THE INFLUENCE OF SOLVENT ON THE MECHANISM AND STEREOCHEMISTRY OF ELECTROHYDRODIMERIZATION - THE REDUCTION OF CINNAMIC ACID-ESTERS IN METHANOL

The kinetics and mechanisms of reactions following electrochemical red uction of members of a series of cinnamic acid esters. Ph-CH=CH-COOR, R = alkyl or aryl, in methanol(MeOH) have been studied. The results ar e compared with those previously obtained for reduction of the same co mpounds in N,N-dimethylformamide (DMF). The change from DMF to MeOH gi ves rise to important differences which are related to the higher acid ity and the hydrogen-bond donating capabilities of MeOH. The changes i nclude: (i) anodic shifts of the formal reduction potentials, in MeOH, by 80-130 mV; (ii) increased rates of reaction of the radical anions in MeOH by 2-3 orders of magnitude; (iii) in contrast with clean 1F pr ocesses for the entire series in DMF, reduction in MeOH gave a 1 F pro cess for the aryl cinnamates and a approximate to 1.5 F process for th e alkyl cinnamates: (iv) the products of preparative scale reduction o f methyl cinnamate in MeOH were a mixture of the saturated ester (2F) and the meso and (+/-) isomers of the linear hydrodimer (1F), whereas reduction in DMF gave exclusively the cyclized 1F product, trans-2-met hoxycarbonyl-3,4-diphenylcyclopentanone [corresponding to (+/-) coupli ng]; (v) the stereochemistry of the product of coupling changes from e xclusively (+/-), in DMF, to equal amounts of meso and (+/-) in MeOH. The changes may be rationalized in a mechanism which crucially involve s initial formation of a radical anion hydrogen-bonded (at the carbony l oxygen) to MeOH. Linear sweep voltammetric experiments reveal an imp ortant dependence of reaction order upon the type of ester reduced. wh ich is strong evidence of competition between dimerization of the hydr ogen-bonded radical anion in a rate-determining second-order reaction favored for the aryl esters and rate-determining protonation in a firs t-order reaction of the carbonyl oxygen (favored for alkyl esters). It is also shown that the neutral radicals formed by protonation of the radical anions at oxygen are more difficult to reduce than the parent ester and may either dimerize (1F) or tautomerize to the more easily r educed carbon protonated radical (2F). The change in mechanism and ste reoselectivity of electroreduction on going from DMF to MeOH electroly te seems to be general for hydrodimerizations.