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Various polymethylbenzenes and anisoles are selectively nitrosated with the electrophilic nitrosonium salt NO+ BF4− in good conversions and yields under mild conditions in which the conventional procedure (based on nitrite neutralization with strong acid) is ineffective. The reactivity patterns in acetonitrile deduced from the various time/conversions in Tables 2 and 3 indicate that aromatic nitrosation is distinctly different from those previously established for electrophilic aromatic nitration. The contrasting behavior of NO+ in aromatic nitrosation is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate, which in the case of aromatic nitration with NO2+ occurs with no deuterium kinetic isotope effect. Aromatic nitroso derivatives (unlike the nitro counterpart) are excellent electron donors that are subject to a reversible one-electron oxidation at positive potentials significantly less than that of the parent polymethylbenzene or anisole. As a result, the series of nitrosobenzenes are also much better Brønsted bases than the corresponding nitro derivatives, and this marked distinction, therefore, accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e. Wheland intermediates).
Bosch et al. (Thu,) studied this question.