UNIQUE SINGLE-ELECTRON TRANSFERS BETWEEN CHEMICALLY INERT TRIPHENYLMETHYL RADICALS AND TRIPHENYLMETHYL ANIONS

A number of SET reactions between inert 4-X-tetradecachlorotriphenylmethyl radicals (X-PTM) and stable tetra-n-butylammonium (Q+) 4-Y-tetradecachlorotriphenylmethides (Q+Y-PTM:-; X, Y = H, Me, NH2CO, Me2NCO, Ph2NCO, MeOCO, PhOCO, NH2, MeO, Cl, Br,-OCO) have been studied in THF, at room temperature, by the ESR technique. These processes are abnormally slow, since no significant ESR linewidth and/or hyperfine coupling changes are observed. In the SET between H-PTM* and NH2-PTM:-the progress has been monitored by ESR, and its second-order rate constant is 5 x 102mol-1L min-1. Such a unique, most remarkable slowness is ascribed to colossal steric hindrance (shielding) caused by chlorine overcrowding in both SET components. The SET equilibrium constants ket have been calculated from the ESR spectrum, using radicals H-PTM-and Me-PTM* as the standards, and they follow the Hammett equation. Exceptions are X = MeO or RCO, due to steric inhibition of resonance. Evidence indicates that the SET process occurs between the radical and the free carbanion, in spite the latter existing predominantly as an ion pair with counterion Q+, as shown by osmometry. The syntheses and isolation in excellent yields of a substantial number of new inert free radicals and new related stable carbanion salts have been effected, most of the latter from the corresponding radicals, using hydroxide ion as a single-electron donor. © 1990, American Chemical Society. All rights reserved.