Photochemistry of Lewis Acid Complexes of Eucarvone

Abstract

<p>This thesis describes several attempts to determine the multiplicity of the excited states involved in the photoisomerization of 2,6,6-trimethylcyclohepta-2,4-dienone, eucarvone. Attempts were made to quench any triplet state involved in the photorearrangement of protonated eucarvone (1H). However, in view of the low triplet energy of 1H, it was not possible by this method to prove or rule out any involvement of a triplet excited state.</p> <p>A further method used was to try and incorporate a heavy atom into eucarvone and so promote intersystem crossing. To this end the preparation, thermal stability and photochemistry of the boron trihalide complexes of eucarvone, 1-BX₃ (X = F, Cl and Br) have been studied. Eucarvone reacts with various Lewis acids to generate o-complexes in which the Lewis acid is bonded to the carbonyl oxygen. The zwitterions have been characterized using pmr, ¹³C nmr and UV spectroscopy. These zwitterions exhibit temperature dependent pmr and ¹³C nmr spectra. This can best be explained in terms of a non-planar seven-membered ring and the interconversions of two conformations.</p> <p>An investigation of the photoisomerizations of 1-BX₃ has shown that a new photoproduct, 2-methyl-6-isopropylphenol, was obtained after quenching the solutions of the complexes. The photoisomerizations of protonated eucarvone and the Bf₃ and BCl₃ complexes of eucarvone are similar at least in an as much as the same types of products are formed. The photoisomerization of the BBr₃ complex of eucarvone is more complicated in view of the extreme photolability of one of the photoproducts, namely the complex of 3,7,7-trimethylbicyclo[4.1.0]hept-2-en-4-one. The photochemical and thermal stability of the other photoproducts of 1-BX₃ were examined.</p> <p>A mechanism has been suggested to account for the formation of photoproducts of 1-BX₃.</p>