Cycloalkenes bromination

Acid-catalyzed epoxide cleavage takes place by back-side attack of] nucleophile on the protonated epoxide in a manner analogous to the fn step of alkene bromination, in which a cyclic bromonium ion is opened 1 nucleophilic attack (Section 7.2). When an epoxycycloalkane is opened 1 aqueous acid, a rans-1,2-diol results, just as a from cycloalkene bromination. 

The reaction of chlorine and bromine with cycloalkenes illustrates an important stereo chemical feature of halogen addition Anti addition is observed the two bromine atoms of Br2 or the two chlorines of CI2 add to opposite faces of the double bond... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

Since (57) had the cis-configuration a detailed NMR spectroscopic investigation of the bromine addition was carried out. In the 13C- and H-NMR spectra at low temperatures as intermediate was found whose signals can be assigned to the un-symmetrically bridged structure (58). However, the corresponding 1,2-dihalo-trans-cycloalkene could not be observed 86). 

Cycloalkenes tethered with a y,5- or 5,8-unsaturated acid side chains react with Brj or I2 to furnish the corresponding halolactones. lodolactonization is more commonly used than bromolactonization since iodine is easier to handle (solid) and is more chemoselective (less reactive) than bromine. Halolactonization with aqueous base is kinetically controlled and proceeds via addition of a Br or B atom to the double bond to form a transient halonium ion. In the absence of strong directing steric effects, formation of the halonium ion may occur at either diastereoface of the double bond. However, only the halonium ion intermediate which allows trans-diaxial Sj. 2 opening by the neighboring carboxylate nucleophile leads, if the intramolecular reaction is sterically favorable, to the lactone. 

From a study of the kinetics of bromination of cycloalkenes, it was concluded that bromination of cyclohexene and cyclopentene proceeds via a cyclic bromonium ion whereas bromination of cyclo-octene proceeds via an a-bromocarbenium ion. Chlorination of conjugated dienes with iodobenzene dichloride gave mixtures of 1,2- and 1,4-dichlorides via radical reactions, e.g. a mixture of cis- and trans-3,4- and -3,8-dichlorocyclo-octenes was obtained from cyclo-octa-1,3-diene. ... 

The addition of bromine and chlorine to a cycloalkene gives a trans dihalocycloal-kane. For example, the addition of bromine to cyclohexene gives trawfrl,2-dibromocyclo-hexane the cis isomer is not formed. Thus, the addition of a halogen to a cycloalkene is stereoselective. A stereoselective reaction is a reaction in which one stereoisomer is formed or destroyed in preference to all others that might be formed or destroyed. We say that addition of bromine to an alkene occurs with anti stereoselectivity. 

The addition of bromine and chlorine to cycloalkenes is stereoselective. Predict the stereochemistry of the product formed in each reaction (See Example 5.7)... 

The addition of bromine or chlorine to a cycloalkene gives a frans-dihalocydoalkane formed as a racemic mixture. The addition of bromine to cyclohexene, for example, gives Irans-1,2-dibromocydohexane ... 

The electron-rich alkene double bond repels the electrons in the bromine molecule to create a partial positive charge on the bromine atom near the double bond. An intermediate bromonium ion is formed, which reacts to give a dibromide derived from anti addition (i.e. the two Br groups add to the alkene from opposite sides). Consequently, reaction of a cycloalkene forms a trans-, 2-dibromocycloalkane. 

There are cases of bromination in which the mechanism shown in Figure 10.9 is exactly correct, but for most alkenes it is not quite right. First, carbocation rearrangements (p. 386) are not observed in most bromination and chlorination reactions. For example, addition of bromine to 3,3-dimethyl-l-butene gives only l,2-dibromo-3,3-dimethylbutane, not l,3-dibromo-2,3-dimethylbutane, as would be expected if an open cation were involved (Fig. 10.10). We already know that if a cycloalkene is used, the product is exclusively the rawr-1,2-dibromide. The cis compound cannot be detected (Fig. 10.8). 

Chlorine and bromine react with cycloalkenes by stereospecific anti addition. 

Acid-catalyzed epoxide opening takes place by protonation of the epoxide to increase its reactivity, followed by nucleophilic addition of water. This nucleophilic addition is analogous to the final step of alkene bromination, in which a cyclic bromonium ion is opened by a nucleophile (Section 8.2). That is, a trans-l,2-diol results when an epoxycycloalkane is opened by aqueous acid, just as a trans-1,2-dibromide results when a cycloalkene is brominated. 

The addition of bromine to cycloalkenes can potentially form two stereoisomeric vicinal dibromides. However, only the trans isomer forms. The reaction is stereospecific. The stereochemistry of the products indicates that the halogen atoms bond from opposite faces of the double bond. 

This cycloalkene can be prepared from the starting material in just two steps. First, radical bromination can be used to selectively install a bromine atom at the tertiary position. And then, the resulting alkyl halide can be treated with a strong base (such as hydroxide, methoxide, or ethoxide) to give an E2 reaction ... 

The necessary cycloalkene can be generated from the starting material in just two steps 1) radical bromination installs a bromine atom in the tertiary position, and 2) subsequent treatment of the resulting tertiary alkyl halide with a strorrg base (such as sodium ethoxide) gives the cycloalkene above ... 

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