Mechanism of the electrophilic iodination of 1,3-diarylallenes

Abstract

Kinetic data are given for the addition of iodine to 12 substituted 1,3-diphenylallenes in 1,2-dichloroethane as solvent. A linear Hammett plot, based on σ⁺ values, is found for diphenylallenes substituted in one ring, providing a ρ value of –3.2 at 5°. This has been taken as evidence that the transition state resembles a planar 1,3-diphenylallyl cation. The conclusion is supported by the observation that the addition is accompanied by racemisation, and that racemisation and addition rate constants are equal at the same temperature. The ρ value for the iodination of 1-phenyl-3-(p-tolyl)allenes, substituted in the phenyl ring is only –2.3 at 5°. The diminished influence of substituents in this series is ascribed to the alteration of the charge distribution in the transition state by the fixed p-methyl substituent in the other ring.