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 OsH 6 (P i Pr 3 ) 2 ( 1 ) activates the C−H α bond of aldehydes. In toluene under reflux, the reactions with benzaldehyde, cyclohexanecarboxaldehyde, and isobutyraldehyde lead to products resulting from two C−H α activation−decarbonylation tandem processes. The reaction with benzaldehyde gives the hydride-phenyl- cis -dicarbonyl derivative OsHPh(CO) 2 (P i Pr 3 ) 2 ( 2 ) and benzene, while cyclohexanecarboxaldehyde and isobutyraldehyde yield the cis -dihydride- cis -dicarbonyl compound OsH 2 (CO) 2 (P i Pr 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 (κ 2 -O 2 CR)(P i Pr 3 ) 2 (R = Cy ( 4 ), (CH 3 ) 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 OsH 3 (κ 2 -O 2 CPh)(P i Pr 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 Δ H ⧧ = 10.2 ± 0.4 kcal·mol - 1 and Δ S ⧧ = −4.3 ± 1.0 cal·mol - 1 ·K - 1 for 4, Δ H ⧧ = 10.6 ± 0.3 kcal·mol - 1 and Δ S ⧧ = 1.1 ± 0.9 cal·mol - 1 ·K - 1 for 5, and Δ H ⧧ = 10.4 ± 0.4 kcal·mol - 1 and Δ S ⧧ = −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.