Nitration of Adamantane and Diamantane with Nitronium Tetrafluoroborate

The slow reaction of adamantane with nitronium tetrafluoroborate at room temperature in purified, nitrile-free nitromethane or in nitroethane gives 1-nitroadamantane in 66% and 74% isolated yield, respectively. Similar reaction of diamantane gives 62% 1- and 5% 4-nitrodiamantane. Initial reaction upon aqueous workup gives 1-adamantanol with 1-fluoroadamantane, 3-fluoro-1-adamantanol, and adamantanone as byproducts. Upon prolonged reaction 1-nitroadamantane is formed in good yield. The experimental data are in accord with intermediate formation of the 1-adamantyl cation (by hydride abstraction or cleavage of initially formed 1-nitroadamantane) followed by reaction with HNO₂ or NO₂^- formed in the reaction, giving 1-adamantyl nitrite, which then undergoes cleavage–rearrangement to give 1-nitroadamantane. A small kinetic hydrogen isotope effect of k_H/k_D = 1.2–1.3 is indicative of a nonlinear highly unsymmetrical transition state in the rate-determining step of the reaction.