STEREOSELECTIVITY AND MECHANISM IN THE ELECTROHYDRODIMERISATION OF ESTERS OF CINNAMIC ACID

Rate constants (k(obs)) and reaction orders have been determined for t he cathodic reduction in DMF solution of 11 cinnamic acid esters inclu ding some derived from chiral alcohols and a dicinnamate derived from trans-cyclohexane-1,2-diol. The cinnamic acid esters typically reduce with high stereoselectivity to all-trans 3,4-diphenylcyclopentanone-2- carboxylate.dagger The enhancement of rates of reaction by addition of water was studied for selected substrates and low energies of activat ion were found, Changes in the alkoxy or aryloxy groups also caused si gnificant changes in rate and log k(obs) correlated linearly with E de grees values, The results from kinetic experiments were complemented b y product studies of reactions aimed at probing reversibility of key r eaction steps. The combined evidence is interpreted as unambiguous sup port for radical anion-radical anion coupling as the key step with com plexation with water, prior to coupling, being crucial. The relative s tereochemistry at C-3 and C-4 is fixed, irreversibly, at the coupling stage and there is strong evidence to suggest that templating in the c omplex between two radical anions and water determines the stereochemi cal outcome.