Detailed kinetic and thermodynamic studies on the cyanation of alkylcobalamins. A generalized mechanistic description

Kinetics and Mechanism of Axial Ligand Substitution of Alkyl Cobaloximes by Substituted Pyridines in Different Solvents

“…[14] The substitution of H 2 O by CN Ϫ for various cobinamides (RCbi) increases in the order (R ϭ) . [31] The pK BHϩ values for 4-NH 2 Py, Py, and 4-CNPy are 9.17, [32] 5.23, [32] and 1.7 [33] (estimated), respectively.…”

“…Some of us recently studied the reaction of cyanide with various alkylcobalamins (RCbl, where R ϭ CN, CNCH 2 , CF 3 , CF 3 CH 2 , CF 2 H, CH 3 , BrCH 2 ) and found that the nature of the alkyl group has an important influence on the thermodynamic equilibrium constants, and the kinetics and mechanism of the substitution reactions of the axial ligand trans to the alkyl group. [13,14] The nature of the axial alkyl group in cobaloximes [2] and in related [RCo{(DO)(DOH)pn}H 2 O] ϩ complexes [15] has an effect on the substitution of the ligand in the trans position. This substitution is very rapid and involves a dissociative activation mode, similar to the substitution of labile tion parameters for the substitution of H 2 O by Py and 4-CNPy in the case where R = CF 3 CH 2 were also found.…”

Coenzyme B12 Model Studies − Kinetics of Axial Ligation of Alkylcobalt Complexes Containing a Tridentate Amino‐Oximate Ligand

“…[18] The kinetic trans effect was found to decrease in the order nPr Ն adenosyl Ն Me Ն CF 3 CH 2 Ͼ CF 2 H Ͼ NCCH 2 Ͼ CF 3 Ͼ CN Ϫ . The relatively slow reaction rate of complex 4 is consistent with the short CoϪN (pyridyl) bond [11] [2.118(3) Å ], and the ∆V ‡ value suggests an CoϪN (pyridyl) bond extension in the transition state similar to that of the Me derivative.…”

“…[16,17] Ligand-substitution reactions of different cobalamins (R ϭ CF 3 , CN Ϫ , NCCH 2 ) with cyanide have been studied, [10,18] and it was found that CN Ϫ substitutes the 5,6-dimethylbenzimidazole (DMBz) moiety in all cases. The activation parameters were interpreted in terms of a limiting D mechanism.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…Of particular interest more recently is the reaction of Co (III) compounds known as cobalamins. 250 Cobalamins can act as a template for reactions of, or models for vitamin B 12 , and typically undergo substitution reactions that are several orders of magnitude faster than those of the Co(III) pentaammine complexes, a labilization brought about by the surrounding 15-member corrin ring and the chelated dimethylimidazole ligand. A huge selection of substitution reactions of various cobalamins, cyanocobalamin (vitamin B 12 ), aquacobalamin (vitamin B 12a ), and 5-deoxyadenosylcobalamin (coenzyme B 12 ) has been thoroughly and comprehensively investigated, as well as have reactions of a complex serving as a model for coenzyme B 12 .…”