Kinetics and Mechanism of Alkyl Transfer from Organocobalt(III) to Nickel(I): Implications for the Synthesis of Acetyl Coenzyme A by CO Dehydrogenase

Reaction of CH 3 Co(dmgBF 2 ) 2 L (dmgBF 2 = (difluoroboryl)dimethylglyoximato); L = py, PEt 3 ) with 2 equiv of [Ni(tmc)]OTf (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf - = CF 3 SO 3 - ) gave Co(dmgBF 2 ) 2 py -, [Ni(tmc)]OTf 2, and [Ni(tmc)CH 3 ]OTf in 80% yield. The overall transformation provides the first model for the transfer of a CH 3 group from methylcobalamin to the Ni-containing enzyme carbon monoxide dehydrogenase during acetyl coenzyme A synthesis. RRSS −[Ni(tmc)CH 3 ](BAr‘ 4 ) (BAr‘ 4 - = B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 - ) has been characterized by X-ray diffraction. The products and 1CH 3 Co(dmgBF 2 ) 2 L:2[Ni(tmc)]OTf stoichiometry of the reaction are consistent with a three-step mechanism initiated by electron transfer from [Ni(tmc)]OTf to CH 3 Co(dmgBF 2 ) 2 L. The second step is rapid CH 3 −Co - bond homolysis yielding Co(dmgBF 2 ) 2 L - and CH 3 •; then the CH 3 radical is captured by the second equivalent of [Ni(tmc)]OTf, yielding [Ni(tmc)CH 3 ]OTf. Radical clock experiments have corroborated the production of free radicals. Reaction of (5-hexenyl)Co(dmgBF 2 ) 2 L with [Ni(tmc)]OTf, followed by hydrolysis of the organonickel products, gave methylcyclopentane, consistent with the formation and cyclization of the 1-hexenyl radical. The second-order rate constants measured by stopped-flow experiments parallel the relative radical stabilities: R = CH 3 (2.43 × 10 3 M - 1 s - 1 ) < C 2 H 5 (7.88 × 10 3 M - 1 s - 1 ) < CH(CH 3 ) 2 (19.2 × 10 3 M - 1 s - 1 ), L = py (2.43 × 10 3 M - 1 s - 1 ) < PEt 3 (5.01 × 10 3 M - 1 s - 1 ). During the course of these studies the following molecules were also characterized by X-ray diffraction, CH 3 Co(dmgBF 2 ) 2 L, L = PEt 3, py, H 2 O.