Hydrolytic dealkylation of organocobaloximes related to coenzyme B-12: Ligand influences

The organocobaloxime complexes R-Co(DH)(2)(L) (R = 5'-deoxyadenosyl, Ado; 2 ',5'-dideoxyadenosyl, 2'dAdo; tetrahydrofurfuryl, THFF; L = 4-tert-butylpyr idine, 4-tBuPy; OH2) were prepared and characterized (H-1 NMR, UV-vis spect roscopy). Rates of Co-C bond hydrolysis were determined as a function of te mperature and pH (range 2.7-5.5), and products were identified. Rates of TH FF-Co(DH)(2)(OH2) and Ado-Co(DH)(2)(OH2) dealkylation were fitted to a supe rposition of acid-dependent and -independent paths, k(obs) k(o) + k'[H+]. B oth pathways are hydrolytic, producing [Co-III(DH)(2)(OH2)(2)](+) and produ cts derived from ring-opening protonation of the organic ligand (4-pentenol or adenine and 2,3-dihydroxy-4-pentenal, respectively). For THFF-Co(DH)(2) (OH2), k(o) = 6.5 x 10(-6) s(-1) and k' = 1.2 M-1 s(-1) at 50.4 degrees C; for Ado-Co(DH)(2)(OH2), k(o) = 1.9 x 10(-5) s(-1) and k' = 0.66 M-1 s(-1) a t 97.0 degrees C. In contrast, decomposition of 2'dAdo-Co(DH)(2)(OH2) proce eds predominantly by cleavage of the nucleosidyl glycosidic bond, k' = 0.34 M-1 s(-1) at 60.2 degrees C. Finally, the use of [Co-II(DH)(2)(OH2)(2)] as a radical trap in the kinetic determination of the Co-C band dissociation enthalpy of coenzyme B-12 is assessed and validated.