Time-resolved infrared spectroscopic study of reactive acyl intermediates relevant to cobalt-catalyzed carbonylations

Time-resolved infrared spectroscopic studies have been used to characterize the reactive intermediate CH3C(O)Co(CO)2PPh3 (ICo), which is relevant to the mechanism of the catalysis of alkene hydroformylation by the phosphine-modified cobalt carbonyls. Step-scan FTIR and (variable) single-frequency time-resolved infrared detection on the microsecond time scale were used to record the spectrum of ICo and to demonstrate that the principal photoproduct of the subsequent reaction of this species at PCO = 1 atm is the methyl cobalt complex CH3Co(CO)3PPh3 (MCo). At higher PCO the trapping of ICo with CO to re-form CH3C(O)Co(CO)3PPh3 (ACo) (rate = kCO[CO][ICo]) was shown to become competitive with the rate of acetyl-to-cobalt methyl migration to give MCo (rate = kM[ICo]). Activation parameters for the competing pathways in benzene were determined to be delta H++CO = 57 +/- 04 kJ mol-1, delta S++CO = -91 +/- 12 J mol-1 K-1 and delta H++M = 40 +/- 2 kJ mol-1, delta S++M = -19 +/- 5 J mol-1 K-1. The effects of varying the solvent on the competitive reactions of ICo were also explored, and the mechanistic implications of these results are discussed.