On the Role of Samarium/HMPA in the Post Electron-Transfer Steps in SmI₂ Reductions

The reaction of p,p‘-dichlorobenzophenone with SmI₂ 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 SmI₂, the initial rate retardation is followed by a dramatic increase in rate.