Hammerich, O; Nielsen, MF; ,
The reactivity of the radical anions derived from cinnamic acid esters decreases in radical ion-radical ion dimerization and increases in protonation when the esters become less easy to reduce. This is reflected by plots of log k vs. E-o' (log k(dim) or log k(prot)) that are linear, bur with slopes of different sign.Results obtained by AM1 calculations show that the unpaired electron density, c(SOMO)(2), at C-3 and the negative charge, -q, at C-2 vary in a fashion similar to that observed for log k(dim) and log k(prot), respectively. The available kinetics and product data for other series of radical anions are reviewed and trends similar io those observed for the cinnamate radical anions are apparent in most cases.More insight into the possible relationship between the electronic properties of radical anions and their reactivity was obtained through AM1 calculations for seven series of model compounds (monosubstituted ethylenes, 1,2-disubstituted ethylenes, monosubstituted benzenes, 1-substituted 2,6-di-tert-butylbenzenes, 9-substituted anthracenes and esters of acrylic and cinnamic acid) encompassing a total of 93 compounds. Calculations were also carried out for the much smaller number of radical anions for which experimental data are available. The results show that the introduction of an alkyl or alkyl-like substituent, or a pi-type substituent that is not in conjugation with the parent pi-system, gives rise to only a small electronic perturbation and in those cases c(SOMO)(2) and q at the atoms of interest are essentially linearly related with the adiabatic gas phase electron affinities, Delta H-f = Delta H-f (radical anion) - Delta H-f(substrate), representing the ease of reduction of the substrate. It is also found that the formation of the new C-C bond during dimerization does not as a rule involve the carbon carrying the highest unpaired electron density and, similarly, protonation does not as a rule take place at the carbon carrying the highest negative charge. For example, for the 9-substituted anthracene radical anions the highest unpaired electron density is located at the carbon carrying the substituent (C-9), whereas dimerization takes place through C-10. The high values of c(SOMO)(2) at C-9 are suggested to be the origin of the attractive forces leading to the pi-complexes earlier proposed to be formed prior to the 10,10' sigma-bond. The results obtained for the other series of compounds indicate that the formation of such intermediate pi-complexes may be a general phenomenon.Finally, the question of radical ion-radical ion (RR) versus radical ion-substrate (RS) mechanisms in reductive dimerizations is addressed.