Reduction of benzophenone by SmI2: the role of proton donors in determining product distribution.

[reaction: see text] Reduction of benzophenone by SmI2 yields benzopinacol. Addition of proton donors results in an initial increase in the amount of the benzhydrol formed. However, the ratio benzhydrol/benzopinacol reaches a maximum, decreases, and then levels off as the proton donor concentration is further increased. The position of the maximum and its height depend on the proton donor concentration and its kinetic acidity. The momentary concentration of the intermediate radicals governs the product distribution.