Bromine addition to alkene

Bromine addition to alkenes is an example of a stereospecific reaction A stereospecific reaction is one m which stereoisomeric starting materials yield products... 

The solvent has no influence on the stereoselectivity of bromine addition to alkenes (Rolston and Yates, 1969b), but it could have some effect on the regioselectivity, since this latter depends not only on polar but also on steric effects. Obviously, it modified the chemoselectivity. For example, in acetic acid Rolston and Yates find that 2-butenes give 98% dibromides and 2% solvent-incorporated products whereas, in methanol with 0.2 m NaBr, dibromide is only about 40% and methoxybromide 60%. There are no extensive data, however, on the solvent effects on the regio- and chemoselectivity which would allow reliable predictions. 

The ability of solvent to influence the rate of formation and subsequent evolution of the bromonium ion or the 0-bromocarbenium ion as key ionic intermediates formed by bromine addition to alkenes are discussed based on kinetic and stereochemical studies as well as theoretical investigations. 

Table 5. Rate coefficients ( br ) of bromine addition to alkenes 1,55,56 and n-ionization energies (tt-IEv) [134]...

Pyridinium bromide, useful for bromine addition to alkenes and a-substitution on aldehydes and ketones, is dangerous to handle. The polymer analog (XXXXI) is easy to handle... 

The mechanism of Figure 10-7 cannot be correct for bromine addition to alkenes in solution for two important reasons. First, notice that this mechanism requires that the two C-Br bonds be formed on the same side of the double bond, and hence produce suprafacial addition. However, there is much evidence to show that bromine and many other reagents add to alkenes to form antarafacial addition products (Figure 10-8). 

To account for the stereospecificity of bromine addition to alkenes, it has been suggested that in the initial electrophilic attack of bromine a cyclic intermediate is formed that has bromine bonded to both carbons of the double bond. Such a bridged ion is called a bromonium ion because the bromine formally carries the positive charge ... 

Instead of a bromonium ion, in certain cases an isomeric acyclic and sufficiently stable cation can occur as an intermediate of the bromine addition to alkenes. This holds true for bromine-containing benzyl cations. Therefore, the bromine addition to /J-methyl styrene shown in Figures 3.5 and 3.6 takes place without stereocontrol. 

In spite of these uncertainties, however, the utility of the reactivity-selectivity principle has been illustrated for a number of diverse areas of mechanistic interest. Such applications are being extended to other areas as well. For example, Olah has recently studied the mechanism of electrophilic addition to multiple bonds using selectivity data and concluded that the transition states of the bromine addition to alkenes are of a 7r-complex nature (Olah and Hockswender, 1974). Finally the large number of reactivity-selectivity relationships which have been discovered offer considerable experimental support for the various expressions and formulations of the Hammond postulate whose profound effect on modem mechanistic chemistry is now beyond question. 

Bromine Addition to Alkenes. Alumina can advantageously replace protic solvents thus avoiding secondary reactions due to their nucleophdicity. This situation is evidenced in the bromation of alkenes [14]. When performed in methanol, bromine addition leads to a mixture of a frans-dibromo adduct and a trans-bromo ether compound. The latter results from competitive attack by pro-tic solvent on the bromonium ion intermediate. This byproduct can be suppressed using Br2/alumina, as the support behaves as a non-nucleophilic polar medium (Scheme 3). 

Bromine addition to alkenes is an example of a stereospecific reaction. A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other. In this case the starting materials, in separate reactions, are the E and Z stereoisomers of 2-butene. The chiral dibromides from (Z)-2-butene are stereoisomers (diastereomers) of the meso dibromide formed from (7 )-2-butene. 

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