Mechanistic insight from activation parameters for the reaction between co-enzyme B₁₂ and cyanide: Further evidence that heterolytic Co-C bond cleavage is solvent-assisted

The potential involvement of the solvent in heterolytic Co-C bond cleavage for vitamin B₁₂ derivatives has been probed by measuring for the first time activation parameters for heterolytic Co-C bond cleavage. If a water molecule is part of the transition state, negative entropies and volumes of activation should be observed. Conversely, if Co-C bond cleavage is a purely dissociative process, these parameters will be positive. Activation parameters have been determined for the reactions of co-enzyme B₁₂ (5′-deoxyadenosylcobalamin, AdoCbl) and the corresponding cobinamide, 5′-deoxyadenosylcobinamide (AdoCbi⁺), with cyanide, since previous studies have shown that these reactions proceed at convenient rates, Co-C heterolytic bond cleavage is rate-determining and the reaction proceeds cleanly (that is, negligible homolytic cleavage). In addition, the kinetics of the reaction of AdoCbl with cyanide in aqueous solution were re-investigated at high concentrations of sodium cyanide (up to 3.00 M) at pH 11.0 and I = 5.0 M (NaClO₄) using UV-visible and ¹H NMR spectroscopy. A significant kinetic saturation was obtained at high CN^- concentrations and the limiting rate constant at 25.0°C was found to be (2.21 ± 0.04) × 10^-2 s^-1, with an overall formation constant for the (β-Ado)(α-CN)Cbl^- intermediate, K_CN/K_Co, of 0.259 ± 0.007 M^-1, ΔH^≠, ΔS^≠ and ΔV^≠ under these conditions were found to be 55 ± 1 kJ mol^-1, -98 ± 3 J K^-1 mol^-1 and -5.7 ± 0.3 cm³ mol^-1, respectively. ΔV^≠ for the reaction of AdoCbi⁺ and CN^- at pH 11.0 and 35.0°C was found to be -4.5 ± 0.4 cm³ mol^-1, while ΔV^≠ for the reaction of AdoCbl and AdoCbi⁺ with tetrabutylammonium cyanide in 92% DMF-8% D₂O at 35°C was found to be -8.2 ± 0.3 and -8.8 ± 0.7 cm³ mol^-1, respectively. The activation parameters data obtained in the present study support the earlier suggestion that the rate determining step for the reaction of AdoCbl and AdoCbi⁺ with CN^- which involves heterolytic cleavage of the Co-C bond of the (β-Ado)(α-CN)Cbl^- and (β-Ado)(α-CN)Cbi intermediates, respectively, is a solvent-assisted, β-elimination process, in which the solvent partially protonates the ribosyl oxygen of the intermediate.