The deprotonation rate constants and kinetic isotope effects of the cation radicals have been determined by combined use of direct electrochemical techniques at micro-and ultramicroelectrodes, redox catalysis, and laser flash photolysis, over a extended series of opposing bases. Significant steric hindrance to deprotonation results from encumbering of the opposing base and of the functional carbon in the cation radical by alkyl groups. Kinetic isotope effects, ranging from 2 to 12 in terms of kH/kD, appear upon substituting H to D at the functional carbon of the cation radical. The modest magnitude of the kinetic isotope effects and the fact that they are insensitive to steric hindrance show that proton (or H-atom) tunneling does not interfere significantly in the deprotonation reaction. All the cation radicals in the methylacridan series are strong acids, with pKa's ranging from 0.8 to 1.7, as determined from thermodynamic cycles involving measured standard potentials and hydride-transfer equilibrium constants.Cation radicals are generally much stronger carbon acids than the molecules from which they derive by electrochemical or homogeneous oxidation. Thus, besides the rather restricted set of molecules where the acidity of hydrogens borne by a carbon atom is enhanced by the proximity of a strongly electronwithdrawing substituent,' a large family of additional carbon acids is made available for studying the dynamics of proton transfer. Together with the importance of cation radical intermediates in the oxidation of organic molecules, this is one reason for the continuous and active attention that is being paid to deprotonation of cation radicals.2 Among these, cation radicals deriving from NADH analogues offer the additional interest that a good understanding of the dynamics of their deprot~nation~,~ is important in the discussion of the mechanism of formal hydride transfers (true hydride transfers or H+ + 2etransfers?) involving NADH itself or its synthetic analogue^.^ Previous studies of deprotonation of NADH analogue cation radicals (BNAH:!, BQAH2, BQCNH2, and N(CH3)MAHz in

The deprotonation of the cation radical of 9-cyanomethylacridane by a series of normal bases is investigated and its pK a and homolytic bond dissociation energy determined experimentally. The latter parameter has the largest value in the NADH analogue series, thanks to the strong destabilization of the corresponding cation by the cyano group. It thus allows a significant extension of the attempted correlation between the intrinsic barriers and homolytic bond dissociation energies (D). Aside from members of the series where bulky substituents cause a decelerating steric effect, the correlation is close to a proportionality to D/4. The same correlation applies for all the other cation radicals where the rate constants of deprotonation by normal bases are available. The respective contributions of the homolytic and ionic states in the dissociation of the two types of acid, cation radicals and the conjugate acid of the normal base, are such that a simple model can be developed which regards the deprotonation reaction as a concerted H atom/one-electron transfer. It explains why, for each cation radical, the deprotonation by normal bases gives rise to a single Brönsted plot and why the intrinsic barriers are proportional to D/4. In the NADH analogue series, the deviations from proportionality observed with bulky substituents, and to a lesser extent, upon changing the extent of charge delocalization over the cation radical molecule are accounted for by product and reactant work terms, respectively.

According to the nature of the alkyl group, cation radicals of NADH analogues alkylated para to the nitrogen atom (AHR), generated by direct or indirect electrochemical means, may undergo C−C fragmentation or deprotonation. The former reaction is dominant with the tert-butyl substituent and the latter with methyl and phenyl substituents. It is shown that the cleavage reaction produces AH+ and the tert-butyl radical which is then rapidly oxidized to form the tert-butyl cation. Changing the A group allows a variation of the C−C fragmentation rate constant (determined by cyclic voltammetry or redox catalysis) by ca. six orders of magnitude for a change of ca. 0.4 eV in the standard free energy of the reaction. The logarithm of the rate constant varies linearly with the standard free energy of the reaction with a slope of 1/(60 meV) showing that fragmentation is kinetically controlled by the diffusion of the two fragments out of the solvent cage rather than by activation. The kinetic data thus allow an easy determination of the thermodynamics of the fragmentation. Analysis of previous rate data concerning an extended series of bibenzylic cation and anion radicals shows that they follow the same behavior.