113120-87-9Relevant academic research and scientific papers
The Thermal Cycloaddition of Substituted Thiobenzophenones with Phenylallene: Mechanism and Linear Free Energy Relationships of Reaction Rates
Kamphuis, Johan,Ruijter, A.Peter,Hendrik, J.T.Bos
, p. 907 - 912 (2007/10/02)
The reaction rates for the thermal cycloadditions of substituted thiobenzophenones with phenylallene in CH3CN at 40 deg C were determined and analysed by linear free energy correlations.Using a Hammett-type relationship, the best correlations were obtained with the ?- -parameter (ρ- = 1.09+/-0.06) indicating that, in the transition state, mesomeric interaction between -M substituents and an electron-releasing reaction centre.The small ρ--value is in agreement with a 1,4-duiradical intermediate in a (?2S + ?2S + ?2S)-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 sensitive to electron-donating substituents than to electron-withdrawing substituents i.e. a ?-repulsive saturation interaction occurs with the ?-acceptor thiocarbonyl function.Additional analysis with Charton's DSP-equation (ρR/ρl = 1.82) shows that the contribution of resonance effect is 69percent and the contribution of inductive effect is 31percent.Bromilow's DSP-non-linear resonance effect equation yields a value of +0.38+/-0.01 for the 'electron demand parameter', indicating the enhanced resonance interaction of electron-acceptor substituents with the thione system in the transition state.An inverse secondary isotope effect on the rate constant of disappearance of thione during the reaction with , , and 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 instead of , 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 (+)- 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 (?2S + ?2S + ?2S)-cycloaddition.
