Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrahalo-p-benzoquinones. Part 2. Reaction with bromanil and iodanil

Abstract

The reaction of tetrahalo-p-benzoquinones with triphenylphosphoniocyclopentadienide yields 6-(triphenylphosphonio-3′-cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-dione (4a–d), a new class of zwitterionic dyes containing phosphorus. The rate-limiting step has been found to be the addition of the ylide to the quinone through a highly polar betaine intermediate. The elimination of hydrogen halide from the betaine, is of the E₂ or E₁cB type for the bromanil system and of the E₁ type for iodanil. SCF–AM1 calculations suggest that the E₁cB path is energetically favoured relative to the E₁ elimination path.