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

The link between stepwise and concerted electron transfer-bond fragmentation 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 standard potentials E-o determined from free energy plots. For all reactions studied 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-sulfur bond upon formation of the radical anion. At the same time, Delta G(o)double dagger increases going from electron-withdrawing to electron-donating substituents which points to a gradual transition between the two reaction pathways within the series of compounds.