REDUCTION OF ACYL LIGAND IN TRANSITION-METAL COMPLEXES BY CATALYTIC HYDROSILYLATION

Hydrosilylation of acyl-transition-metal complexes M-C(O)R (1) with H2SiPh2 in the presence of a catalytic amount of RhCl(PPh3)3 leads to the formation of the corresponding alpha-siloxyalkyl complexes M-CH(OSiHPh2)R (2). In case 1 contains a ligand or a substituent which stabilizes a transient cationic alkylidene intermediate M+ = CHR (5), overreduction takes place to afford M-CH2R (3) via 5. The mechanism of the overreduction has been studied by using alpha-methoxyalkyl complexes M-CH(OMe)R in place of 2. In addition, the chain propagation step in the Pichler-Schulz mechanism is mimiced by combination of the overreduction and carbonylation. Thus, the acetyl ligand in CpFe(CO)(PPh3)[C(O)CH3] (1f) is elongated to the pentanoyl ligand by repeating the homologation three times. Transformation of 1 to an alpha-alkoxyalkyl complex and an alpha-hydroxycarboxylic acid ester is also described.