Role of the NH2 functionality and solvent in terdentate CNN alkoxide ruthenium complexes for the fast transfer hydrogenation of ketones in 2-propanol

The reaction of [RuCl-(CNN)(dppb)) (1; HCNN = 6-(4-methylphenyl)-2-pyridylmethylamine) with NaOiPr in 2-propanol/C6D6 affords the alcohol adduct alkoxide [Ru(OiPr)-(CNN)(dppb)]center dot niPrOH (5), containing the Ru-NH2 linkage. The alkoxide (Ru(OiPr)(CNN)(dppb)] (4) is formed by treatment of the hydride [Ru(H)(CNN)(dppb)] (2) with acetone in C6D6. Complex 5 in 2-propanol/C6D6 equilibrates quickly with hydride 2 and acetone with an exchange rate of (5.4 +/- 0.2) s(-1) at 25 degrees C, higher than that found between 4 and 2 ((2.9 +/- 0.4) s(-1)). This fast process, involving a beta-hydrogen elimination versus ketone insertion into the Ru-H bond, occurs within a hydrogen-bonding network favored by the Ru-NH2 motif. The cationic alcohol complex [Ru(CNN)(dppb)(iPrOH)](BAr4f) (6; Ar-f = 3,5-C6H3-(CF3)(2)), obtained from 1, Na[BAr4f], and 2-propanol, reacts with NaOiPr to afford 5. Complex 5 reacts with either 4,4'-difluorobenzophenone through hydride 2 or with 4,4'-difluorobenzhydrol through protonation, affording the alkoxide [Ru{OCH(4-C6H4F)(2)}(CNN)(dppb)] (7) in 90 and 85 % yield of the isolated product. The chiral CNN-ruthenium compound [RuCl(CNN)-{(S,S)-Skewphos}] (8), obtained by the reaction of [RuCl2(PPh3)(3)] with (S,S)-Skewphos and orthometalation of HCNN in the presence of NEt3, is a highly active catalyst for the enantioselective transfer hydrogenation of methylaryl ketones (turnover frequencies (TOFs) of up to 1.4 x 10(6) h(-1) at reflux were obtained) with up to 89 % ee. Also the ketone CF3CO(4-C6H4F), containing the strong electron-withdrawing CF3 group, is reduced to the R alcohol with 64 % ee and a TOF of 1.5 x 10(4) h(-1). The chiral alkoxide [Ru(OiPr)(CNN)-(S,S)-Skewphos)]center dot niPrOH (9), obtained from 8 and NaOiPr in the presence of 2-propanol, reacts with CF3CO-(4-C6H4F) to afford a mixture of the diastereomer alkoxides [Ru{OCH-(CF3)(4-C6H4F)}(CNN){(S,S)-Skewphos}] (10/11; 74 % yield) with 67 % de. This value is very close to the enantiomeric excess of the alcohol (R)-CF3CH(OH)(4-C6H4F) formed in catalysis, thus suggesting that diastereoisomeric alkoxides with the Ru-NH2 linkage are key species in the catalytic asymmetric transfer hydrogenation reaction.