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

摘要

The hexahydride complex OsH6(PiPr3)2 (1) activates the C−Hα bond of aldehydes. In toluene under reflux, the reactions with benzaldehyde, cyclohexanecarboxaldehyde, and isobutyraldehyde lead to products resulting from two C−Hα activation−decarbonylation tandem processes. The reaction with benzaldehyde gives the hydride-phenyl-cis-dicarbonyl derivative OsHPh(CO)2(PiPr3)2 (2) and benzene, while cyclohexanecarboxaldehyde and isobutyraldehyde yield the cis-dihydride-cis-dicarbonyl compound OsH2(CO)2(PiPr3)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 OsH3(κ2-O2CR)(PiPr3)2 (R = Cy (4), (CH3)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 OsH3 (κ2-O2CPh)(PiPr3)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 ΔH = 10.2 ± 0.4 kcal·mol-1 and ΔS = −4.3 ± 1.0 cal·mol-1·K-1 for 4, ΔH = 10.6 ± 0.3 kcal·mol-1 and ΔS = 1.1 ± 0.9 cal·mol-1·K-1 for 5, and ΔH = 10.4 ± 0.4 kcal·mol-1 and ΔS = −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.