A model system based on photodecompositions of alkylcobaloximes for the conversion of 1,2-diols to aldehydes catalysed by diol dehydrase

Abstract

Attempts to model the conversion of 1,2-diols to aldehydes catalysed by the adenosylcobalamin-dependent enzyme diol dehydrase utilising the photohomolysis of alkylcobaloximes, have been carried out in two ways. First, the alkyl radical released from such photolysis has been shown to convert ethane-1,2-diol to CH₃CHO at pH 2 with an efficiency of ca. 10%. Second, photolysis of several compounds of the type HOCH₂CHOH[CH₂]_n-Co(dmgH)₂(C₅H₅N) yields products (when n= 3 or 4) suggestive of the sequence: (a) photohomolysis followed by (b) an intramolecular 1,5-hydrogen shift (n= 3 or 4) or 1,6-hydrogen shift (n= 4) to produce a 1,2-dihydroxyalkyl radical which in turn undergoes (c) further transformations, including either an acid-catalysed 1,2-hydroxy shift [to give a 1-(dihydroxymethyl)alkyl radical], or acid-catalysed dehydration to R¹ĊHCOR²(R¹= Prⁿ, R²= H, R¹= H, R²= Buⁿ, or R¹= Buⁿ, R²= H), finally yielding R¹CH₂COR². When n= 3, the exclusive carbonylcontaining product from the dihydroxyalkyl group is pentanal (via a 1,5-hydrogen shift). These results enable us to counter claims that radical chemistry cannot simulate the diol dehydrase reactions, and to provide a critique of the present discussion of models for adenosylcobalamin-dependent enzymatic reactions.