Cβ−H Activation of Aldehydes Promoted by an Osmium Complex

Abstract

Evidence for the Cβ−H bond activation of aldehydes is shown. Complex (1) reacts with benzaldehyde and p- and m-substituted benzaldehydes to give (R = H (2), OCH3 (3, 5, 6), CF3 (4, 7)), which result from the C−H activation of one ortho-C−H bond. Reactions of 1 with 3-furaldehyde and 1-cyclohexene-1-carboxaldehyde lead to (8) and (9), respectively. 1H NMR spectroscopic studies indicate that the OsH2 unit of 2−9 forms elongated dihydrogen systems with H−H separations ranging between 1.41 and 1.50 A. Furthermore, the hydrogen atoms undergo thermally activated site exchange processes with ΔG⧧ between 9 and 12 kcal·mol-1 and quantum exchange coupling. Complexes 2, 5, and 8 have been characterized by X-ray diffraction analysis.