Hydrogen bromide conjugated dienes

Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism ... 

This procedure illustrates a recently published, simple, general method for the synthesis of conjugated dienes from olefins. The scope of the reaction is shown in Table I.5 In most of these examples hydrogen bromide. .elimination can be effected by stirring a solution of the olefin-bromo-methanesulfonyl bromide adduct in methylene chloride with one equivalent of triethylamine at room temperature. Only two equivalents of the more costly potassium tert-butoxide are then needed in the second elimination step the yields using the two-base procedure are generally superior to that obtained using only potassium tert-butoxide. 

The first step is a 1,4 addition of hydrogen bromide to the conjugated diene system of isoprene. 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

Allyl radicals substituted at only one of the terminal carbon centers usually react predominantly at the unsubstituted terminus in reactions with nonradicals. This has been shown in reactions of simple dienes such as butadiene, which react with hydrogen bromide, tetrachloromethane or bromotrichloromethane to yield overall 1,4-addition products . The reaction of allyl radicals with hydrogen donors such as thiols or tin hydrides has been investigated and reviewed repeatedly. In most cases, the thermodynamically more favorable product is formed predominantly. This accords with formation of either the higher substituted alkene or the formation of conjugated tt-systems. Not in all cases, however, is the formation of the thermodynamically more favorable product identical to overall 1,4-addition to the diene. In those cases in which allyl radicals are formed through reaction of dienes with tin hydrides or thiols, the... 

Does the structure of conjugated dienes affect their reactivity Although more stable thermodynamically than dienes with isolated double bonds, conjugated dienes are actually more reactive kinetically in the presence of electrophiles and other reagents. 1,3-Butadiene, for example, readily adds 1 mol of cold hydrogen bromide. Two isomeric addition products are formed 3-bromo-1-butene and l-bromo-2-butene. 

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