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

Article abstract of DOI:10.1002/chem.201304987

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 (E_1/2(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. Light on C-C coupling: Tetraguanidino-substituted pyridines are used for metal-free C-C coupling reactions with visible light (see figure).