Reactions of a hexahydride-osmium complex with aldehydes:: Double C-Hα activation-decarbonylation and single C-Hα activation-hydroxylation tandem processes and catalytic Tishchenko reactions

The hexahydride complex OsH6((PPr3)-Pr-i)(2) (1) activates the C-H.. bond of aldehydes. In toluene under reflux, the reactions with benzaldehyde, cyclohexanecarboxaldehyde, and isobutyraldehyde 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)((PPr3)-Pr-i)(2) (2) and benzene, while cyclohexanecarboxaldehyde and isobutyraldehyde yield the cis-dihydride-cis-dicarbonyl compound OsH2(CO)(2)((PPr3)-Pr-i)(2) (3) and the corresponding alkane. In the presence of water I reacts with benzaldehyde to give 2. However, cyclohexanecarboxaldehyde and isobutyraldehyde afford the carboxylate complexes OsH3(kappa(2)-O2CR)((PPr3)-Pr-i)(2) (R = Cy (4), (CH3)(2)CH (5)), which can also be obtained by reaction of 1 and the corresponding carboxylic acid. Similarly, the treatment of 1 with benzoic acid gives OsH3(kappa(2)-O2CPh)((PPr3)-Pr-i)(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(double dagger) = 10.2 +/- 0.4 kcal(.)mol(-1) and DeltaS(double dagger) = -4.3 +/- 1.0 cal(.)mol(-1.)K(-1) for 4, DeltaH(double dagger) = 10.6 +/- 0.3 kcal(.)mol(-1) and DeltaS(double dagger) = 1.1 +/- 0.9 cal(.)mol(-1.)K(-1) for 5, and DeltaH(double dagger) = 10.4 +/- 0.4 kcal(.)mol(-1) and DeltaS(double dagger) = -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.