Vinyl 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,... 

Mixtures of isomeric allylic chlorides are formed when 1,1-disubstituted allenes react with HC1.1,3-Disubstituted allenes yield products of both central and terminal carbon attacks. In contrast, selective transformations occur with HBr. The gas-phase, photocatalytic addition of HBr gives selectively vinylic bromides129 [Eq. (6.20)], while hydrobromination in solution phase yields allylic bromides130 [Eq. (6.21)] ... 

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 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... 

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... 

Dehydrobromination.3 One approach to trialkylsiloxyalkynes involves de-hydrobromination of 1, prepared from tribromoethanol, with BuLi (2 equiv.) to form (Z)-2-bromovinyl silyl ethers 2, which can be converted by LDA followed by alkylation into siloxyalkynes (4). These products undergo cycloaddition with vinyl... 

Step CF2 CF2 O CF2 CF2 NF + Av CF2 CF2 0 CF2 CF2-N. -f F-). Pyrolysis (700 °Q of the cyclobutyl derivative (32) provides perfluoro-(iV-vinyl-morpholine) (35) (86%), photochemical bromination and hydrobromination of which yields the ethyl derivatives (33) and (34), respectively. Reinvestigation of the thmmal decomposition of perfluoro-iV-fluoromorpholine in platinum at 600 °C has shown that the product comprises perfluoro-fiV-methyloxa2 lidine) (37) (17%),... 

Vinyl Halides. The Wittig reaction of bromomethylene triphenylphosphorane with aldehydes is an acceptable route to mainly (Z)-l-brome-alkenes if potassium t-butoxide is used as base to form the ylide. ° 2-Bromo-l-atkenes have been prepared by Markovnikov hydrobromination of 1-alkynes with tetraethylammonium hydrogen dibromide (Et4N HBr2)."°... 

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