Solid State Bromination with the Sulfonium Tribromide Crystals

Abstract

Sulfonium tribromides have been found to brominate trans-stilbene effectively in solid state reactions. When the ground mixture of 1-methylthianium tribromide and trans-stilbene in an agate mortar was stored in a thermostat, at 30°C for 2 d, meso-1, 2-dibromo-1, 2-diphenylethane (meso-stilbene dibromide) was obtained quantitatively and stereoselectively (transaddition). Several other sulfonium tribromides also gave similar results, though the reaction time needed for completion varied. Examination of the progress of the reaction of 1rnethylthianium tribromide by infrared absorption spectrometry and powder X-ray diffraction indicated that the crystals of trans-stilbene gradually change to those of meso-stilbene dibromide, while those of the sulfonium tribromide to those of the sulfonium bromide.