Orbital Symmetry and the Photochemical Ring Opening of Cyclobutene

Abstract

Unlike most excited state pericyclic reactions, the photochemical electrocycloreversion of substituted cyclobutene derivatives proceeds non-stereoselectively.' Orbital symmetry selection rules2 and numerous computational ~ t u d i e s ~ ~ predict that, for a four-electron system such as cyclobutene/l,3butadiene, photochemical interconversion should proceed in disrotatory fashion. While substituted 1,3-butadienes do indeed undergo photochemical ring closure with predominant disrotatory stereochemistry,'0-'2 direct irradiation of alkyl-substituted cyclobutenes results in the formation of mixtures of both the formally-allowed and -forbidden diene isomers, in relative yields that vary substantially from system to system.-l3 There are a number of possible mechanistic explanations for this, but the one that is most compatible with orbital symmetry selection rules involves stereospecific, disrotatory ring opening entirely on the excited singlet state surface to yield a single, electronically excited diene isomer which decays to the ground state by cis,trans-isomerization. It is well established that the photochemical ring opening of Dewar aromatics follows an analogous p a t h ~ a y . ' ~ . ' ~ Furthermore, spectroscopic resultsI6 and an analysis of the effects of steric factors on the quantum yields for ring opening of monocyclic cyclobutene~'~ both support the suggestion that, as this mechanism demands, orbital symmetry is obeyed in the early stages of the reaction on the excited singlet state surface. We have published results which are consistent with the adiabatic ring opening mechanism, but within a rather large margin of error owing to the necessity of comparing product ratios from cyclobutene ring opening with ratios calculated from absolute quantum yields for diene cis,trans-photoisomerization.Is This comparison is valid only with systems whose isomeric dienes exist exclusively in the s-cis conformation (the conformation in which they would be formed during ring opening), since the excited singlet state decay characteristics of conjugated dienes are conformation-dependent..'2 We now report the