Benzylic Fragmentation (BFR), an Aromatic Analogue of E1cB Including a Homolytic Variant, as either Nucleofugal or Homolytic Path to Aryl Stabilized Carbanions or Ketyls, respectively

BFR is the decisive step of novel reductions at benzyl-type carbons that may complement available methods. BFR is mechanistically important for understanding and control of curious findings in radical anion chemistry. Driving force of BFR is rearomatization of a dihydroarene species usually derived from anthracene. This intermediate 1 arises (solvent THF) either from coupling of anthracenidyl A°− with a benzyl radical or from nucleophilic reaction of anthracene hydride AH− with a benzylic electrophile followed by CH 2 deprotonation (excess AH− ) of the generated 9-benzyl-dihydroanthracene. 1 spontaneously undergoes BFR either heterolytically or homolytically depending on the stabilities of carbanion 2 (or even dianion 2, e.g. from chalcone) and ketyl 3. BFR is more rapid with counterion Na+ than with Li+ . The overall reaction is a selective one electron (innersphere eletron transfer) or two electron reduction of a benzylic electrophile, sometimes under expected subsequent rearrangement. This account reports on mechanism and scope of these reductions; a few related cases are described.