Oxidative Aromatic Nitration with Charge-Transfer Complexes of Arenes and Nitrosonium Salts

Brightly colored solutions are obtained immediately upon the exposure of various arenes (ArH) to nitrosonium (NO⁺) salts. The colors arise from the charge-transfer transitions of 1:1 complexes [ArH,NO⁺] that are reversibly formed as persistent intermediates. However the yellow-red charge-transfer (CT) colors are readily bleached by dioxygen, and the corresponding nitroarenes (ArN0₂) can be isolated in excellent yields from acetonitrile solutions. Such an oxidative aromatic nitration of aromatic donors proceeds via the initial autooxidation of the charge-transfer complex. The collapse of the resulting radical ion pair [ArH'⁺, N0₂] to the σ-adduct, followed by the loss of proton, affords ArN0₂. Direct evidence for electron transfer in the initial step when anthracene is treated with NO⁺PF₆- stems for the isolation of (a) the anthracene ion radical salt [(ArH)₂^.+PF₆-] along with nitric oxide in dichloromethane solution and (b) the formation of 9-nitroanthracene (admixed with anthraquinone) in the more polar acetonitrile. The aromatic products (and isomer distribution) from oxidative aromatic nitration are highly reminiscent of those from electrophilic aromatic nitration. The possibility of common reactive intermediates in these two distinctive pathways for aromatic nitration is discussed.