Vinyl Cation Intermediates in Electrophilic Additions to Triple Bonds. 1. Chlorination of Arylacetylenes

Article abstract of DOI:10.1021/jo01300a023

The products of ionic addition of chlorine to phenylacetylene, β-methylphenylacetylene, 4-methylphenylacetylene, β-ethylphenylacetylene, and tolan have been investigated in anhydrous acetic acid.The major products are the α,β-dichlorostyrenes arising from simple 1,2-addition, but significant yields of solvent-incorporated products are also found.In some cases significant yields of β-chlorophenylacetylenes are found, presumably arising from an addition-elimination process.No products arising from addition of 2 mol of Cl2 were observed.The reactions are clearlynonstereospecific and show only weak stereoselectivity varying from predominant syn to predominant anti addition.However, the reactions are all completely regiospecific in the Markovnikov sense.In the presence of low concentrations of added salts such as lithium chloride, acetate, and perchlorate, the product distribution and stereochemistry are hardly affected.Only at high concentrations of these salts is there any significant change in product distribution.The second-order rates of addition have been measured for five additional phenyl-substituted compounds.The seven ring-substituted phenylacetylenes show an excellent correlation with ?⁺, giving a large negative ρ value (-4.19).The effects of β-substitution on the rate of chlorination are very small.The results are interpreted in terms of a simple Ad-E2 process, in which the rate-determining transition state is an open vinyl-cation-like species, with most of the positive charge being developed at C_α.The subsequent product-determining intermediate is considered to be a tight ion pair between an open α-phenylvinyl cation and a chloride counterion.This ion pair can react by ion-pair collapse, solvent attack, or internal proton elimination.Activation parameters determined for three of the above compounds show that the higher rates of chlorination (over bromination) of the acetylene system are due almost entirely to lower Δ H(excit.) values.