Dehydrohalogenation stilbenes

The procedure described is essentially that of Ballard and Dehn.1 Stilbene has also been prepared by reduction of desoxy-benzoin,20 benzaldehyde,23 and benzil 2o-2c by dehydrogenation of ethyl benzene,30 toluene,30- 33- 3, and bibenzyl 33-3alkaline reduction of phenylnitromethane,40 phenylnitroacetonitrile,40 and desoxybenzoin 43 by distillation of benzyl sulfone,50 benzyl sulfide,60-63 calcium cinnamate,5 cinnamic acid,5d phenyl cinna-mate,6e-6/ and diphenyl fumarate ie by dehydrohalogenation of a,a -dichlorobibenzyl60 and benzyl chloride 63 by dehalogenation of a,a,c/,a -tetrachlorobibenzyl70 and benzal chloride 73 by the coupling of cinnamic acid and phenyldiazonium chloride 8 by de-... 

The method used in this procedure involves dehydrohalogenation of mcio-stilbene dibromide. An earlier procedure called for refluxing the dibromide with 43% ethanolic potassium hydroxide in an oil bath at 140°C for 24 h. In the following procedure the reaction time is reduced to a few minutes by use of the high-boiling triethylene glycol as solvent to permit operation at a higher reaction temperature. 

Improvements in the preparation and dehydrohalogenation of w o-stilbene dibromide [Org. Syntheses, Coll. Vol. 3, 350 (1955)] are as follows. ra 5-Stilbene (20 g.) is heated with 400 ml. of acetic acid on the steam bath until dissolved, 40 g. of pyridinium bromide perbromide is added, and the mixture is heated on the steam bath and swirled for 5 minutes, meso-Stilbene dibromide separates at once in pearly white plates. The mixture is cooled to room temperature, and the product is collected and washed with methanol. The yield of dibromide, m.p. 

Diphenylacetylene can be synthesized by the double dehydrohalogenation of 1,2-dibromo-l,2-diphenylethene. The sequence starting from ( )-l,2-diphenylethene (stilbene) consists of bromination to give the dibromide, followed by dehydrohalogenation to give a vinylic bromide, then a second dehydrohalogenation to give diphenylacetylene. 

For many decades the standard method of preparation of diphenylacetylenes (tolanes) based upon the dehydrohalogenation of diphenylbromoethenes or diphenyldibro-moethanes [89], which are easily available by hydrobromination and bromination, respectively, of the corresponding stilbenes... 

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