Acid catalysis, particularly by a Brønsted acid, has been extensively applied to diverse organic reactions. In a mechanistic context, proton-transfer to an organic substrate (D) is tantamount to a two electron oxidation of D, and catalytic activation generally derives from the enhanced electrophilic reactivity of the cationic intermediates. There is an alternative mode of activation that derives via one-electron oxidation of the organic substrate and this catalytic process has been termed as electron-transfer chain catalysis (ETC) by Chanon and hole catalysis by Bauld. Catalytic activation induced by electron-transfer arises from the enhanced electrophilic (as well as homolytic) reactivity of cation radical intermediates.
Catalysis by proton transfer differs from the electron transfer catalysis in that the reactive intermediates are diamagnetic cations as opposed to paramagnetic cation radicals. Moreover, unfamiliarity with cation radical intermediates and unavailability of the ready methods to generate them has led to the sparse use of ETC in organic chemistry. We recently introduced a series of highly robust cation radicals as excellent 1-electron oxidants [E (red) = 1.11-1.50 V vs. SCE, see structures below] which can be readily prepared in multi-gram quantities from cheap starting materials:
For the easy separation of the reduced cation radical species from the reaction media, we have synthesized high molecular weight cation radicals that contain multiple holes, i.e.
We have successfully employed these cation radical salts in the development of high-yielding synthetic methodologies for the hydrogenation of sterically hindered olefins, synthesis of diarylmethanes, autoxidation of hydroquinones to quinones using molecular oxygen, oxidation of aromatic donors using nitrogen oxides, etc. For example: