Metal-free C-C coupling reactions with tetraguanidino-functionalized pyridines and light

Abstract

Herein we report on metal-free C-C coupling reactions mediated by the pyridine derivative 2,3,6,7-tetrakis(tetramethylguanidino)pyridine under the action of visible light. The rate-determining step is the homolytic N-C bond cleavage of the initially formed N-alkyl pyridinium ion upon excitation with visible light. The released alkyl radicals subsequently dimerize to the C-C coupling product. 2,3,6,7-Tetrakis(tetramethylguanidino)pyridine, which is a strong electron donor (E1/2(CH2Cl2) = -0.76 V vs. ferrocene) is oxidized to the dication. For alkyl = benzyl and allyl, relatively high first-order rate constants of 0.23±0.03 and 0.13±0.03 s(-1) were determined. Regeneration of neutral 2,3,6,7-tetrakis(tetramethylguanidino)-pyridine by reduction allows to drive the process in a cycle.

Keywords: CC coupling; guanidine; photoreactions; radicals; reduction.