Chapter 5 - 4n Systems Based on 1,3-Electrocyclization

Abstract

The photochromic behavior of many classes of compounds is based on electrocyclizations. This chapter describes photochromic systems with four electrons which undergo 1,3-electrocyclic reactions and describes 1,5-electrocyclizations involving six electrons. An outstanding feature of electrocyclic reactions is their stereospecificity. The orbital symmetry conservation principle in the form of the Woodward-Hoffmann rules predicts the following behavior for symmetrical hydrocarbons: when the number of interacting electrons in the cyclic array is 4n (n = 1,2,..) the thermal electrocyclic reactions proceed via conrotatory pathways, the electrocyclic reactions, which occur as primary photochemical processes, via disrotatory pathways. The rules may be applied to slightly disturbed systems. The question that arises, concerns the application of the Woodward- Hoffmann rules to unsymmetrical substances including both highly substituted hydrocarbons as well as compounds containing heteroatoms.