Olefins halogen substituents

In the case of mono-olefins halogen substituents again reduce the basicity, as in the case of benzene. Unfortunately no information is available which would permit a comment as to whether the halogen atom causes this effect only if it is directly linked to the C=C double bond or whether an effect is also still exerted by an adjacent saturated halogen-substituted C-atom. With multiple halogen substitution the basicity falls even more as may be seen from the examples of chlorine-substituted ethylenes in Table 17. 

In contrast to this, very little is known about the ozonolysis of olefins which bear halogen substituents at the double bond. This is somewhat surprising since compounds containing vinyl halide moieties are important technical products whose properties could be adversely affected by ozone degradation. A case in point is neoprene rubber, whose performance as an elastomer could suffer considerably by ozone attack and concurrent crack formation (1). 

Irradiation of Fe(CO) (olefin) complexes leads to olefin exchange . When the olefins bear halogen substituents, the transformation ... 

Certain sample groups i have been excluded from the plots of Fig. 10-10 because they merit special attention. These include olefinic or aromatic carbon atoms (—CH=), halogen substituents, acids, and moderately strong bases (pKj, < 5). The preferential adsorption of olefins and aromatic hydrocarbons on magnesia has already been noted (Section 7-3B). It is of interest to compare the values of an unsaturated carbon group (—CH=) for the different adsorbents with values calculated from Fig. 

Olefinic groups carrying oxygen and halogen substituents are susceptible to... 

The reason for this anomalous behavior of halogenated olefins lies not in antarafacial cycloaddition, but rather in the fact that these reactions are not concerted. They are stepwise processes that proceed by way of a diradical intermediate stabilized by the halogen substituents ... 

When designing substances that contain oleftnic or acetylenic moieties, chemists should keep in mind the potential for these substituents to be bioactivated to electrophilic species and, whenever possible, incorporate other structural changes that lessen the likelihood for bioactivation. Terminal carbon-carbon double or triple bonds should be avoided or at least contain an alkyl substituent on the C-2 carbon (in the case of olefins) and alkyl substituents on the allylic or propargylic carbons. Halogens on terminal unsaturated carbons should be avoided. Aromatic substituents at the allylic or propargylic positions should also be avoided if allylic or propargylic hydrogens are present. 

Stereocontrol in intermolecular cyclopropanation also depends on the structure of the unsaturated substrate. Early work concerning the influence of substrate on stereoselectivity has been summarized by Doyle2. In general, cyclopropanation of ciy-disubstituted alkenes results in higher stereoselectivity than with monosubstituted alkenes and the steric bulk of the olefinic substituent enhances the stereoselectivity. However, the stereocontrol appears not simply to be caused by a steric factor. In comparable cases, the presence of halogen as an alkene substituent may cause a reversal of the normal stereoselectivity. A few examples which illustrate these effects are shown in equations 124167 172, 74. 

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