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 reduction 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 are compared with those previously obtained for reduction of the same compounds in N,N-dimethylformamide (DMF). The change from DMF to MeOH gives rise to important differences which are related to the higher acidity and the hydrogen-bond donating capabilities of MeOH. The changes include: (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 processes for the entire series in DMF, reduction in MeOH gave a 1 F process for the aryl cinnamates and a approximate to 1.5 F process for the alkyl cinnamates: (iv) the products of preparative scale reduction of 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, all-trans-2-methoxycarbonyl-3,4-diphenylcyclopentanone [corresponding to (+/-) coupling]; (v) the stereochemistry of the product of coupling changes from exclusively (+/-), in DMF, to equal amounts of meso and (+/-) in MeOH. The changes may be rationalized in a mechanism which crucially involves initial formation of a radical anion hydrogen-bonded (at the carbonyl oxygen) to MeOH. Linear sweep voltammetric experiments reveal an important dependence of reaction order upon the type of ester reduced. which is strong evidence of competition between dimerization of the hydrogen-bonded radical anion in a rate-determining second-order reaction favored for the aryl esters and rate-determining protonation in a first-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 reduced carbon protonated radical (2F). The change in mechanism and stereoselectivity of electroreduction on going from DMF to MeOH electrolyte seems to be general for hydrodimerizations.