Allenes hydrobromination

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)] ... 

Similar differences may be observed in hydrobromination of tetrasubstituted allenes. Ring size and reaction conditions determine the outcome of hydrobromination of cyclic allenes.92... 

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 extreme readiness with which tetrafluoroallene homopolymerises has prevented a study of the orientation of free-radical attack on the allenic system. Perfluoropenta-1,2-diene, however, does not polymerise readily, and so its photochemical chlorination and hydrobromination have been studied. ... 

Though inert towards chlorine at room temperature in the dark, perfluoropenta-1,2-diene is rapidly converted into the saturated tetrachloride, 1,2,2,3-tetrachloro-octafluom-n-pentane (95 %), and a small amount of mixed dimers, (CjFjIj (4%), when photolysed in the presence of an excess of chlorine. Photochemical hydrobromination yields mainly 3/f-2-bromo-octafluoropent-1-ene (34) and 1 W-2-bromo-octafluoropent-2-ene (35), a result which indicates that bromine atom preferentially attacks the central carbon of the allenic system to give an intermediate radical (36) which exists long enough to permit 90° rotation of the orbital containing the unpaired electron, so that it is transformed into an allylic radical (37) (see Scheme 14). 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , , ]

See also in sourсe #XX -- [ , , ]

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