Vicinal groups dihalides

Similar kinetics are exhibited by the reduction of vicinal dihalides by Cr(II) , which proceeds 10-40 times faster than that of the allylic halides. Such activation by a second halogen atom suggests a neighbouring group effect, viz. 

Alkynes are prepared from alkyl dihalides via elimination of atoms or groups from adjacent carbons. Dehydrohalogenation of vicinal- or gemiwaZ-dihahdes is a particularly useful method for the preparation of alkynes (see Section 5.4.5). 

The stereochemistry of formation and rearrangement of vicinal dihalides has been elucidated, chiefly by Barton s group.221,266,267 Trans diaxial addition occurs but the product may then equilibrate with the more stable... 

The reaction of an alkene with a halogen like bromine and chlorine generally gives a vicinal dihalide. However, if the reaction is carried out in water as solvent, the product obtained is a halohydrin where the halogen adds to one end of the double bond and a hydroxyl group from water adds to the other. 

Based on the limited number of vicinal dihalides tested thus far, it appears that negative groups on both carbons bearing halogen atoms are necessary for dehalogenation. Thus, methyl 2,3-dichloro- (2,160) and dibromopropionate (2), 2,3-dichloro- and dibromopropionitrile (2), and... 

In each of the examples of 1,2-elimination reactions discussed above, one of the groups eliminated was a proton. Another iype of 1,2-elimination reaction is the dehalogenation of vicinal dihalides. Iodide ion can be used as the dehalogenating agent in both protic and aprotic solvents (equation 10.40). 

The addition of an acid such as HX to an alkene leads to formation of an alkyl halide. If the acid catalyzed addition of water or an alcohol is examined, the product is an alcohol or an ether. Oxymercuration also leads to addition of water to an alkene. The functional group exchange can be generalized as shown, where X = Cl, Br, I, OH, OR. When an alkene reacts with a halogen, the product is a vicinal dihalide. The functional group exchange is shown where X = Cl, Br, I. When bromine in water or chlorine in water is reacted with an... 

Olefin inversion via epoxides is reported by two groups of workers. One sequence involves epoxidation with peracid, which occurs with retention of stereochemistry, followed by deoxygenation with hexamethyldisilane and potassium methoxide in HMPT at 65 °C. The alternative procedure utilizes reaction of the episode with triphenylphosphine dihalides to give vicinal dihalides. Zinc reduction of the dihalides is specifically trans, and thus the sequence epoxidation-bromination-reduction gives overall inversion of olefin configuration. ... 

Some synthetic methods introduce the carbon-carbon triple bond by functional group transformations such as the elimination reaction of vicinal dihalides. Other methods construct complex alkynes by forming carbon-carbon single bonds between two molecules, one of which already contains a triple bond. We discuss both synthetic methods in this section. 

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