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

Hendrikus J. Gamelkoorn, M. W. G. De Bolster, S. Balt

Open publisher page 9 citations

Abstract

Abstract The effect of pressure on the rates of anaerobic thermal homolytic scission of the CoC bond in alkylcobalamins (RB12, 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 cm3 ṁ mol−1 for AdoB12, 17.0 cm3 ṁ mol−1 for MeB12 and 7.9 cm3 ṁ mol−1 for neopentylB12, 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.

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Abstract The effect of pressure on the rates of anaerobic thermal homolytic scission of the CoC bond in alkylcobalamins (RB12, 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 cm3 ṁ mol−1 for AdoB12, 17.0 cm3 ṁ mol−1 for MeB12 and 7.9 cm3 ṁ mol−1 for neopentylB12, 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.

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Available abstract

Abstract The effect of pressure on the rates of anaerobic thermal homolytic scission of the CoC bond in alkylcobalamins (RB12, 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 cm3 ṁ mol−1 for AdoB12, 17.0 cm3 ṁ mol−1 for MeB12 and 7.9 cm3 ṁ mol−1 for neopentylB12, 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.

Key concepts: Homolysis, Heterolysis, Bond cleavage, Chemistry, Ethylene glycol, Cobalt, Photochemistry, Fission

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