Bromine with methylcyclohexane

A(-bromosuccinimide (NBS) is, however, one of the best known bromine donors. For exanq>le, it can brominate cyclohexane to give a 30% yield of the corresponding bromide. 7 NBS/dibenzoyl peroxide is not very selective, giving mixtures with methylcyclohexane, although decalin gives tetrabromides in a reasonably well-defined manner (equation 81). 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

A problem arises when l-bromo-2-methylcyclohexane is treated with base. There are two diastereomers one where the bromine and methyl groups are irons... 

While relatively little work has been done on the stereochemistry of 5 2 reactions, the evidence suggests that they do in fact lead to retention of configuration. A good example is provided by the reactions of cis- and trans-methylcyclohexylmercuric bromides with bromine and pyridine to form l-bromo-4-methylcyclohexane, the cis isomer giving almost exclusively the cis product while the trans isomer gives 91% of the trans product. The reactions probably involve an electrophilic (S 2) attack by bromine on the pyridine complexes, as indicated below. 

Reaction of (H Si—CH2)3 with bromine allows selective formation of partially brominated derivatives, because formation of an SiBr group reduces the reactivity of the remaining Si—H bond in the SiHBr group. If appropriate amounts of bromine, dissolved in methylcyclohexane are added dropwise at —50 °C to a 0.2 molar solution of (H2Si—CH2)3 in methylcyclohexane, three bromination products are obtained ... 

---