Chlorine to alkenes

Attempts to control the stereochemistry of the addition of chlorine to alkenes using ephedrinium and cinchoninium salts give only low optical activity [31]. 

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

Dichlorination of cyclopentane under free-radical conditions is not a realistic approach to the introduction of two chlorines in a trans-1,2 relationship without contamination by isomeric dichlorides. Vicinal dichlorides are prepared by electrophilic addition of chlorine to alkenes. The stereochemistry of addition is anti. 

The study of the addition of chlorine to alkenes is even older than the study of bromine addition. The synthesis of 1,2-dichloroethane was first reported by a group of Dutch chemists in 1795, and Michael Faraday investigated the addition of chlorine to ethene in 1821 In many ways the addition of chlorine to alkenes is similar to the addition of bromine to alkenes. For example, each additional alkyl group increases the relative reactivity of an olefin by a factor of 50 to 100 (Table 9.3). A Taft p a correlation is observed, and the p for chlorine addition is —2.9, which is similar to the value of —3.1 for addition of bromine. This result suggests that charges on the alkenyl carbon atoms in the transition structures for addition of chlorine are similar to those in the addition of bromine. ... 

There are significant differences between the bromine addition and chlorine addition reactions, however. The addition reaction of ethene and chlorine is exothermic by 44 kcal/mol, which is 15 kcal/mol more exothermic than the addition of ethene and bromine. Poutsma noted that addition of chlorine to alkenes in nonpolar solvents can occur by either radical or ionic pathways, but that oxygen inhibits the radical reaction. For example, addition of chlorine to neat (i.e., not diluted by solvent) cyclohexene (19) gave frans-l,2-dichlorocyclohexane (20), 3-chlorocyclohexene (21), and 4-chloro-cyclohexene (22) in a 1.95 1.00 0.60 ratio when the reaction was carried out imder a nitrogen atmosphere. In the presence of oxygen, the ratio was 3-4 1.00 0. ... 

A small amount of asymmetric induction is observed in the additon of chlorine to alkenes and in the s)mthesis of 0-acetylcyanohydrins using chiral quaternary ammonium salts. ... 

We explain the addition of bromine and chlorine to alkenes and its anti stereoselectivity by the following two-step mechanism. 

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