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