Stilbene dibromide

Stilbene decolorises bromine only on heating. Proceed as above, but keep the stilbene solution hot during the addition of the CCI4 solution of bromine. Stilbene dibromide has m.p. 237 . 

Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

X-ray diffraction has settled the conformation of stilbene dibromide. The meso form is that when both phenyl groups and bromine atoms are anti so that steric and dipolar repulsions are at a minimum. The dl form crystallizes in conformation in which the bromines are approximately gauche, and thus the steric repulsion of the bulky phenyl groups is at a minimum, even though there is steric and dipolar repulsions of the bromine atoms. 

The claims of exclusive formation of rac-stilbene dichloride upon gas-solid addition of chlorine to frans-stilbene (103) [71] and of meso-stilbene dibromide in the gas-solid addition of bromine to trans- or czs-stilbene [54] could not be verified. Scheme 12 shows the results of more detailed studies indicating the mesolrac ratios on the solid-state chlorination and bromination of trflns-stilbene (103) and some variations when the crystal size was changed [58, 60-61]. There is a risk of partial transient liquefaction if the chlorine is added too rapidly, due to initially heavy reaction. But even at the start with a stoichiometric amount of chlorine at 0.1 bar and 0 °C, a persistent product layer forms on the unground crystal powder of 103 that cannot be disintegrated by the ultrasound of a cleaning bath at 20 °C for 60 h (only 7% conversion with mesolrac ratio of 11 89 under these conditions) [22]. It is therefore unavoidable to mill the crystals of 103 to sizes <1 pm in order to overcome these rare diffi-... 

Let us now turn to the experimental results to see if these predictions are borne out in fact. It has long been known that E2H reactions normally give preferentially anti elimination. For example, reaction of ffz fo-stilbene dibromide with potassium ethoxide gives a j-bromostilbene (Reaction 7.39), whereas reaction of the D,L-dibromide gives the trans product (Reaction 7.40).102 A multitude of other examples exist—see, for example, note 64 (p. 355) and note 82 (p. 362). 

A similar sequence probably accounts for the formation of 50% (40%) 1-clilo-ro- (1-bromo-) -2-methoxy-2-phenylpropionic acid by electrolysis of cinnamic acid in CH30H/NH4C1 (NH4Br)as SSE 271 Electrolysis of irans-stilbene in me-thanol/NH4Br produces eryr ro-stilbene dibromide and l-methoxy-2-bromo-1,2-diphenylethane 1 °4-1 in fair yields. 

Though a choice of paths is available, the additions and eliminations of (154) and (155) are anti stereospecific. Two different eliminations occur with the stilbene dibromides in (156) (Kwok and Miller, 1967b), but they are both anti which path is taken is determined by conformational energy factors to be discussed later. 

A more general application of (210) and (211) is found in unpublished data from the author s laboratory, on debrominations of the stilbene dibromides with lithium bromide or stannous chloride. Fig. 27 is a... 

A different effect of these product-like transition states is seen in the fact that the overall debromination of df-stilbene dibromide with lithium bromide may proceed in the syn sense. That is, the rate ratio at 59° k(meso)/k(dl) = 50 is composite. In the dl compound, electronic factors which favor anti elimination collide with steric or conformational factors, which favor either bimolecular syn eliminations or some other path to frans-stilbene. The rate ratio for production of trans-stilbene k meso)jk dl) 60, while that for anti elimination is k(meso)lk(dl) 310. 

A solution of 45 g. (0.25 mole) of /raws-stilbene in 750 cc. of ether is prepared in a 1-1. three-necked, round-bottomed flask fitted with a reflux condenser, an efficient mechanical stirrer, and a dropping funnel. To the well-stirred solution there is added 13.8 cc. (43 g. 0.27 mole) of bromine, during the course of ten minutes. A solid begins to separate in five minutes, but stirring is continued for one hour. The product is collected on a Buchner funnel and washed with ether until it is white. The yield of stilbene dibromide, melting at 235-237°, is 65.8-69.1 g. (77-81 per cent of the theoretical amount). 

A solution of 90 g. of potassium hydroxide in 150 cc. of absolute ethanol is prepared in a 500-cc. round-bottomed flask fitted with a reflux condenser (Note 1). The solution is cooled somewhat, and the stilbene dibromide is added in several portions (Note 2). The mixture is then refluxed for twenty-four hours in an oil bath (Note 3). The hot mixture is poured into 750 cc. of cold water, and the product is removed by filtration and washed with 50 cc. of water. The crude diphenylacetylene is dried over calcium chloride in a vacuum desiccator for eighteen hours at room temperature. The chunky, pale yellow crystals melt at 58-60° and weigh 33.7-37.8 g. By recrystallization of this product from 50 cc. of 95 per cent ethanol there is obtained 29.2-30.5 g. (66-69 per cent of the theoretical amount based on the stilbene)... 

The addition of the stilbene dibromide causes an immediate, vigorous reaction, with the evolution of heat. It is necessary to replace the reflux condenser after each addition, until boiling has ceased. 

Whereas in nonpolar solvents such as CS2 and CCI4, bromine adds to cw-stilbene in a highly stereoselective manner to give 81.4 cmol/mol racemic stilbene dibromide, and to traw-stilbene to give 94.5 cmol/mol meso-stilhenc dibromide, in polar solvents, the degree of stereoselectivity is considerably reduced in both cases. Similar results have... 

Question. Starting with 150 mg of benzaldehyde and assuming an 80% yield on each step, what yield of diphenylacetylene, in grams, might you expect From the information given and assuming an 80% yield on the last two reactions, what yield of stilbene dibromide would you expect employing 150 mg of benzaldehyde ... 

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. 

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