Reaction of aromatic N-oxides with dipolarophiles. X. Role of charge-transfer complexes in 1,3-dipolar cycloaddition of pyridine N-oxides to phenyl isocyanate

In the 1, 3-dipolar cycloaddition reaction of 3-methylpyridine N-oxide with phenyl isocyanate, spectroscopic evidence indicates that phenyl isocyanate forms charge-transfer complexes with both pyridine N-oxide and aromatic solvents such as pyridine. In connection with the charge-transfer complex formation, the solvent effect on the site selectivity was investigated. The most prominent solvent effect was observed in the reaction using 3-methylpyridine as a solvent. The equilibrium constants for the interaction of 3-methylpyridine N-oxide with phenyl isocyanate in several solvents were found to be quite large, indicating that the reaction mixtures favor complex formation. The proton nuclear magnetic resonance (1H-NMR) spectra of the mixtures show a high field shift of the methyl signal of 3-methylpyridine N-oxide due to charge-transfer complexation. Based on these results, the structure and role of the complexes are discussed.