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.

Each reactant forms a bromonium ion.

cisMeBr.pdb transbromonium.pdb

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.)

cisbromohydrin1.pdb cisbromohydrin2.pdb

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

trbromohydrin1.pdb trbromohydrin2.pdb
Reference: Rolston, J. H.; Yates, K., J. Am. Chem. Soc., 1969, 91, 1469-1476
Rolston, J. H.; Yates, K., J. Am. Chem. Soc., 1969, 91, 1477-1483.

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Last updated: -5/07/2014
Comments to K. Gable