Reduction of benzophenones with SmI₂. Post electron transfer processes

The kinetics of the reaction of several substituted benzophenones with SmI₂ in THF was studied by using stopped flow spectrometry. The electron transfer takes place during the dead time of mixing and for most derivatives it is nearly quantitative. In the presence of an excess of substrate and in the absence of proton donors the dimerization reaction to pinacol is second order in the radical anion and has a negative order in SmI₂. Similarly, in reactions in the presence of excess SmI₂, the reaction shows negative order in the concentration of the substrate. It is concluded that the radical anions exist as a mixture of monomeric ion pairs and Streitwieser dimers. In the presence of trifluoroethanol, protonation of the Streitwieser dimer occurs with a rate constant an order of magnitude larger than that of the monomer.