Electro-organic reactions. Part 15. The mechanism of cathodic cleavage and elimination from vicinal diacetates and an unusual cleavage from 9,10-diacyloxyphenanthrenes

Abstract

The products of controlled-potential electrolysis of cis- and trans-diacetoxystilbene are dependent on solvent composition; in dry dimethylformamide the major product is diphenylacetylene (formed by reductive elimination) whereas in aqueous dimethylformamide hydrogenolysis of one of the acetate groups may be achieved. This trapping of the carbanionic intermediate is compelling evidence for a stepwise mechanism.

In the structurally related 9,10-diacyloxyphenanthrenes elimination is precluded and they are cathodically cleaved in an unexpected fashion. Fission of the acyl–oxygen bonds occurs to give phenanthraquinone radical-anion. After aqueous work-up, phenanthraquinone and the appropriate carboxylic acid are formed. It is suggested that the acid is formed by addition of water to the corresponding keten which, in turn, is probably the product of initial cleavage.

The mechanistic proposals are supported by detailed product analyses, coulometry, and cyclic voltammetric experiments.