| Abstract
| - The title hydrido amido binap ruthenium complex containing the new app ligand is crystallographically characterized and tested as a catalyst for the asymmetric hydrogenation of ketones. The product alcohol lowers the transition state for dihydrogen splitting by a proton shuttle mechanism.
- The 2-(aminomethyl)pyridine (ampy) ligand is known to activate ruthenium complexes for the catalytichydrogenation of ketones. Here we prepare well-defined catalysts using the new ligand 2-amino-2-(2-pyridyl)propane (appH) in order to elucidate the role of the pyridyl group. The ligand has two methylgroups on the α-carbon to block β-hydride elimination reactions. It reacts with RuHCl(S-binap)(PPh3) toproduce the orange-yellow complex RuHCl(S-binap)(appH) (2). In the presence of a strong base (KOtBu), complex 2 is converted into an active catalyst for the H2-hydrogenation of acetophenone in benzeneunder mild conditions (20 °C, 5 atm H2). Solutions of 2 rapidly react with KOtBu under an argonatmosphere to produce a deep red amidohydrido complex RuH(S-binap)(app) (3), which is an activecatalyst. A crystal structure determination of 3 represents the first structure of a Ru-binap hydrido-amidocomplex. It reveals a five-coordinate Ru(II) center with a short Ru−N(amido) distance (1.962(3) Å) anda trigonal planar geometry at the amido nitrogen. The kinetic experiments using 3 as a catalyst andacetophenone as a substrate in benzene show that the rate of 1-phenylethanol production is dependent onboth catalyst and H2 concentrations. These results parallel the behavior of the conventional Noyori-typeRu(II) catalysts with diamine ligands. However, unique features of catalysis with 3 are as follows: (1)the formation of a dihydride is thermodynamically unfavorable at 1 atm H2, 20 °C; (2) the rate shows adependence on the product concentration since it increases as the product builds up during the reactionin an autocatalytic fashion. A significant increase in the initial rate is observed when a critical concentrationof rac-1-phenylethanol is present at the beginning of the reaction. The addition of 2-propanol in benzeneraises the rate as well, and the fastest H2-hydrogenation is achieved if 2-propanol is used as a solvent.This “alcohol effect” is favored by the pyridyl ligand app since it was not observed for the similar catalystRuH(NHCMe2CMe2NH2)(binap). While 3 is an exceptional catalyst for H2-hydrogenation in 2-propanol(TOF > 6700 h-1 at 20 °C, 5 atm H2), it has a lower activity in transfer hydrogenation from the samesolvent under comparable conditions (TOF 110 h-1 at 20 °C, 1 atm Ar). DFT calculations on the modelamido complex Ru(H)(PH3)2(HNCH2C5H4N) (4) confirm that the splitting of H2 to give the trans dihydrideis the turnover-limiting step and lies 9 kcal/mol in free energy above the transition state for the ketonehydrogenation step. The formation of the dihydride is entropically unfavorable. The theoretical activationbarrier for H2 splitting is lowered by 5 kcal/mol by an alcohol-assisted mechanism but still remainshigher in energy than the ketone hydrogenation step. This latter step can also be alcohol-assisted and canresult in a different ee in the product alcohol than without alcohol assistance, as observed experimentallyfor reactions using 2-propanol versus benzene as the solvent. With alcohol present, an alkoxohydridoruthenium(II) complex is calculated to be the catalyst resting state.
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