Halogenation of olefins

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

The fact that the partially saturated derivative (64) is obtained only in the trans form (these reactions are regio- and sterespecific) indicates that the bromonium ion, 65, which is similar to that usually reported for halogenations of olefins, precedes the saturation of the C=C double bond. 

Ethers, particularly cyclic ethers, are another group of nucleophilic solvents engaged in cohalogenations. Halogenation of olefins in presence of epoxides leads to the formation of / ,/ -dihaloethers (equation 52)432. 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

Important reactions belonging to this class have been studied sonochemically, the epoxidation and halogenation of olefins. 

All the products gave low optical yields with 0-34% ee. Furthermore, asymmetric halogenation of olefins with CD as a chiral matrix has not been investigated in the past apart from our reports [8-10]. 

The halogenation of olefins has been studied with trichloramine in nonpolar solvents with complexed copper(ii) halides with iodobenzene... 

Sergeev, G.B. and Smirnov, V.V., 1985, Molecular Halogenation of Olefins, (in Russian), Izdatelstvo Moscow University, Moscow. 

Although the field of organocatalysis has flourished over the past decade, there are still some areas that remain virtually untouched, such as the asymmetric a-halogenation of olefins. Several enanhoselective halolactonization reactions of exocyclic carboxylic acids have been reported since 2010 [41]. Borhan and coworkers reported the first highly enanhoselective chlorolactonization of alkenoic acids... 

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