Chemoselectivity, bromination alkenes

COMPLETELY REGIO- AND CHEMOSELECTIVE BROMINATION OF fflGHLY CONJUGATED ALKENES... 

The Markovnikov regioselectivity of the gem-alkenes is associated with a chemoselectivity. in favour of methanol attack, significantly greater than that observed for the other alkenes. If no sodium bromide is added to the reaction medium, no dibromide is observed for this series. Therefore, these alkenes behave as highly conjugated olefins, as regards their regio- and chemo-selectivity. In other words, the bromination intermediates of gem-alkenes resemble P-bromocarbocations, rather than bromonium ions. Theoretical calculations (ref. 8) but not kinetic data (ref. 14) support this conclusion. 

The chemoselectivity of bromination going through bromocarbocations (highly conjugated olefins and also gem-alkenes ) is 100 % in favour of methanol, a nucleophile stronger than bromide ions. However, when the intermediates are bromonium ions, the chemoselectivity is poor. Branched substituents seem to favour the dibromide over the solvent-incorporated adduct, although the bromide ion is considered to be a bulkier nucleophile than methanol. 

The solvent has no influence on the stereoselectivity of bromine addition to alkenes (Rolston and Yates, 1969b), but it could have some effect on the regioselectivity, since this latter depends not only on polar but also on steric effects. Obviously, it modified the chemoselectivity. For example, in acetic acid Rolston and Yates find that 2-butenes give 98% dibromides and 2% solvent-incorporated products whereas, in methanol with 0.2 m NaBr, dibromide is only about 40% and methoxybromide 60%. There are no extensive data, however, on the solvent effects on the regio- and chemoselectivity which would allow reliable predictions. 

Hydrogen atoms in the benzylic position can be replaced by elemental bromine as shown. This is not true for hydrogen atoms in the allylic position. The alkene reacts rapidly with molecular bromine via addition and allylic bromination is not observed (Figure 1.25, left). A chemoselective allylic bromination of alkenes succeeds only according to the Wohl-Ziegler process (Figure 1.25, right), that is, with A-bromosuccinimide (NBS). 

Chemoselectivity is often easy to control in alkenes because the alkene is a weak functional group. It does have inherent reactivity with electrophiles such as bromine but when another functional group, whether electron-donating or electron-withdrawing, is conjugated to the alkene, the reactivity is normally dominated by the other functional group. Compounds 2-4 would normally not be described as alkenes but as benzene, an enone, and an enol ether. But they are all alkenes. You will already know many reactions that would happen with one of these compounds and not at all with another. The triene 5 has three alkenes in the same molecule and we shall want to react just one and not the others. Chemoselectivity. 

TABLE 9.2 Chemoselectivity and Regioselectivity in Addition of Bromine to Alkenes ... 

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