Abstract
NO 2 + BF 4 − nitration of 2,6-dibromo-1 and 2,6-dichloropyridine 2 in CH 3 CN results in predominant C-nitration, whereas in CH 2 Cl 2 , 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
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