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PHOTOOXYGENOLYSIS OF VITAMIN B 12 and RELATED CORRINS: COBALT(III)CORRINS AS SUBSTRATES and QUENCHERS FOR SINGLET OXYGEN ( 1Δ g )*
Author(s) -
Kraeutler Bernhard,
Stepanek Rene
Publication year - 1991
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1991.tb02060.x
Subject(s) - chemistry , singlet oxygen , methylene blue , yield (engineering) , methylene , regioselectivity , adduct , oxygen , stoichiometry , medicinal chemistry , cobalt , photochemistry , photodissociation , vitamin , catalysis , nuclear chemistry , inorganic chemistry , organic chemistry , photocatalysis , materials science , metallurgy , biochemistry
Under photooxygenolytic conditions, vitamin B 12 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 B 12 ( 1 ), of KCN and of methylene blue (molar ratio (1:1:0.005) in CD 3 OD at ca 200 K with visible light led to a selective oxygenolysis of the vitamin. The two products, potassium Co α Co β ‐dicyano‐5,6‐dioxo‐5,6‐seco‐5′6′‐dimethylbenzimidazolyl‐cobam‐idate ( 3 , 10% yield) and potassium Co α Co 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 α Co β ‐dicyano‐heptamethyl‐ dioxosecocobyrinates (6 and 7). When irradiated with visible light in any oxygen saturated methanolic solution containing methylene blue, vitamin B 12 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 ( 1 Δ g ). In support of this interpretation, the preparative photolysis of 2 at 200 K also turned out to be about 7 times slower when CH 3 OH was used instead of CD 3 OD.