The thermal cycloaddition of substituted thiobenzophenones with phenylallene: mechanism and linear free energy relationships of reaction rates

Abstract

The reaction rates for the thermal cycloadditions of substituted thiobenzophenones with phenylallene in CH₃CN at 40 °C were determined and analysed by linear free energy correlations. Using a Hammett-type relationship, the best correlations were obtained with the σ^–-parameter (ρ^–= 1.09 ± 0.06) indicating that, in the transition state, mesomeric interaction occurs between –M substituents and an electron-releasing reaction centre. The small ρ^–-value is in agreement with a 1,4-diradical intermediate in a (_π2_s+_π2_s+_π2_s)-cycloaddition.

Separate correlation of electron-donating and electron-withdrawing substituents gives rise to two lines with different slopes, each with a much better correlation coefficient and ψ. This observation indicates that the thione system is relatively more senstive to electron-donating substituents than to electron-withdrawing substituents i.e. a π-repulsive saturation interaction occurs with the π-acceptor thiocarbonyl function. Additional analysis with Charton's DSP-equation (ρ_R/ρ₁= 1.82) shows that the contribution of resonance effect is 69% and the contribution of inductive effect is 31%. Bromilow's DSP–non-linear resonance effect equation yields a value of +0.38 ± 0.01 for the electron demand parameter', indicating the enhanced responance interaction of electron-acceptor substituents with the thione system in the transition state. An inverse secondary isotope effect on the rate constant of disapperance of thione during the reaction with PhCHCCH₂, PhCDCCH₂, and PhCDCCD₂ was observed; k₂(H_α)/k₂(D_α)= 0.91 ± 0.02 and k₂(2H_γ)/k₂(2D_γ)= 0.75 ± 0.01. These values are less than unity and consistent with attack at C-β and indicative for a change in hybridization at C-α and C-γ. The observed isotope effect when using D₃CO-CHCCH₂ instead of H₃CO-CHCCH₂, k₂(H)/k₂(D)= 1.20 ± 0.02 represents a rotational isotope effect arising from an increase in mass on substitution of hydrogen by deuterium. Experiments with an optically active allene (+)-PhC(H)CCHBu^t yielded products which were optically inactive, which points to a non-chiral intermediate i.e. the allylic diradical. We observed also a low solvent effect which is in agreement with the reaction mechanism i.e. a 1,4-diradical-mediated (_π2_s+_π2_s+_π2_s)-cycloaddition.