Olefins 1,2-dihalogenation

Complexes of Dihalogen Acceptors with Aromatic and Olefinic ti-Donors. 146... 

The dihalogen complexes with olefin donors were first identified spectroscopically in the mid-1960s [42-45] and extensive experimental and computational studies have been carried out by Chiappe, Lenoir and coworkers in recent years [46 - 48 ]. These systems are highly unstable, since the complexation of dihalogens with olefins is followed rapidly by the formation of ionic intermediates and further chemical transformations. Therefore, attention in the corresponding work has mostly focused on hindered olefins, although the spectral characteristics of complexes with less sterically crowded and alkyl- as well as chloro-substituted and cyclic olefins are also reported [44]. The absorption maxima for the dihalogen complexes with olefins (evaluated by the subtraction... 

In spite of the numerous spectral observations of complex formation between aromatic and olefinic donors with the dihalogens, the preparations of the corresponding crystalline complexes have been hindered by their enhanced reactivity (as well as the relatively weak bonding). As such, only few examples of the X-ray structural characterization of the corresponding intermolecular associates are reported, the most notable exception being the dibromine complex with benzene. 

It is should be noted that high reactivity precludes the X-ray structural characterization of the ir-complexcs between dihalogens and olefinic acceptors. Indeed, quantum mechanical calculation of the interaction between... 

In the case of particular olefins115, such as 1,2,3-tri-r-butylbutadiene (56), which is strongly sterically hindered, the usual dihalogenated products cannot be obtained. Instead of the addition process, the elimination of the positive charge by a proton removal takes place73, as Scheme 26 illustrates. This reaction can be classified as an electrophilic substitution of the olefins. 

A conjugated double bond system undergoes 1,4-addition (Thiele s rule) for example, butadiene and an equimolar quantity of bromine yield 1,4-dibromo-2-butene (90%). On the other hand, chlorination of butadiene in the liquid or vapor phase furnishes about equal amounts of 1,2-and 1,4-addition products. Other polyfunctional compounds resulting from this method of preparation include dihalogenated acids, esters, aldehydes, and ketones. < The addition of bromine to unsaturated ethers yields dibromo ethers which are used as intermediates in the synthesis of olefins (method 21) and olefinic alcohols (method 99) ... 

Hydrogen halides can be eliminated either from 1,1-, or 1,2-dihalogenated hydrocarbons or from 1-halo olefins to yield acetylenes in good yields. 

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