Photoisomerization of the boron trihalide complexes of eucarvone

Abstract

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.

Comparison of the charge distributions and barriers to ring inversion of protonated eucarvone and its boron trihalide adducts

“…In the following paper we present the results tively few complexes of ketones and boron triof our investigation into the effect on the photochloride or tribromide have been described (4, chemistry of eucarvone on its complexation with 7-9). Before the excited state behaviour of the various boron trihalides (1). There are numerous complexed or protonated cycloheptadienones reports of the protonation of aldehydes and can be examined, it is necessary to establish their ketones and the spectroscopic characterization of the resulting hydroxy substituted carbocations (2).…”