Acid-induced broadening of 1H n.m.r. signals in the 6-hydroxychroman and 5-hydroxydihydrobenzofuran series

Abstract

Acids at least as strong as trichloroacetic acid induce line broadening in the ¹H n.m.r. spectra of 6-hydroxychroman and 5-hydroxy-2,3-dihydrobenzofuran derivatives. The effect is specific for derivatives of hydroquinone although it is weak in the absence of the heterocyclic rings. The dihydrofuran ring confers greater sensitivity than the dihydropyran ring. A second heterocyclic ring increases the sensitivity greatly. Nuclear (aromatic) methyl groups increase the sensitivity but acyl groups remove it. The effect is attributed to the formation of traces of cation radicals and is destroyed by bases or water. It varies in the same way as hyperfine coupling constants where these are known for the cation radical species. It can be used to simplify complex spectra because only protons very close to the carbon or oxygen atoms of the hydroquinone nucleus are affected strongly; conversely, it can be used for diagnostic purposes. The syntheses of some oxygen heterocycles needed for the survey are discussed briefly.