Aryl substituent effects on the thermal interconversion of cyclobutenediones and 1,2-bisketenes

Abstract

The rates and activation parameters are reported for ring closure of 1,2 bisketenes OCC(C₆H₄X)C(SiMe₃)CO (7), generated by photolysis of the corresponding cyclobutenediones 6. The rates at 70 °C are correlated with the σ_p⁺ constants of the aryl substituents by the relationship log k=–1.10 σ_p⁺–3.60 (r= 1.000). The greater reactivity in ring closure to 6 for electron-donating aryl substituents is consistent with stabilization of the electron-deficient cyclobutenedione ring by electron donor groups, ketene stabilization by π-acceptor groups and ketene destabilization by electron-π-donor groups, as has been previously postulated based on theoretical calculations. The equilibrium concentration of 7d(X = CH₃ CO) was also measured and varied from 1.6% at 69.2 °C to 9.7% at 143.1 °C, exceeding by factors of 5.4–7.5 those for 7a (X = H). This favouring of the bisketene by CH₃CO correlates closely with the effect on the reaction rates.