Photoisomerization of the boron trihalide complexes of eucarvone

The photoisomerization of the BF3, BCl3 and BBr3 complexes of eucarvone (2,6,6-trimethylcyclohepta-2,4-dienone), 1, have been examined. Irradiation of these zwitterions at low temperature gave, after subsequent recovery of decomplexed materials, 3,7,7-trimethylbicyclo-[4.1.0]hept-2-en-4-one, 2, 2-methyl-5-isopropylphenol, 3, 2-methyl-6-isopropylphenol, 4, dehydrocamphor, 5, and carvone, 6. In the case of 1•BF3 and 1•BCl3 the initial photoproducts were shown to be the complexes of 2, 3, 4, and 5. The complexes of 6 were shown to arise from a secondary photoisomerization of 2•BX3. It was not possible to establish the primary product distribution, obtained from 1•BBr3 as further photoisomerization of 2•BBr3 occurred even at very low degrees of conversion. The mechanisms and synthetic utility of these reactions is discussed.