Reactions of a Hexahydride-Osmium Complex with Aldehydes: Double C-H_(alpha) Activation-Decarbonylation and Single C-H_(alpha) Activation-Hydroxylation Tandem Processes and Catalytic Tishchenko Reactions

摘要

The hexahydride complex OsH_6(P~iPr_3)_2 (1) activates the C-H_(alpha) bond of aldehydes.In toluene under reflux,the reactions with benzaldehyde,cyclohexanecarboxaldehyde,and isobutyral-dehyde lead to products resulting from two C-H_(alpha) activation-decarbonylation tandem processes.The reaction with benzaldehyde gives the hydride-phenyl-cis-dicarbonyl derivative OsHPh(CO)_2(P~iPr_3)_2 (2) and benzene,while cyclohexanecarboxaldehyde and isobutyraldehyde yield the cis-dihydride-cis-dicarbonyl compound OsH_2(CO)_2(P~iPr_3)_2 (3) and the corresponding alkane.In the presence of water 1 reacts with benzaldehyde to give 2.However,cyclohexanecarboxaldehyde and isobutyraldehyde afford the carboxylate complexes OsH_3(kappa~2-O_2CR)-(P~iPr_3)_2 (R=Cy (4),(CH_3)_2CH (5)),which can also be obtained by reaction of 1 and the corresponding carboxylic acid.Similarly,the treatment of 1 with benzoic acid gives OsH_3 (kappa~2-O_2CPh)(P~iPr_3)_2 (6).The structure of 5 in the solid state has been determined by X-ray diffraction analysis.The coordination geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphorus atoms of the phosphines occupying axial positions.In solution,the central hydride ligand of 4-6 exchanges its position with that of the hydrides disposed cisoid to the oxygen atoms.The activation parameters for the exchange process are DELTAH~(not=)=10.2 +- 0.4 kcal-mol~(-1) and DELTAS~(not=)=-4.3 +- 1.0 cal.mol~(-1).K~(-1) for 4,DELTAH~(not=)=10.6 +- 0.3 kcal.mol~(-1)_ and DELTAS~(not=)=1.1 +- 0.9 cal.mol~(-1)K~(-1) for 5,and DELTAH~(not =)=10.4 +-0.4 kcal-mol~(-1) and DELTAS~(not=)=-1.3 +- 1.2 cal-mol~(-1)K~(-1) for 6.Complexes 1,3,and 4 are active catalyst precursors for classical Tishchenko dimerization of cyclohexanecarboxaldehyde.Complexes 1 and 3 are also active catalyst precursors for the classical Tishchenko dimerization of benzaldehyde and for the homo aldo-Tishchenko trimerization of isobutyraldehyde to (3-hydroxy-2,2,4-trimethylpentyl)-2-methylpropionate.