Ketyl Radicals Formed in Grignard Reaction. IV. Sterically Hindered Ketyl Radicals in Nuclear Replacement and Conjugate Addition

The reaction in which 4′-methoxy and 4′-cyano substituents on 2,3,5,6-tetramethylbenzophenone are effectively replaced by benzyl- and t-butylmagnesium chloride was examined by means of ESR measurements. Both the original and the “replaced” ketyl radicals were identified, and an electron-transfer mechanism similar to that of SRN−-type reactions was proposed. Some sterically hindered αβ-enones were also found to form detectable amounts of ketyl radicals. The factors governing the production ratio of the normal and the conjugate adduct were discussed.