On the role of Samarium/HMPA in the post electron-transfer steps in Sml(2) reductions

The reaction of p,p '-dichlorobenzophenone with Sml(2) was studied in the presence of variable amounts of HMPA. The electron-transfer step takes place instantaneously. In the presence of excess substrate, the addition of HMPA retarded the rate of coupling to pinacol by a factor of 250. In the presence of an excess of Sml(2), the initial rate retardation is followed by a dramatic increase in rate.