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 (hvCT) 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.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry