Bromides from alkenes

Preparation of allylic bromides from alkenes (Section 12.2)... 

Preparation from Other Precursors. Alkyl cations can be formed not only from halide precursors (the earlier investigation of generation from alkyl fluorides was later extended to alkyl chlorides, bromides, and even iodides) but also from alkenes in superacids like HF-SbF5 [Eq. (3.10)]. 

Considering the problem retrosynthetically, we can see that a key intermediate having the carbon skeleton of the desired product is 3-methyl-3-pentanol. This becomes apparent from the fact that alkanes may be prepared from alkenes, which in turn are available from alcohols. The desired alcohol may be prepared from reaction of an acetate ester with a Grignard reagent, ethylmagnesium bromide. 

The trans acetate 25 is the pheromone used to trap pea moths.6 Changing the -alkene into the acetylene allows the disconnection next to what was the double bond. The only problem is how to make a mono-bromide from the symmetrical diol 30. 

Epoxides give rise to many 1,2-difunctionalised compounds such as 48 with control over stereochemistry. Reactions of the epoxide 49 from 44 give the anti stereochemistry in 48 in contrast to the syn stereochemistry in 43. Other compounds made from alkenes include 1,2-bromides and bromohydrins from reaction with bromine alone or bromine and water. 

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. 

An example of an addition of a mineral acid to an alkene that takes place via a tertiary carbenium ion is the formation of a tertiary alkyl bromide from a 1,1-dialkylethylene and HBr (Figure 3.52). 

The key synthetic steps in the formation of the desired unnatural amino acids, 5-8, involved the preparation of appropriately substituted cyclobutanones followed by a Btlcherer-Strecker amino acid synthesis.12 The syntheses of the boron-containing amino acids were initiated utlizing alkenes 16 -19. Alkene 17 was prepared by a SN2 displacement of bromide from 4-bromobenzyl bromide (Scheme 3). In the syntheses of alkenes 18 and 19, a catalytic... 

Aromatic hydrocarbons with saturated side chains are distinguished from alkenes by their failure to decolorize bromine in carbon tetrachloride (without evolution of hydrogen bromide) and by their failure to decolorize cold, dilute, neutral permanganate solutions. (Oxidation of the side chains requires more vigorous conditions see Sec. 12.10.)... 

Base-promoted a-elimination of hydrogen bromide from dibromofluoromethane in the presence of an alkene affords 1-bromo-l-fluorocyclopropanes (see Houben-Weyl, Vol. E19b, pp 1497-1499). 

Addition of bromochlorocarbene (carbenoid) to an alkene results in the formation of 1-bromo-1-chlorocyclopropanes. The preparative value of this process depends on the structure of the carbene source, the type of base used for a-elimination of hydrogen bromide from the haloform, reaction conditions etc. 

Now, recall that the alkyl bromide from which this alkene was obtained must be tertiary. The only way this could happen is if the bromine is positioned on the starred ( ) carbon, that is, the structure must be ... 

Finally, the weak base bicarbonate (HCO3") is enough to remove a proton from the nitrogen atom and allow participation in nitrogen migration by displacement of bromide. This alkene is formed because the C-N+ bond to tertiary carbon is broken preferentially. 

In some cases the addition of bromide to alkene radical cations is reversible. For example, the addition of bromide to the p-methyl-4-methoxystyrene radical cation occurs reversibly, as demonstrated by the formation of the radical cation when the P-bromo radical is generated independently by photolysis of l-(4-methoxyphenyl)-l,2-dibtomopropane (Eq. 18). An equilibrium constant of 2 x 10 M has been measured for the loss of bromide from this radical in acetonitrile. The apparent lack of reactivity of 1,3-dioxole radical cations with bromide ion in water has also been explained on the basis of reversible addition with rapid loss of bromide from the product radical. However, on the basis of the solvent effects noted above, it is also possible that the lack of reactivity in water is a solvent effect since decreases in reactivity of 4 to 5 orders of magnitude have been observed for reactions of bromide ion with styrene radical cations in largely aqueous solvent mixtures. - ... 

In Chapter 24 we introduced the fact that bromine radicals react regioselectively with alk-enes. Let us remind you of one reaction you met then radical addition to an alkene. The product is an alkyl bromide, and is a different alkyl bromide from the one formed when HBr adds to an alkene in an ionic manner. 

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