Organized photodimerization of unsaturated carboxylates. Selectivity control by normal and reversed micelles

Abstract

Aromatic unsaturated carboxylates such as cinnamate and 2-indenecarboxylate form, in the presence of long-chain alkylammonium ion, short rod-like micelles with an aggregation number of an order of 10³ in water and premicelle aggregates including 2–3 surfactant molecules in carbon tetrachloride. It was found that these aggregates undergo efficient and stereoselective photodimerizations. Photoreactions in aqueous rod-like micellar solutions resulted in the predominant formation of anti-head-to-head dimers (anti-HH), while the reaction in small aggregated premicelles yielded thermodynamically least stable syn-head-to-head dimers (syn-HH). The addition of a small amount of methanol to the premicelle aggregate shifted the major product from syn-HH to anti-HH. Control experiments showed that the photoreaction of the corresponding methyl ester afforded mixtures of four isomeric dimers, syn- and anti-HH and HT (head-to-tail), of statistical distribution. These stereochemical selectivities are discussed in terms of the organized olefin orientation and correlated to the results of light scattering and NMR probe techniques.