Hydrolytic Dealkylation of Organocobaloximes Related to Coenzyme B₁₂:  Ligand Influences

The organocobaloxime complexes R−Co(DH) 2 (L) (R = 5‘-deoxyadenosyl, Ado; 2‘,5‘-dideoxyadenosyl, 2‘dAdo; tetrahydrofurfuryl, THFF; L = 4- tert -butylpyridine, 4- t BuPy; OH 2 ) were prepared and characterized ( 1 H NMR, UV−vis spectroscopy). Rates of Co−C bond hydrolysis were determined as a function of temperature and pH (range 2.7−5.5), and products were identified. Rates of THFF−Co(DH) 2 (OH 2 ) and Ado−Co(DH) 2 (OH 2 ) dealkylation were fitted to a superposition of acid-dependent and -independent paths, k obs = k 0 + k ‘[H + ]. Both pathways are hydrolytic, producing [Co III (DH) 2 (OH 2 ) 2 ] + and products 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 (OH 2 ), k 0 = 6.5 × 10 - 6 s - 1 and k ‘ = 1.2 M - 1 s - 1 at 50.4 °C; for Ado−Co(DH) 2 (OH 2 ), k 0 = 1.9 × 10 - 5 s - 1 and k ‘ = 0.66 M - 1 s - 1 at 97.0 °C. In contrast, decomposition of 2‘dAdo−Co(DH) 2 (OH 2 ) proceeds predominantly by cleavage of the nucleosidyl glycosidic bond, k ‘ = 0.34 M - 1 s - 1 at 60.2 °C. Finally, the use of [Co II (DH) 2 (OH 2 ) 2 ] as a radical trap in the kinetic determination of the Co−C bond dissociation enthalpy of coenzyme B 12 is assessed and validated.