Electrochemical reduction of phenyl-substituted cyclopentadienes: a case of an 'indirect father-son' self-protonation process

The electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene an d 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene has been investigated in DMF. A t low temperatures (less than or equal to -30 degrees C) and under anhydrou s conditions cyclic voltammetry experiments indicate that both substrates a re reducible in two successive, chemically reversible, one-electron steps, affording the corresponding radical anions and dianions. More complex volta mmetric behaviour is detected at higher temperatures, when the radical anio ns are protonated by the substrate itself, giving rise, via the so-called ' father-son' self-protonation process, to further reducible compounds genera ting additional basic intermediates. In exhaustive electrolyses, 1/3 of the starting substrate is converted into dihydroreduction products and 2/3 int o its conjugate bases. On the other hand, the quantitative formation of the dihydroreduction products is observed when exhaustive electrolyses are car ried out in the presence of phenol, which acts as the proton donor instead of the substrate. Under voltammetric conditions, however, evidence of depro tonation of the substrate by the phenoxide anions thus generated is obtaine d, indicating the occurrence of an 'indirect father-son' self-protonation p rocess, which takes place since phenol is kinetically more acidic, but ther modynamically less acidic, than the substrate itself. The mechanisms of dec ay of the radical anions are proposed on the basis of the comparison of exp erimental and simulated voltammetric data: this approach allowed also the d etermination of the characteristic kinetic rate constants, some of which ar e strongly dependent on the structure of the substrate, and of the importan ce of the homoconjugation reaction involving phenol and phenoxide anion. Th e stereochemistry of the dihydroreduction process and the voltammetric beha viour of the corresponding products are considered also, even in relation t o the voltammetric behaviour of the starting substrates. (C) 1999 Elsevier Science S.A. All rights reserved.