Cβ−H Activation of Aldehydes Promoted by an Osmium Complex

Evidence for the C β −H bond activation of aldehydes is shown. Complex ( 1 ) reacts with benzaldehyde and p - and m -substituted benzaldehydes to give (R = H ( 2 ), OCH 3 ( 3, 5, 6 ), CF 3 ( 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. 1 H NMR spectroscopic studies indicate that the OsH 2 unit of 2 − 9 forms elongated dihydrogen systems with H−H separations ranging between 1.41 and 1.50 Å. Furthermore, the hydrogen atoms undergo thermally activated site exchange processes with Δ G ⧧ between 9 and 12 kcal·mol - 1 and quantum exchange coupling. Complexes 2, 5, and 8 have been characterized by X-ray diffraction analysis.