Isomerization of d-limonene oxide over solid acids and bases

Abstract

The reaction of d-limonene oxide (a mixture of cis and trans forms) (I) over solid acids and bases was found to give the following products; cymene (II), 1-methyl-3-isopropenylcyclo-pentyl-1-carboxaldehyde (III), 1-methyl-3-isopropylidenecyelopentyl-1-carboxaldehyde (IV), carvenone (V), carvone (VI), cis- and trans-dihydrocarvone (VII), cis- and trans-exo-carveol (VIII) and cis- and trans-endo-carveol (IX). A large amount of II was formed over H 2SO 4 SiO 2 , III and IV over SiO 2-Al 2O 3, V over H 2SO 4 SiO 2 and SiO 2-Al 2O 3, VII over SiO 2-Al 2O 3 and BF 3 and VIII and IX over Al 2O 3. Solid H 3PO 4 heat-treated at low temperature gave comparatively uniform yields of all the compounds. Solid LiClO 4 was surprisingly active for the formation of VII, but Li 3PO 4 was quite inactive. ZnO, MgO, and CaO were almost inactive. From the time variations of yields, IV and V were found to be formed by the rearrangments of III and the cis form of VII, respectively. The formations of carbonyl compounds (III and VII) and cymene (II) were explained by the isomerization over strong acid sites, while the allyl alcohol formation (VIII and IX) by an acid-base bifunctional mechanism over A1 2O 3.