Aromatic nitration with ion radical pairs [ArH.cntdot.+,NO2.cntdot.] as reactive intermediates. Time-resolved studies of charge-transfer activation of dialkoxybenzenes

Aromatic nitrations carried out both under electrophilic conditions and by charge-transfer activation afford the same yields and isomer distributions of nitration products from a common series of aromatic ethers (ArH). Time-resolved spectroscopy establishes the charge-transfer nitration to proceed via the ion radical pair (ArH/sup +/,NO/sub 2/), generated by the deliberate excitation of the electron donor-acceptor or ..pi.. complex of the arene with C(NO/sub 2/)/sub 4/. Laser flash photolysis of the charge-transfer band defines the evolution of the arene cation radical ArH/sup +/ and allows its decay kinetics to be delineated in various solvents and with added salts. The internal trapping of ArH/sup +/ is examined in the substituted p-dimethoxybenzenes CH/sub 3/OC/sub 6/H/sub 4/OCH/sub 2/X with X = CO/sub 2/H, CO/sub 2//sup -/, CO/sub 2/Et, and CH/sub 2/OH as the pendant functional groups. The mechanistic relevance of the collapse of (ArH/sup +/, NO/sub 2/) to the Wheland intermediate is discussed in the context of electrophilic aromatic nitrations.