Pressure effects as mechanistic probes for homolytic thermal cobaltcarbon bond cleavage of cobalamins

Abstract The effect of pressure on the rates of anaerobic thermal homolytic scission of the CoC bond in alkylcobalamins (RB 12 , R = adenosyl, methyl and neopentyl) has been determined in different solvents of varying viscosities. Homolytic fission is compared to bond breaking using βH elimination (isopropylcobalamin in ethylene‐glycol + water mixtures) and heterolysis. These two mechanisms are characterized by a volume of activation that is close to zero, whereas homolysis shows a substantial positive value (18.5 cm 3 ṁ mol −1 for AdoB 12 , 17.0 cm 3 ṁ mol −1 for MeB 12 and 7.9 cm 3 ṁ mol −1 for neopentylB 12 , all in ethylene glycol) for this parameter. Consequently, pressure effects can be used, with caution, as mechanistic probes to identify CoC bond fission mechanisms for the reactions studied.