Hydride affinities of arylcarbenium ions and iminium ions in dimethyl sulfoxide and acetonitrile

Gibbs free energies for scission of C-H bonds leading to carbanion and proton (mode a), radical pair (mode b), and carbenium ion and hydride (mode c) have been determined for a series of acidic C-H bonds in ca. 45 weak acids. This involved the use of the equilibrium acidities (or homolytic bond dissociation enthalpies), redox potentials in DMSO or MeCN solution, and the appropriate thermodynamic cycles in the two solvents. The introduction of electron-donating groups generally results in small-to-negligible effects on Delta G(R)- values (mode a scission) but in a relatively large stabilizing influence on the Delta G(R)+ values for heterolytic cleavage to hydride and carbenium ion (mode c). Electron-withdrawing groups exert large stabilizing effects on Delta G(R)-, but their effects on Delta G(R)+ are dependent on the nature of the substituents. The heterolytic processes a are usually favored in solution due to the strong solvation of the proton. Homolytic scission (mode b) is usually more favorable than the corresponding heterolytic process c, but they can be comparable when strong electron-donating groups such as dialkylamino are present. Indeed, heterolytic cleavage to hydride and carbenium ion was ca. 10 kcal/mol more favorable than homolytic cleavage for. series of highly stabilized 2-benzoyl-N,N'-dialkylperhydropyrimidines.