AROMATIC NITRATION WITH ELECTROPHILIC N-NITROPYRIDINIUM CATIONS - TRANSITORY CHARGE-TRANSFER COMPLEXES AS KEY INTERMEDIATES

Electrophilic aromatic nitration of various arenes (ArH) is shown to be critically dependent on labile charge-transfer complexes derived from N-nitropyridinium cations. The electrophiles XPyNO2+, with X = CN, CO2CH3, Cl, H, CH3, and OCH3, form a highly graded series of electron acceptors that produce divers [ArH,XPyNO2+] complexes, with charge-transfer excitation energies (h-nu(CT)) spanning a range of almost 50 kcal mol-1. The latter underlie an equally broad spectrum of aromatic substrate selectivities from the different nitrating agents (XPyNO2+), but they all yield an isomeric product distribution from toluene that is singularly insensitive to the X substituent. The strong correlation of the nitration rates with the HOMO-LUMO gap in the [ArH,XPyNO2+] complex is presented (Scheme III) in the context of a stepwise process in which the charge-transfer activation process is cleanly decoupled from the product-determining step-as earlier defined by Olah's requirement of several discrete intermediates. This charge-transfer formulation thus provides a readily visualized as well as a unifying mechanistic basis for the striking comparison of XPyNO2+ With other nitrating agents, including the coordinatively unsaturated nitronium cation (NO2+BF4-), despite their highly differentiated reactivities.