Stereogenic centers alkene bromination

BMI. Radical bromination to give (9) (eq 3), Arbuzov reaction, and alkenation lead to ( )-5-alkylidene-Boc-BMI, to which cuprates add highly diastereoselectively with formation of the imidazolidinones (10) containing two new stereogenic centers (eq4). ... 

The stereochemistry of 43 (38) is easy to see because the cyclohexane ring cannot rotate about individual C-C bonds. Does this same stereochemical bias occur with acyclic alkenes When an acyclic alkene reacts with a halogen, the product is an acyclic dihalide, and free rotation is possible about those bonds. However, the answer is yes Acyclic alkenes react with the same stereochemical bias because the mechanism of reaction of an alkene and diatomic halogen is the same for both acyclic and cyclic alkenes. This selectivity is demonstrated with the simple acyclic alkene, 2-butene however, an analysis requires an examination of each stereoisomer, cts-2-butene and rans-2-butene (see Chapter 9, Section 9.4). When cts-2-butene reacts with bromine, the product is a racemic mixture, (2S,3iS)-dibromobutane along with (2i2,3i2)-dibromobutane (see 44). Two new stereogenic centers are created by this reaction. (See Chapter 9, Section 9.3, to review absolute configuration.) When rans-2-butene reacts with bromine, however, the product is a racemic mixture of (2S,3i2)-dibromobutane and (2i ,3S)-dibromobutane (see 45), which are drawn a second time as the eclipsed rotamer (Chapter 8, Section 8.1) to show their relationship to 44. Dibromides 44 and 45 are diastereomers (Chapter 9, Section 9.5). 

We recall that the reaction of bromine with an alkene gives a product with bromine atoms on adjacent carbon atoms (Section 6.6). For example, 2-butene reacts with bromine to give 2,3-dibro-mobutane. Two equivalently substituted stereogenic centers form in this reaction. There are three stereoisomers for such compounds, a pair of enantiomers and a meso compound. Which products would we predict based on the reaction mechanism we discussed in Section 7.6 Put another way, how do the observed products support the proposed mechanism of the reaction ... 

We now can confidently accept the mechanism for addition of bromine to alkenes because it agrees with the experimental facts. We have again found that achiral reactants—in this case either cis- or trans-2-h xtcnc and bromine—always form optically inactive products. Remember the products have two stereogenic centers the reaction produces either a racemic mixture or a meso compound. 

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See also in sourсe #XX -- [ ]

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