N-bonded enolatorhenium(I) complexes having dimethylphenylphosphine ligands as active key intermediates in catalytic Knoevenagel and Michael reactions

Abstract

Enolatorhenium(I) complexes cis-Re(NCCRCO 2R′)(NCCHRCO 2R′)(PMe 2Ph) 4 (R=H, R′=Me ( 2a); R=H, R′=Et ( 2b); R=H, R′= n-Bu ( 2c); R=Me, R′=Et ( 2d)) are prepared by the reaction of ReH(N 2)(PMe 2Ph) 4 ( 1) with alkyl cyanoalkyl carboxylate. X-ray structure analysis of 2b shows that it has an octahedral Re geometry, where mutually cis enolato and ester ligands bind to the rhenium via cyano groups. Reaction of 2b with benzaldehyde gives Re(NCCHCO 2Et)[NC(EtO 2C)CCHPh]–(PMe 2Ph) 4 ( 4), which is also derived from the ligand exchange reaction of 2b with ethyl ( E)-2-cyano-3-phenylpropenoate. These rhenium(I) complexes 1, 2, and 4 catalyze Knoevenagel and Michael reactions under neutral and mild conditions. A possible mechanism for the Knoevenagel reaction has been proposed.