Alkenes from 1,2-dibromides

Ambrettohde is obtained from hibiscus and has a musk like odor Its preparation from a compound A is outlined in the table that follows Write structural formulas ignonng stereochem istry for compounds B through G in this synthesis Hint Zinc as used in step 4 converts vici nal dibromides to alkenes )... 

So we should be able to recognize that we know how to make dibromides from double bonds. We draw the alkene that would have been used to form the product ... 

Higher alkynes can be synthesised from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base(Following fig.). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required. 

We have already mentioned the bulky f-butoxide—ideal for promoting E2 as it s both bulky and a strong base (p aH - 18). Here it is at work converting a dibromide to a diene with two successive E2 eliminations. Since dibromides can be made from alkenes (you will see how in the next chapter), this is a useful two-step conversion of an alkene to a diene, synthesis of a diene by a double E2 elimination... 

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

Bromine addition to alkenes is an example of a stereospecific reaction. A stereospecific reaction is one in which stereoisomeric starting materials yield products that are stereoisomers of each other. In this case the starting materials, in separate reactions, are the E and Z stereoisomers of 2-butene. The chiral dibromides from (Z)-2-butene are stereoisomers (diastereomers) of the meso dibromide formed from (7 )-2-butene. 

This reaction illustrates the double dehydrohalogenation of a vicinal dibromo compound to form an alkyne. It is a useful reaction for the synthesis of alkynes, because the starting dibromides are readily available from alkenes (see, e.g.. Experiment [A2b]). 

That benzene has different chemical properties from ordinary polyenes was well known to the early chemists. Polyenes, or even alkenes, react with a variety of nucleophiles, electrophiles, and free radicals, nnder various circumstances, to yield addition products. For example, bromine reacts more or less instantaneously with alkenes and polyenes, which is readily apparent when one drips bromine into an alkene solution, and the color disappears. The product in the simplest case will be simply a 1,2-dibromide. Benzene, on the other hand, requires prolonged reflnxing with bromine, in the presence of a catalyst, to convert it to bromobenzene. And the latter reaction is not a simple bromine addition, it is a substitution of one hydrogen by one bromine. And a variety of compounds related to benzene (snch as those discnssed previously in this chapter) similarly are aromatic compounds, and chemically quite different from alkenes and polyenes. Why is this ... 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

A more recent report describes the regio- and stereoselective addition of aryltellurenyl iodides (prepared in situ from the corresponding ditellurides and iodine) to alkynes to afford the ( )-l-iodo-2-aryltelluro-l-alkenes, which treated with bromine give the corresponding dibromides. ... 

Reductive elimination from 1,2-dibromides generates the alkene in excellent yields. Conformationally rigid, periplanar tro 5-diaxial, also staggered trans-diequatorial, cyclohexane dibromides all afford the alkene at a mercury cathode [110]. In the bicyclo[2,2,2]octane series, the rra s-2,3-dibromide forms the alkene on dissolving metal reduction [111]. The rigid cis-periplanar 1,2 dibromobicy-clo[2,2,l]heptane, at a mercury cathode, also gives the strained alkene which can be trapped as a furan adduct [112]. 

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