Carbonyl insertion reactions in iridium(III) complexes, : II. Mechanism and stereochemistry

Abstract

A study of the stereochemistry of the ‘carbonyl insertion’ reactions: [IrCl 2Et(CO) 2L]+L′[IrCl 2(COEt)(CO)LL′] shows that the rate-determining step involves the combination of ethyl and carbonyl ligands to give a five-coordinate intermediate, probably of square-pyramidal geometry, with the propionyl group in the apical position. The combination is a symmetrical process for which the two terms ‘carbonyl insertion’ and ‘alkyl migration’ are equally misleading. Subsequent attack by the incoming ligand L′ occurs at the vacant octahedral coordination site trans to the propionyl group. That the stereochemistry of the products is a direct result of the mechanism of the reactions is shown by the subsequent rearrangement of the products to a more thermodynamically stable isomer.