The Photolysis of Methylcobalamin^*

Abstract

The major secondary reactions involving the methyl radical appear to be reaction with oxygen to yield a methyl peroxide radical or formaldehyde and a hydroxyl radical.

Stoichiometric reduction of primary alkyl monohalides with electrogenerated nickel(I) salen: Formation of aldehydes

“…Because of the likelihood that it is atmospheric O 2 which reacts with post-electrolysis species to produce 1-octanal, other experiments were performed by passage of extra-dry-grade O 2 into the cell for 15 min after an injection of 1-bromooctane, after which product analysis was performed, but without any exposure of the solution to air. As shown in entry 2 of Table 1, our results support the assertion that O 2 (introduced as either air or pure oxygen) causes the formation of 1-octanal, a finding in parallel with the ability of O 2 to produce an aldehyde via photodecomposition of alkylcobalt(III) species [27][28][29]. Moreover, additional experiments involving GC analysis of reaction mixtures exposed to air after being saturated with pure O 2 revealed that the product distribution does not change further.…”

“…Recently, however, we reported [26] that reaction of a primary alkyl monohalide with pre-electrogenerated nickel(I) salen in dimethylformamide (DMF) containing deliberately added water, followed by exposure of the reaction mixture to either air or oxygen (O 2 ), generates an aldehyde in substantial yield (along with other classic products involving alkyl radicals); the aldehyde that is obtained has the same number of carbon atoms as the starting material. It is interesting to compare our findings with earlier results for cobaltbased systems, where aldehydes can be produced via the photodecomposition of organocobalt(III) species in the presence of O 2 , as shown in Scheme 1 [27][28][29].…”

“…To compare further our work with the process by which an aldehyde is produced via photolytic decomposition of alkylcobalt(III) complexes in the presence of O 2 [28,29], we carried out some experiments in a totally dark cell (Table 1, entry 3). Upon excluding light at every step of these experiments, we still obtained 1-octanal in a yield of 13%, along with the other products.…”

“…In 1966, Hogenkamp reported that methylcobalamin underwent photolysis in the presence of excess oxygen to afford aquocobalamin and formaldehyde (in 76% yield) [28]. Unlike the formation of aldehyde via photodecomposition of methylcobalt(III) species with O 2 (Scheme 1), our research has shown that an aldehyde can arise from the reaction between stoichiometric amounts of electrogenerated nickel(I) salen and a 1-bromo-or 1-iodoalkane in the absence of light, even though irradiation of the reaction mixture with ultraviolet-visible light improves the yield of aldehyde.…”

“…In the reduction of an alkyl halide by nickel(I) salen, the pathway for production of an aldehyde was first thought to mimic the photodecomposition of an organocobalt(III) complex in the presence of O 2 [28,29]. However, recent results from CV-CPE as well as HPLC experiments suggest that, instead of an alkylnickel(III) salen intermediate, a different nickel-containing species is involved in the formation of aldehyde.…”