Investigation of rotational isomerism in thiophen- and furan-2-carbonyl halides by nuclear magnetic resonance and infrared spectrometry

Abstract

The possible occurrence of rotational isomerism in 35 thiophen- and furan-2-carbonyl fluorides and chlorides has been investigated by n.m.r. and i.r. spectrometry. At room temperature, solutions of the fluorides in trichlorofluoromethane, carbon tetrachloride, and acetonitrile contain appreciable amounts of the syn- and anti-rotamers. With furan-2-carbonyl fluoride, the compound investigated in most detail, the rotamer giving the higher field ¹⁹F signal in non-polar solvents at low temperature corresponds to the rotamer of the 5-deuterio-analogue which has the higher wavenumber CO band; these are the predominant forms (ca. 70% in trichiorofluoromethane) and are thought to be the syn-rotamers. It is probable that the chlorides also exhibit rotational isomerism, but the absence of halogen n.m.r. evidence makes the interpretation less certain.