Vinyl Cation Intermediates in Electrophilic Additions to Triple Bonds. 2. Chlorination of Alkylacetylenes

Article abstract of DOI:10.1021/jo01300a024

The addition of molecular chlorine to 12 alkyl-substituted acetylenes has been studied in anhydrous acetic acid.Disubstituted acetylenes react by predominant anti addition to give both dichlorides and chloro acetates.Monosubstituted acetylenes react by exclusively syn addition and give only 1,2-dichlorides.Acetylene itself does not react at all under the same conditions.Acetylenes substituted with tert-butyl groups react by a variety of pathways to give skeletally rearranged products and none of the expected simple addition products.The rates of chlorination are very fast for the disubstituted compounds, and the second-order rate constants follow Taft's polar substituent constant scale with a ρ of -3.2.However, the rates for monosubstituted compounds are slower than would be expected from this relationship.The tert-butyl derivatives also react more slowly, and some show evidence of steric retardation.The results are consistent with bridge vinyl-cation-like intermediates and transition states only in the case of disubstituted triple bonds.Acetylenes with only one or no alkyl substituents are presumed to react via open vinyl-cation-like species, if at all.Both types of intermediate can react by a variety of pathways to give final products.These include ion-pair collapse, solvent attack, proton abstraction, and 1,2-shifts.The overall mechanisms of chlorination of both alkyl- and aryl-substituted triple bonds are summarized.