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A Mechanism Displaying Autocatalysis:  The Hydrogenation of Acetophenone Catalyzed by RuH(S-binap)(app) Where app Is the Amido Ligand Derived from 2-Amino-2-(2-pyridyl)propane

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posted on 03.04.2020, 16:54 authored by Alen Hadzovic, Datong Song, Christina M. MacLaughlin, Robert H. Morris
The 2-(aminomethyl)pyridine (ampy) ligand is known to activate ruthenium complexes for the catalytic hydrogenation 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 methyl groups on the α-carbon to block β-hydride elimination reactions. It reacts with RuHCl(S-binap)(PPh3) to produce 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 benzene under mild conditions (20 °C, 5 atm H2). Solutions of 2 rapidly react with KOtBu under an argon atmosphere to produce a deep red amidohydrido complex RuH(S-binap)(app) (3), which is an active catalyst. A crystal structure determination of 3 represents the first structure of a Ru-binap hydrido-amido complex. It reveals a five-coordinate Ru(II) center with a short Ru−N(amido) distance (1.962(3) Å) and a trigonal planar geometry at the amido nitrogen. The kinetic experiments using 3 as a catalyst and acetophenone as a substrate in benzene show that the rate of 1-phenylethanol production is dependent on both catalyst and H2 concentrations. These results parallel the behavior of the conventional Noyori-type Ru(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 a dependence on the product concentration since it increases as the product builds up during the reaction in an autocatalytic fashion. A significant increase in the initial rate is observed when a critical concentration of rac-1-phenylethanol is present at the beginning of the reaction. The addition of 2-propanol in benzene raises 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 catalyst RuH(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 same solvent under comparable conditions (TOF 110 h-1 at 20 °C, 1 atm Ar). DFT calculations on the model amido complex Ru(H)(PH3)2(HNCH2C5H4N) (4) confirm that the splitting of H2 to give the trans dihydride is the turnover-limiting step and lies 9 kcal/mol in free energy above the transition state for the ketone hydrogenation step. The formation of the dihydride is entropically unfavorable. The theoretical activation barrier for H2 splitting is lowered by 5 kcal/mol by an alcohol-assisted mechanism but still remains higher in energy than the ketone hydrogenation step. This latter step can also be alcohol-assisted and can result in a different ee in the product alcohol than without alcohol assistance, as observed experimentally for reactions using 2-propanol versus benzene as the solvent. With alcohol present, an alkoxohydridoruthenium(II) complex is calculated to be the catalyst resting state.