NEWER ASPECTS OF ELECTRON-TRANSFER IN ELECTROPHILIC AROMATIC NITRATION - RADICAL PAIRS AS REACTIVE INTERMEDIATES BY TIME-RESOLVED SPECTROSCOPY

Aromatic nitration is effectively carried out by the deliberate irradiation of the charge-transfer absorption band (hnu(CT)) of the transient electron donor-acceptor complex [ArH, NO2Y+], where NO2Y+ represents common nitrating agents. Time-resolved spectroscopy demonstrates that the charge-transfer nitration effected in this way proceeds via an electron-transfer mechanism, in which the collapse of the reactive triad, [ArH+., NO2., Y] is critical to the formation of the Wheland intermediate. Comparative product analyses of toluene and anisole nitrations together with the 'non-conventional' products arising from NO+ catalysis, ipso attack and addition/elimination show that the activation process leading to charge-transfer (photochemical) nitration is indistinguishable from that leading to electrophilic (thermal) aromatic nitration.