Alkenes, bromination

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 7.2). That is,... 

Table 1. Regio- ) and Chemoselectivityb) of Alkene Bromination =) in Methanol, 0.2 M NaBr, at25°C... 

As regards the regioselectivity of monosubstituted, cis and trans disubstituted alkenes, bromination most frequently exhibits anti-Markovnikov behaviour. Only in the case of propene and 1-butene, i.e. when the double bond bears only one linear substituent, bromination in methanol is predominantly but not completely Markovnikov. Steric effects obviously play an important role in determining the... 

Values of mBr in alkene bromination nucleophilic and electrophilic assistance by protic solvents 272... 

Some years later, Kochi et al (Fukuzumi and Kochi, 1981) applied their theory on electron and charge transfers to electrophilic alkene bromination (Kochi, 1988) by comparing the reactivities of various alkenes in bromination and in mercuration. Although the substituent effect trends in the two reactions are totally different, a linear relationship (7) is observed when the reactivities... 

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.

The additivity of substituent effects, demonstrated by (16), was the first kinetic evidence for a symmetrical charge distribution in the rate-limiting transition states of alkene bromination. 

Finally, as shown in Table 13, p for an aromatic ring is also strongly dependent on the other substituents at the double bond it varies from —1.6 to — 5.5 on going from a-methoxystyrenes to stilbenes. This variation, which is related to the well-known non-additivity of multiple substituent effects, and contrasts with what is observed for alkene bromination, is discussed in the next paragraph, devoted to substituent interaction and selectivity relationships in bromination. 

Table 20 Solvent effects in alkene bromination dependence of the electrophilic (KSIEs) and nucleophilic assistances (R) on the alkene.

Fig. 14 Typical log k/YBr plots for assisted and unassisted alkene brominations. Allylbenzene and 1-pentene, less crowded than cis-methyl-t-butylethylene and methylideneadamantane, exhibit the smallest m-values. The points corresponding to acetic acid (O) and trifluoroethanol (A), two weakly nucleophilic solvents, are below the regression line for water, methanol, ethanol and their aqueous mixtures ( ) of similar nucleophilicity. In contrast, they are on the line for the branched alkenes where steric crowding inhibits nucleophilic assistance by alcoholic solvents (Ruasse et al, 1991, Ruasse and Motallebi, 1991).

VALUES OF mBr IN ALKENE BROMINATION NUCLEOPHILIC AND ELECTROPHILIC ASSISTANCE BY PROTIC SOLVENTS... 

In the solvolysis of secondary alkyl sulfonates, competition between nucleophilic solvation and electron donation by the substituents results in a significantly solvent-dependent p, which varies from — 9 to — 1 on going from the non-nucleophilic hexafluoro-2-propanol to 80% aqueous ethanol (Bentley et al, 1981). In contrast, the p -invariance for alkene bromination in H20, M70, MeOH and AcOH [equations (22)-(25)] seems to imply a perfect balance between the two types of charge stabilization. However, this conclusion is probably risky since the nucleophilicities of the solvents implied in (22)-(25) do not vary markedly. Data in non-nucleophilic fluorinated solvents would therefore help to fill the gap in our knowledge. 

Indeed, more intermediates are involved in alkenes bromination than were previously considered. The first intermediates formed in the early steps are the alkene-halogen molecular complexes, whose ionization gives the corresponding bromonium or p-bromocarbenium bromide (tribromide) ion pairs. (2) The reversibility of the ionization step has been widely discussed in the last years... 

Scheme 2. Mechanisms for alkene bromination in aprotic and protic solvents. 

As observed with alkenes, bromine addition to sterically hindered dienes shows a peculiar behavior. Highly substituted dienes, existing predominantly in non-planar conformations, often present a chemical reactivity distinctly different from that of planar... 

The addition of bromine to alkenes is a rapid, exothermic reaction usually taking place at room temperature. In contrast to chlorination, the rate law in bromination depends on the solvent used. On passing from hydroxylic to nonpolar aprotic solvents, the overall second-order changes to a rate law that is first-order in alkene and second-order in bromine.226 Alkene-bromine complexes with varying compositions were shown to form under reaction conditions3,218,227 228(Scheme 6.5). At low bromine concentration in protic solvents the reaction proceeds via a 1 1 complex (23). A 1 2 alkene-bromine complex (25) is involved at high bromine concentration in nonprotic solvents. The ionic intermediates (24, 26) were shown to exist as contact ion pairs, solvent-separated ions, or dissociated ions. 

Micellar effects on alkene bromination have been further studied and strong inhibition (105-106 fold) of the second-order reaction rate constants relative to those in water has been observed. The kinetics and the product distribution suggest that different olefins have different locations at the micellar surface. Kinetics in the presence of added NaBr and n-decane support this hypothesis160. Selective bromination of alkenes using bromine... 

Alkenes Bromine test Permanganate test Solubility in cone, sulfuric acid Positive for all alkenes Positive for all alkenes All alkenes dissolve... 

Heterogeneous reactions lend themselves to continuous flow reactors, which are desirable as they minimise the reacting volume. This reduces operation risks, and allows smaller, more efficient plants to be built. Flow reactors designed for fluorous reactions with both liquid and gaseous substrates have been demonstrated to be effective, at least on a bench scale [49]. Fluorous solvents have also recently found applications as liquid membranes to control the rate of addition of reagents and so control exothermic reactions such as alkene bromination (Fig. 6), and demethylation of anisoles by reaction with boron tribromide [50], This has potential as a clean route as the kinetic control gives improved selectivity. 

Fig. 6. Alkene bromination using a fluorous liquid membrane to control diffusion rates.

These mechanisms should remind you of the mechanism of alkene bromination (p. 504)—except that here the attack on the bromine is assisted by an electron pair on oxygen. The product, instead of being a bromonium ion (which would undergo further reactions), loses a proton (or the Lewis acid) to give a ketone. 

The anti addition is more easily explained it is the result of formation of a bromonium ion, similar, in fact, to the normal mechanism for the bromination of alkenes. Bromine adds from one side of ie alkene and the bromide ion must necessarily form the E-dibromo product regardless of which... 

The bis(trimethylsiloxy)alkene bromination procedure is a large-scale preparation that gives excellent yields of cyclic and acyclic 1,2-diones however, when enolizable 1,2-diketones are produced, some complications can be encountered.7,8... 

Alkene brominations have been widely studied by the Dubois and Ruasse group, and the aryl substituent effects and their dependence upon the alkene structures have been precisely analysed, using the Y-T equation as well as other structure-reactivity relationships. The results have been summarized by Ruasse in her review article (Ruasse, 1993). The results of our analysis by the Y-T equation and related equations are summarized in Table 11. 

It appears that in the rate-determining step of alkene brominations there is also a small specific nucleophilic solvent contribution [513]. In addition it should be mentioned that, not only the rate, but also the stereospecifity of this halogen addition is strongly solvent-dependent cf. Section 5.5.7) [79, 81]. 

Alkene brominations, reactions in fluorous [b]... [f] biphasic systems (FBS)... 

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