Regio- and Stereochemistry of Bromochlorinations of Alkynes with molecular Bromine Chloride and Dichlorobromate(1-) Ion

Article abstract of DOI:10.1246/bcsj.59.3515

The bromochlorination of phenyl- and alkyl-substituted acetylenes with tetrabutylammonium dichlorobromate(1-) (1) in dichloromethane was found to be anti-stereospecific and nonregiospecific (regiospecific in the case of phenylacetylene).Whereas the addition of molecular bromine chloride (2) to phenyl-substituted acetylenes was found to give nonstereospecific and regiospecific adducts, the reaction of alkyl-substituted acetylenes gave anti-stereospecific and nonregiospecific adducts.These results suggest that the addition of 1 involves an attack of chloride ion to a three-centered ?-complex in the product-forming stage, and that the addition of 2 to phenyl-substituted acetylenes involves a vinyl cation intermediate (but a bridged bromonium ion intermediate in the case of alkyl-substituted acetylenes).