DONOR-ACCEPTOR EFFECTS ON THE STABILITY OF RADICALS DERIVED FROM ALPHA-DIALKYLAMINO KETONES

The bond dissociation energies (BDEs) for the acidic C-H bonds in CH3COCH2NMe2 and CH3COCH2NEt2 ketones, estimated in DMSO from pK(HA) and oxidation potential data, are 3-4 kcal/mol higher than that for the acidic C-H bond in PhCOCH2NMe2. It is suggested that the electronic and steric effects of the Ph group dictate a Z structure for the precursor enolate ion and the generation of a radical that is strongly stabilized by electrostatic effects, which are responsible for the apparent synergistic effects of the PhCO and NMe2 groups in stabilizing the PhCOCHNMe2 radical. This suggestion was supported by the observation that the BDEs of the acidic C-H bonds in ketocyclic analogues of the alpha-dialkylamino open-chain ketones, wherein the enolate ion is locked in an E structure, have higher BDEs by 9-11.5 kcal/mol.