Stereochemical Consequences
of Neighboring Group Participation

Addition of HOBr to a symmetrical alkene is a stereospecific reaction:  different isomers of the alkene lead to different isomers of product.  Convince yourself of this by examining the outcome.  (Note:  this page requires the Chemscape Chime plugin or the WebLab viewer.)

Each reactant forms a bromonium ion.

Each bromonium ion is symmetric, and can be attacked at either ring carbon.  However, the trans-dimethylbromonium ion is chiral; both enantiomers are formed in equal amounts.

The cis isomer leads to two enantiomeric products, the (2R, 3R) and the (2S, 3S):
(Which is which?  Assign the configurations.)

The trans isomer leads to a slightly different situation, the pair is now (2R, 3S) and (2S, 3R):

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Last updated: 09/21/00
Comments to K. Gable