Alkene group bromination

Even when the alkene is conjugated with an electron-withdrawing group, bromine addition still occurs, though less readily. As we said, alkenes are nucleophilic. 

Introduction of a bromoalkyl group (derived from an alkene and bromine) to the aromatic ring via titanium intermediate sets the stage for elaboration of an indole nucleus. ... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

In aqueous solution chlorine and bromine react with alkenes to form vicinal halohy drins, compounds that have a halogen and a hydroxyl group on adjacent carbons... 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

This neighbouring group participation by bromine (cf. p. 93) does not of course prove that addition to alkenes proceeds via cyclic bromonium ions, but it does mean that such species are no longer merely ad hoc assumptions, and to that extent are correspondingly more plausible as intermediates. 

For alkene bromination, present kinetic data show that their transition states are always bridged, whatever the number of alkyl groups on the double... 

Fig. 4 Symmetrical charge distribution in bromonium-ion like transition states in alkene bromination (data from Bienvenue-Goetz and Dubois, 1978). The effect of the R-substituent depends neither on the number nor on the position of methyl groups on the double bond.

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

Fig. 9 Comparison of polar and steric effects of alkyl groups on bromination rates of linear ( ), branched (O) and adamantyl (A) alkenes in acetic acid and in methanol (Ruasse and Zhang, 1984 Ruasse et al., 1990). Polar effects are identical in both solvents [full line, eq. (24)], but steric effects differ. Deviations of branched alkenes are attributed to steric inhibition of nucleophilic solvation by methanol.

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

An extended form of (39) has been used to analyse kinetic substituent effects on bromination of alkenes GRaC=CR()R) , where G is a conjugatively electron-donating group and R is alkyl (Bienvenue-Goetz and Dubois, 1981). 

Bromination rates of aliphatic enol ethers have been included in the interactive treatment of alkenes GRIC=CR R, with G being a conjugated group most of them fit the multiparameter equation (41) satisfactorily. A more detailed analysis of reactivity-selectivity effects in the reaction of 1-ethoxyethylene [22] and its a- and / -methyl analogues [23] and [24] has been carried out,... 

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