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

Walter Baratta, Maurizio Ballico, Gennaro Esposito, Pierluigi Rigo

Research output: Contribution to journalArticlepeer-review


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)]·n iPrOH (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°C, higher than that found between 4 and 2 ((2.9±0.4)s-1). This fast process, involving a β-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)](BArf4) (6; Arf = 3,5-C6H3-(CF3)2), obtained from 1, Na[BArf4], 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 × 106h -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 × 104 h-1. The chiral alkoxide [Ru(OiPr)(CNN)-((S,S)-Skewphos)]·n iPrOH (9), obtained from 8 and NaO(Pr in the presence of 2-propanol, reacts with CF3CO-(4-C 6H4F) to afford a mixture of the diastereomer alkoxides [Ru(OCH-(CF3)(4-C6H4F)}(CNN){(S,S)-Skewphosj] (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.

Original languageEnglish (US)
Pages (from-to)5588-5595
Number of pages8
JournalChemistry - A European Journal
Issue number18
StatePublished - Jun 20 2008


  • Alkoxides
  • Asymmetric catalysis
  • Hydrogen transfer
  • Ruthenium
  • Solvent effects

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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