Chapter 4. Reaction mechanisms. Part (i) Pericyclic reactions

Abstract

The pericyclic reactions of singlet oxygen' and of the photoexcited states of organic anions2 have been reviewed.The effect of substituents on pericyclic reactions has long remained a source of difficulty. This year two interesting approaches to the problem have been described. Carpenter and Wilcox3 have used a simple HMO model. They have shown that for electrocyclic reactions, 1,5-hydrogen shifts, cyclopropane stereomutations, and reactions involving carbon-carbon bond homolysis, there is a linear relationship (1) between AAH* (the measured effect of the substituent on the enthalpy of activation in kcal mol-l) and AAE, (the calculated difference between the effect of the substituent on the HMO .rr-energy of the substrate and that of a suitable model for the transition state, both in units of p. Mobius cyclobutadiene is taken as a model for the transition state in the conrotatory opening of cyclobutene).