PHOTOOXYGENOLYSIS OF VITAMIN B12 and RELATED CORRINS: COBALT(III)CORRINS AS SUBSTRATES and QUENCHERS FOR SINGLET OXYGEN (1Δg)*

Under photooxygenolytic conditions, vitamin B₁₂ can be degraded to two isomeric dioxosec-ocorrins by a regioselective oxygenolytic cleavage of the corrin macrocycle (preliminary report in Krautler and Stepanek (1985), Angew. Chem. 97 ,71–73; Angew. Chem. Intl. Ed. Engl. 24 ,62–64): irradiation of an oxygen saturated solution of vitamin B₁₂ ( 1 ), of KCN and of methylene blue (molar ratio (1:1:0.005) in CD₃OD at ca 200 K with visible light led to a selective oxygenolysis of the vitamin. The two products, potassium Co_αCo_β-dicyano-5,6-dioxo-5,6-seco-5′6′-dimethylbenzimidazolyl-cobam-idate ( 3 , 10% yield) and potassium Co_αCo_β-dicyano-14,15-dioxo-14,15-seco-5′6′-dimetliylbenzirnida-zolyl-cobamidate (4, 25% yield), and 1 (31% yield) were chromatographically separated and isolated. The structures were established spectroanalytically, and by the help of an acid catalyzed methanolysis of 3 and 4 to the related Co_αCo_β -dicyano-heptamethyl- dioxosecocobyrinates (6 and 7). When irradiated with visible light in any oxygen saturated methanolic solution containing methylene blue, vitamin B₁₂ itself exhibited considerable inertness. Only upon addition of a stoichiometric amount of KCN to such a solution (to afford the adduct 2) did the photolysis lead to oxygenation of the cobaltcorrin within a useful time. The regioselectivity of the oxygenolysis of 2 and the ratio of products formed are comparable to the outcome of the photooxygenolysis of the structurally related heptamethylcobyrinate 5 (Kräutler (1982) Helv. Chim. Acta 65 ,1941–1948). The reaction at the methyl-substituted meso-positions of the corrin macrocycle is indicated to involve singlet oxygen (¹Δ_g). In support of this interpretation, the preparative photolysis of 2 at 200 K also turned out to be about 7 times slower when CH₃OH was used instead of CD₃OD.