Hexatriene bromination

The first synthesis of thiepin 1,1-dioxide (131) was performed by Mock in 1967 75>. With exess bromine in chloroform 2,7-dihydrothiepin 1,1-dioxide (129), prepared by 1,6-addition of sulfur dioxide to m-hexatriene, gave a dibromide 130 which, on treatment with two equivalents of triethylamine, afforded 131. Upon catalytic reduction, 131 rapidly absorbed three molar equivalents of hydrogen to yield hexahydrothiepin 1,1-dioxide (132). The 1,1-dioxide 131 is a fairly stable compound ... 

ProNcm 8.11 Addition of one mole of bromine to 1,3,5-hexatriene yields oply 5,6-dibromo-l,3-hexadiene and l,6-dibromo 2,4-hexadiene. (a) Are these prodUicts... 

The increased reactivity of this extended tt system is due to the low activation barriers for electrophilic additions, which proceed through highly delocalized carbocations. For example, the bromination of 1,3,5-hexatriene produces a substituted pentadienyl cation intermediate that can be described by three resonance structures. 

Upon treatment with two equivalents of bromine, 1,3,5-hexatriene has been reported to give moderate amounts of l,2,5,6-tetrabromo-3-hexene. Write a mechanism for the formation of this product. 

Hexatriene reacts with IM equivalent of bromine to give only 1,2- and 1,6-addition products. Write the structures of these products. Why there is no 1,4-addition product ... 

Use for Alkene Preparation. Replacement of two alcohol groups by bromine, followed by zinc dehalogenation, provides an interesting alkene synthesis methodology (eq 12). The preparation of 1,3,5-hexatriene is readily accomplished via a PBrs step (eq 13). In the presence of DMF, PBry has been used to eliminate a hindered diallylic tertiary alcohol (eq 14). See also Phospho-rus(HI) Bromide-Copperil) Bromide-Zinc. 

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