Charge-transfer structures of aromatic electron donor-acceptor complexes leading to electron transfer with the electrophilic nitrosonium cation

Benzene and hexamethylbenzene partake in the rapid formation of unique electron donor-acceptor (EDA) complexes with the nitrosonium cation, as reflected in the association constants that vary markedly from K=0.5 to 31 000 M −1 , respectively. The increase of Z (the degree of charge transfer) from benzene (0.52) to hexamethylbenzene (0.97) in the charge-transfer complex tracks their increasing proclivity to undergo electron transfer and afford arene cation radicals (ArH •+ ) as reactive intermediates. A general mechanistic formulation based on electron transfer from the charge-transfer complex is developed, in which the facility of the followup reactions (such as fragmentation, cycloreversion, rearrangement, ion-pair annihilation, etc.) of the labile ArH •+ is critical. When the electron transfer is reversible, this 1-electron mechanism is shown not to be readily distinguished from the more conventional electrophilic (2-electron) pathways