Mechanistic studies on reaction of [ReH4(η2-H2)(Cyttp)]+ with ketones to give the hydrido-oxo complex [ReH2(O)(Cyttp)]+ (Cyttp = PhP(CH2CH2CH2PCy2)2)

Mechanistic studies were conducted on reaction of [ReH4(η2-H2)(Cyttp)]OTf (1(OTf); Cyttp = PhP(CH2CH2CH2PCy2)2, OTf = O3SCF3) with ketones, both neat and in solution. Treatment of 1(OTf) with excess acetone at 60–65 °C affords [ReH2(O)(Cyttp)]OTf (2(OTf)) in high yield, nearly 1 equiv. of H2, 2 equiv. of 2-propanol, 1 equiv. of each of 4-hydroxy-4-methyl-2-pentanone (B) and 4-methylpent-3-en-2-one (C), and smaller amounts of other organic products derived by condensation or related reactions of acetone. The presence of C, apparently arising by dehydration of B, points to the formation of 1 equiv. of H2O in the reaction system. Use of acetone-d6 in conjunction with 1(OTf) gives 2(OTf) containing no deuterium, as well as 1 equiv. of each of (CD3)2CHOH/OD and (CD3)2CDOD/OH. Reactions of 1(OTf) with cyclohexanone, including cyclohexanone-2,2,6,6-d4, under comparable conditions, give analogous results. The ketones cyclopentanone, 2-butanone, and 3-pentanone also convert 1(OTf) to 2(OTf) upon heating, as does isobutyraldehyde, but only in the presence of the stabilizer BHT. In contrast, the more robust ketones 2,4-dimethyl-3-pentanone, 2,6-dimethylcyclohexanone, and 2-adamantanone, which do not undergo condensation, failed to effect this transformation. Other organooxygen compounds, i.e., methanol, cyclohexanol, 1,2-butene oxide, cyclohexene oxide, DMSO, and Me3NO, also are ineffective. A mechanism is proposed which begins with loss of H2 by 2 to give a 16-electron “[ReH4(Cyttp)]+” which, depending on the experimental conditions, binds a solvent or ligand molecule. A [ReH4(R2Cdouble bond; length as m-dashO)(Cyttp)]+ intermediate generated in this manner reacts spontaneously by elimination of R2CHOH (containing methine hydrogen even when deuteriated ketone is used), which results from transfer of two hydride ligands to coordinated ketone. Continued reaction leads to the formation of 2 and another molecule of R2CHOH (containing methine deuterium when deuteriated ketone is employed), with the added hydrogens coming from H2O, which derives from solvent/reactant ketone.