Stepwise versus concerted electron transfer-bolted fragmentation in the reduction of phenyl triphenylmethyl sulfides

The link between stepwise and concerted electron transfer-bond fragmentatio n processes is described for the homogeneous reduction process of four para -substituted phenyl triphenylmethyl sulfides in N,N-dimethylformamide. The description is based on intrinsic barriers Delta G(o)double dagger and stan dard potentials E-o determined from free energy plots. For all reactions st udied aG(o)double dagger is remarkable high and E-o is not far from what is expected for a concerted pathway, even if the mechanism is stepwise. This is attributed to a substantial elongation and weakening of the carbon-sulfu r bond upon formation of the radical anion. At the same time, Delta G(o)dou ble dagger increases going from electron-withdrawing to electron-donating s ubstituents which points to a gradual transition between the two reaction p athways within the series of compounds.