REACTIVE INTERMEDIATES IN THE CARBONYLATION OF METAL ALKYL BONDS - TIME-RESOLVED INFRARED SPECTRAL TECHNIQUES

In this chapter, we describe flash photolysis experiments using time-resolved infrared (TRIR) detection techniques to probe the reactivities of key intermediates proposed for the mechanisms of the thermal ''migratory insertion'' of CO into a metal-alkyl bond. The intermediates studied were generated by the photodissociation of CO from the metal complexes CpFe(CO)L(COCH3) (Cp is eta5-C5H5, L is CO or phosphine) and Mn(CO)5(COCH3). Kinetic and spectroscopic evidence points to the formation of a solvated species in each case, which undergoes migration of the acyl methyl group to the metal center, competitive with trapping by addition of a ligand. In cyclohexane, the CpFe intermediate CpFe(CO)(sol)(COCH3) (sol is solvent) undergoes methyl migration at a rate (k1 = 5.6 x 10(4) M-1 s-1) several orders of magnitude faster than does the remarkably slow manganese analog (k1 = 60 M-1 s-1). Comparisons are also made to the reactivity of the unsaturated intermediate Mn(CO)4(sol)(CH3) formed by CO photodissociation from Mn(CO)5(CH3).