Stepwise versus Concerted Electron Transfer-Bond 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 Δ and standard potentials E o determined from free energy plots. For all reactions studied Δ 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, Δ 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.