1.3- Dibromocyclopentane

Step 2 of the mechanism m Figure 6 12 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion For reasons that will be explained m Chapter 8 reactions of this type normally take place via a transition state m which the nude ophile approaches carbon from the side opposite the bond that is to be broken Recall mg that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only trans 1 2 dibromocyclopentane m accordance with the experimental observations... 

The malonic ester required for synthesis of cyclopal (107) can be obtained by alkylation of diethyl allylmalonate (115) with 1,2-dibromocyclopentane in the presence of excess base. It is probable that the reaction proceeds by elimination of hydrogen bromide from the dihalide as the first step. The resulting allilic halide (116) would be the most reactive electrophile in the reaction mixture and thus would quickly alkylate the anion of the malonate to afford 117. 

Problem 9.28 Starting with cyclopentanol, show the reactions and reagents needed to prepare (a) cy-clopentene, (b) 3-bromocyclopentane, (c) 1,3-cycIopentadiene, (d) tranj-1,2-dibromocyclopentane, (e) cyclopentane. ... 

Cyclopentene Bromine Br rrans-1,2-Dibromocyclopentane (80% yield none of the cis isomer is formed) ... 

Although there are two enantiomers of trans-1,2-dibromocyclopentane, there is only one epimer of it. Inversion of one chirality centre in either of the two enantiomers always leads to the same cis-configured compound. 

What reagents are needed to convert cyclopentene into (a) bromocyclopentane (b) trans-1,2-dibromocyclopentane-(c) 3-bromocyclopentene ... 

With molecular models, verify that an analogous nucleophilic attack at C(2) of 36 gives the same mesa compound. Confirm also that the configuration of 37 is (2R,3S). Show that cyclopentene (38) reacts similarly via a bromonium ion intermediate to give trans-1,2-dibromocyclopentane (39). 

Bromine is a diatomic molecule that reacts with cyclopentene to form 1,2-dibromocyclopentane. Nitrogen is also diatomic but does not give a similar reaction. Why not ... 

FIGURE 10.13 This anti addition of the two bromine atoms requires the formation of trans-1,2-dibromocyclopentane. 

In many systems, variations in the abundances of certain fragment ions in the mass spectra of stereoisomers have been ascribed to anchimeric assistance. The loss of bromine from the molecular ions of stereoisomeric 1,2-dibromocyclopentanes... 

FIGURE 1.4. Cis- and ira/ii-isomers of 1,2-dibromocyclopentane and cyclopropane-1,2-dicarboxylic acid. 

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