Benzyl Radicals from Toluene by Photosensitization with Naphthalene-1,4-dicarbonitrile – Benzylation and Hydroxymethylation of Unsaturated Compounds

Abstract

In aprotic media, photoinduced electron transfer from toluene to 1,4-naphthalenedicarbonitrile (DCN) is followed by in-cage proton transfer and radical coupling. However, in the presence of a protic co-solvent, the radical ions diffuse out of the cage and deprotonation of the toluene radical cation takes place from the free solvated species. With tBuOH, this results in reductive benzylation of DCN through coupling of benzyl radicals with DCN–·. With MeOH, secondary hydrogen abstraction by PhCH2· yields hydroxymethyl radicals, which couple with DCN–·. Both benzyl and hydroxymethyl radicals are efficiently trapped by electrophilic alkenes. Thus, in the presence of dimethyl maleate, DCN-photosensitized benzylation – or hydroxymethylation – of the substrate occurs through direct activation of the C–H bond. The same reaction has been accomplished with acrylonitrile, albeit with a lower yield.