1-bromo-2,2-diphenylethylene, reaction

A mixture of (triisopropyl phosphito)copper(I) bromide (17.6 g, 0.05 mol) and l-bromo-2,2-diphenylethylene (9.1 g, 0.035 mol) was heated at 200°C for 1 h under a nitrogen atmosphere in a flask equipped with a Vigreaux column topped by a Dean-Stark trap. The alkyl halide produced in the reaction was collected in the trap. After cooling, the reaction mixture was poured into toluene (60 ml), and ethylenedi-amine was added (5 ml). After filtering and washing the precipitate with toluene, the combined toluene solutions were washed with 10% hydrochloric acid (10 ml) and water (10 ml), dried over magnesium... 

Korneev and Kaufmann successfully lithiated 2-bromo-l,l-diphenylethylene (46) by bromide-lithium exchange to form 2-lithio-l,l-diphenylethylene (47). A second lithia-tion could be effected in four hours at room temperature by deprotonation of the aromatic ring with w-butyllithium in the presence of TMEDA (Scheme 17). Like in the synthesis of compound 23, the first lithiation activates the ortho-hydrogen atom of the Z-phenyl substituent to give 1,4-dilithium compound 48. In total, three equivalents of the alkyl-lithium base are required the third equivalent is consumed in the trapping reaction of w-bromobutane with generation of octane. 

The competitive nature of these two reactions is determined primarily by the leaving group. l-chloro-2,2-diphenylethylene yielding mainly the acetylene, whilst the bromo and iodo substrates afford mainly the carboxylic acid when the reaction mixture is treated with carbon dioxide. 

The E2 reaction of most compounds is known to proceed preferentially by removal of a proton anti-periplanar to the leaving group. Based upon this generalization, predict the geometry of the l-bromo-l,2-diphenylethylene that is produced as the intermediate in the double dehydrobromination of meso-stilbene dibromide. 

If meso-stilbene dibromide is treated with KOH in ethanol, it is possible to isolate the l-bromo-l,2-diphenylethylene that is formed from the first dehydrobromination. The E2 elimination of the second molecule of hydrogen bromide from this intermediate alkene to give diphenylacetylene has a higher activation enthalpy than the first elimination and thus requires a higher reaction temperature. Explain. 

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