O. Hammerich et Mf. Nielsen, THE COMPETITION BETWEEN THE DIMERIZATION OF RADICAL-ANIONS AND THEIR REACTIONS WITH ELECTROPHILES, Acta chemica Scandinavica, 52(7), 1998, pp. 831-857
The reactivity of the radical anions derived from cinnamic acid esters
decreases in radical ion-radical ion dimerization and increases in pr
otonation when the esters become less easy to reduce. This is reflecte
d by plots of log k vs. E-o' (log k(dim) or log k(prot)) that are line
ar, bur with slopes of different sign. Results obtained by AM1 calcula
tions 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 obse
rved for log k(dim) and log k(prot), respectively. The available kinet
ics and product data for other series of radical anions are reviewed a
nd trends similar io those observed for the cinnamate radical anions a
re apparent in most cases. More insight into the possible relationship
between the electronic properties of radical anions and their reactiv
ity was obtained through AM1 calculations for seven series of model co
mpounds (monosubstituted ethylenes, 1,2-disubstituted ethylenes, monos
ubstituted benzenes, 1-substituted 2,6-di-tert-butylbenzenes, 9-substi
tuted anthracenes and esters of acrylic and cinnamic acid) encompassin
g 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 pert
urbation 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(substra
te), representing the ease of reduction of the substrate. It is also f
ound that the formation of the new C-C bond during dimerization does n
ot 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), where
as 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 leadi
ng to the pi-complexes earlier proposed to be formed prior to the 10,1
0' 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 dimeriz
ations is addressed.