Mechanistic studies on reaction of [ReH 4(η 2-H 2)(Cyttp)] + with ketones to give the hydrido-oxo complex [ReH 2(O)(Cyttp)] + (Cyttp = PhP(CH 2CH 2CH 2PCy 2) 2)

Abstract

Mechanistic studies were conducted on reaction of [ReH 4(η 2-H 2)(Cyttp)]OTf ( 1(OTf); Cyttp = PhP(CH 2CH 2CH 2PCy 2) 2, OTf = O 3SCF 3) with ketones, both neat and in solution. Treatment of 1(OTf) with excess acetone at 60–65 °C affords [ReH 2(O)(Cyttp)]OTf ( 2(OTf)) in high yield, nearly 1 equiv. of H 2, 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 H 2O in the reaction system. Use of acetone-d 6 in conjunction with 1(OTf) gives 2(OTf) containing no deuterium, as well as 1 equiv. of each of (CD 3) 2CHOH/OD and (CD 3) 2CDOD/OH. Reactions of 1(OTf) with cyclohexanone, including cyclohexanone-2,2,6,6-d 4, 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 Me 3NO, also are ineffective. A mechanism is proposed which begins with loss of H 2 by 2 to give a 16-electron “[ReH 4(Cyttp)] +” which, depending on the experimental conditions, binds a solvent or ligand molecule. A [ReH 4(R 2C O)(Cyttp)] + intermediate generated in this manner reacts spontaneously by elimination of R 2CHOH (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 R 2CHOH (containing methine deuterium when deuteriated ketone is employed), with the added hydrogens coming from H 2O, which derives from solvent/reactant ketone.