Ir pincer. 42 In the present study, the energy of A3 Bs/H is 8.3 kcal/mol above the A2 Bs/H + CO 2, which indicates that the dehydrogenation reaction with associated release of carbon dioxide has a favorable free energy under standard conditions.…”

“…26,27 Morris et al reported the reaction of formic acid with a 16e Ru II amido-amine complex producing a crystallographically characterized formate complex in which a proton is transferred to NH and a hydrogen bond is formed between the NH 2 group 8 and the carbonyl oxygen. 11 Koike and Ikariya reported a related reaction of formic acid with the 16e complex, [(p-cymene)Ru(TsNCHPhCHPhNH)]. 33 The kinetics of conversion of the resulting formate complex to a hydride complex and CO 2 were determined.…”

“…2, 3 Considerable effort has been devoted to understand these mechanisms and experimental and computational studies have concurred to show the prevalence of the outer sphere mechanism. 2, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] In the latter case, the proton is transferred from the hydrogen donor to a ligand that has an electron rich atom such as nitrogen or oxygen and the hydride is transferred to the metal. The catalyst is considered as bifunctional since it involves the metal and the ligand in the transformation between its unsaturated 16 electron (16e) and saturated 18e metal hydride forms.…”

Net Hydrogenation of Pt−NHPh Bond Is Catalyzed by Elemental Pt

“…We initially presumed this straightforward pathway, which is consistent with the general notion of reactions with related systems. 5,7,8,13,19,20 Monitoring ( 1 H NMR) the reaction of 2 with H 2 and a plot of [2] versus time reveals an induction period ( Figure 2A). The observation of an induction period led us to consider Pt(s) as a heterogeneous catalyst.…”

Ligand Design for Catalytic Asymmetric Reduction

“…86,[88][89][90] As shown in Figure 1.18, trans-RuH(Z 1 -BH 4 )(tolbinap)(dpen) (18A) (TolBINAP; see Figure 1.2), a precatalyst, is converted to 16-electron cationic species 18B in 2-propanol. 88,91,92 It accepts an H 2 molecule to form 18C, which undergoes deprotonation with an alcoholic solvent giving the Ru dihydride 18D.…”