Adsorption of Intermediates and Stereochemical Effects

Considerable interest arises in electrochemical reactions in which a chemisorbed radical may be formed from an initially optically active center. In certain cases, chirality of the radical produced may be preserved if chemisorption occurs and leads to stereochemically different products from those which may arise in the corresponding homogeneous reaction. Examples which have been considered are products arising from reduction of certain ketones and in decarboxylation. For example, the stereochemistry at the C center in a radical RR R C derived from an acid RR R C COOH may be preserved if the radical is chemisorbed. Also, the radio of dl to meso diastereoisomers in the case of dimeric products may differ according to whether the products are formed heterogeneously or homogeneously. Steric effects can also influence the reduction of cis and trans reactants or the formation of cis and trans products in appropriate cases. 

Various cases of the above effects have been considered by Brew-ster, including the question of reduction of cycloalkyl ketones to equatorial or axial alcohols depending on the orientation at the electrode surface and the pH. In neutral medium, the equatorial alcohol is produced according to the scheme 

These effects are observed in a more pronounced fashion in reduction of steroid ketones where the structure is rigid. 

In many cases of organic reductions, direct participation of metal-organic intermediates is indicated, e.g., in the Clemmensen reduction of ketones at Pb and Hg where hydrocarbons are formed as well as pinacols or alcohols. 

With ketones in acid media, Brewster suggests the following path with interaction with the electrode surface  

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