Diphenylacetylene bromination

Chromone-3-carbonitrile oxide undergoes cycloaddition reactions with phenyl-and diphenylacetylenes to give isoxazoles 205 (R = H, Ph). The nitrile oxide is obtained from 3-formylchromone oxime by bromination and subsequent dehy-drobromination (175). 

When diphenylacetylene dissolved in cold chloroform reacts with iodine monofluoride suspended in trichlorofluoromethane, the iodine atoms in the primary addition product are easily replaced by fluorine to give l,1,2,2-tetrafluoro-1,2-diphenylethane (60%) along with benzil (10%). Since the C —Br bond is stronger than the C —I bond, the reaction of bromine monofluoride with diphenylacetylene gives 1,1-dibromo-2,2-difluoro-1,2-diphenylethane (65%) and benzil (15%). 

It is evident from the above arguments that the addition of halogens to alkynes may occur by a multiplicity of mechanisms. One more argument is offered by a kinetic study of the addition of bromine to diphenylacetylene in bromobenzene reported by Sinn et al. (1965). In this case kinetic data are better interpreted on the basis of a nucleophilic mechanism involving the intermediacy of a vinyl anion, rather than an electrophilic reaction. 

Synthesis of an Alkyne from an Alkene. Bromination and Dehydrobromination Stilbene and Diphenylacetylene... 

In this experiment, benzoin, prepared in Chapter 54, is converted to the alkene fraws-stilbene ( -stilbene), which is in turn brominated and dehy-drobrominated to form the alkyne, diphenylacetylene. 

Dehalogenation is not important for preparation of acetylenic compounds. A few vicinal dibromoalkenes have been dehalogenated by metals — usually finely divided metals in an appropriate solvent. For example, diphenylacetylene may be made by treating l,2-dibromo-l,2-diphenylethylene with zinc in acetone (yield 85%) 200 and about 80% of fluoroacetylene is formed on removal of bromine from 1,2-dibromo-l-fluoroethylene with magnesium in tetra-hydrofuran 201 M... 

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. 

Discuss the differences observed in the NMR spectra of nxeso-stilbene dibro-mide and diphenylacetylene that are consistent with the double dehydro-bromination in this experiment. 

Purpose. You will synthesize the second intermediate in the b series of Sequential Reactions by carrying out the bromination of (E)-stilbene to obtain meso-stHbene dibromide. This product is the precursor to diphenylacetylene, the next synthetic intermediate in the b series. A further purpose of this experiment is to demonstrate the stereospecific addition of bromine to alkenes. 

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... 

The complex (OT-Ph5Cs)2Mo is of interest since the unsubstituted analogue, molybdenocene, is unknown. It is one of the products of the reactions between diphenylacetylene and molybdenum hexacarbonyl [116], and it forms red crystals which are rather unreactive. Oxidation with bromine gives a green, paramagnetic (3 5 B.M.) salt of stoicheiometry... 

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