Vinyl bromides hydrobromination

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

In gas-phase hydrobromination, where a radical mechanism is operative, the bromine atom always adds to the central carbon atom of the allenic system. As a result, vinylic bromides are formed through the stable allylic radical. In the solution phase under ionic addition conditions, either the vinylic or the allylic cation may be the intermediate, resulting in nonselective hydrobromination. Allylic rearrangement or free-radical processes may also affect product distributions. 

The hydrobromination of arylalkynes in acetic acid produces good yields of vinylic bromides in which syn addition is favored over anti addition by -3 to 1 (equation 112)-90 6 - 62... 

The hydrobromination of functionally substituted allenes has received little attention. Vinylic allenes generate mixtures of bromides,76 while tetrafluoroallene affords the allylic bromide,83 and allenic acids produce 3-bromo-3-alkenoic acids (equation 109). 37,138... 

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See also in sourсe #XX -- [ ]

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