Photoisomerizations of Cyclohepta-2,4-dienones

Abstract

<p>This thesis has resolved some rather fundamental and rudimentary questions concerning the photochemistry of cyclohepta-2,4-dienones.</p> <p>A new approach has been taken to assess the role of (π,π*) excited states in the photoisomerizations of 2,6,6-trimethylcyclohepta-2,4-dienone, eucarvone. It involved an investigation of the low temperature photorearrangements of protonated eucarvone in fluorosulphuric acid. The results, in additions to those obtained from a re-examination of neutral eucarvone photochemistry, have demonstrated that both (π,π*) and (π,π*) states are involved in the formation of primary photoproducts. A scheme has been formulated which relates these products to these excited states.</p> <p>An investigation of the photoisomerizations of cyclohepta-2,4-dienone and 2-methylcyclohepta-2,4-dienone has revealed that substituents have a profound influence upon the mode of reaction of the (π,π*) states of this dienone system. Whereas eucarvone was found to produce 3,7,7-trimethylbicyclo[4.1.0]hept-2-en-4-one from this state, the less substituted systems yielded only bicyclo [2.2.1]hept-2-en-7-ones. This substituent effect has been interpreted mechanistically in terms of an intermediate in the eucarvone reaction in which positive charge is developed upon the carbon bearing the two methyl groups.</p> <p>A novel synthesis of cyclohepta-2,4-dienone has been developed which involved as the key step the thermal isomerization of protonated cyclohepta-3,5-dienone.</p>