Aprotic Nitration (NO2+BF4-) of 2-Halo- and 2,6-Dihalopyridines and Transfer-Nitration Chemistry of Their N-Nitropyridinium Cations

Article abstract of DOI:10.1021/jo00009a015

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 oniums 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.