Aromatic nitration with electrophilic N-nitropyridinium cations. Transitory charge-transfer complexes as key intermediates

Article abstract of DOI:10.1021/ja00031a034

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 (hv_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.