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 (π2s+π2s+π2s)-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 (ρR/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 PhCHC
CH2, PhCD
C
CH2, and PhCD
C
CD2 was observed; k2(Hα)/k2(Dα)= 0.91 ± 0.02 and k2(2Hγ)/k2(2Dγ)= 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-CH
C
CH2 instead of H3CO-CH
C
CH2, 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)
C
CHBut 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 (π2s+π2s+π2s)-cycloaddition.