Nitrosoanisoles. Sensitive indicators of dimerisation criteria for C-nitrosoarenes

Abstract

A series of mono- and di-methyl substituted p-nitrosoanisoles was synthesised either by direct nitrosation of the anisoles with NO⁺HSO₄⁻ or by mild oxidation (H₂O₂–Mo catalyst) of the appropriate amines. The nature of these compounds in solution was established by ¹H and ¹³C NMR spectroscopy. Variable temperature ¹H dynamic NMR studies provided thermodynamic data for the cases of dimer–monomer solution equilibria, and ΔG^‡ data for the restricted rotation of the NO group in the monomers. Magnitudes of ΔG^‡ were in the range 28–41 kJ mol⁻¹ and were shown to be sensitive predictors of self-dimerisation tendencies of these compounds. This activation parameter, together with the ^14,15N and ¹⁷O chemical shifts of the NO group, was shown to be a reliable predictor of self-dimerisation of nitroso compounds in general. The NO bond length and the wavenumber of the IR-active NO stretching mode were found to be somewhat less reliable predictors of this behaviour.