APROTIC NITRATION (NO2+BF4-) OF 2-HALO-PYRIDINES AND 2,6-DIHALOPYRIDINES AND TRANSFER-NITRATION CHEMISTRY OF THEIR N-NITROPYRIDINIUM CATIONS

NO2+BF4-nitration of 2,6-dibromo-1 and 2,6-dichloropyridine 2 in CH3CN results in predominant C-nitration, whereas in CH2Cl2, N-nitration is predominant. With 2,6-difluoropyridine 3 only C-nitration was observed. Dehalogenation of the C-nitrated 1 and 2 affords 3-nitropyridine (3-NP) in moderate but greatly improved yields over conventional protic nitration of pyridine. Despite favorable presence of steric inhibition to resonance and the -I effect of halogens, N-nitrated pyridinium salts 1b and 2b do not transfer-nitrate to aromatics even under forcing conditions. The lack of transfer-nitration reactivity is not due to in situ rearrangement of the nitro onium to nitrito onium ions. A mechanism involving neighboring group participation by the 2,6-halogens is proposed. The monohalo-N-nitropyridinium cations transfer-nitrate toluene and benzene. Transfer-nitration selectivity of the 2-bromo-N-nitro- and 2-chloro-N-nitropyridinium cations are comparable (K(T)/K(B) = 41-44), but the 2-fluoro-N-nitro cation is much less selective (more reactive) (K(T)/K(B) = 15.4), indicative of a stronger-I effect, weakening the N+-N+ bond.