Thermal Rearrangements of Monoterpenes and Monoterpenoids

Abstract

On the occasion of the 70th anniversary of the Nobel Prize in Chemistry awarded to Leopold Ružička for his work on polymethylenes and higher terpenes 1 )The thermal conversions of monoterpenes and monoterpenoids are an interesting field of research with respect to mechanistic, kinetic, and theoretical issues. Since the beginning of the 20th century, these reactions have attracted the interest of many research groups, and even today there are sufficient problems and questions to deal with. This review covers the thermal isomerization chemistry of pinanes, pinenes, carenes, and thujenes over the past 70 years. Categorization of these compounds into groups, each of them being represented by a small parent molecule (cyclobutane, vinylcyclobutane, vinylcyclopropane), allows systematization of multitude of publications.

Kinetic study of the thermal rearrangement of cis- and trans-2-pinanol

“…Kinetic models were constructed for the rearrangement of cisand trans-2-pinanol which are able to predict the reactant reactivities, product yields and selectivities. Both kinetic models are based on the mechanism for thermal rearrangement of bicyclic monoterpenes containing a [3.1.1]bicycloheptane system that has been reported in literature [26,39]. A stepwise fragmentation of the cyclobutane ring with biradical intermediates is generally accepted as the initial decomposition of the reactant molecules.…”

“…Stolle et al investigated the mechanism and kinetics of the gasphase isomerization of monoterpenes [26,31] and showed that there are mechanistic analogies in the isomerization of pinaneand pinene-type structures. Moreover, the authors were able to model reactivity differences between cis-pinane and trans-pinane.…”

“…The reaction mechanism for the thermal isomerization of pinane-and pinene-type compounds has been extensively discussed in literature and it is generally agreed upon that the fragmentation of the four-membered ring is the initial step in the thermal isomerization of bicyclic monoterpenes consisting of a bicyclo[3.1.1]heptane system [26][27][28][29][30][31][32][33][34][35][36]. Two types of cyclobutane rupture mechanisms were proposed to explain the formation of the different primary acyclic and monocyclic pyrolysis products: a concerted mechanism of the cyclobutane fragmentation containing a 4--electron anti-aromatic transition state and a stepwise mechanism with biradical intermediates.…”

Environmentally Benign Transformations of Monoterpenes and Monoterpenoids in Supercritical Fluids

“…Two recent reviews [7,8] have described only the thermal transformations of some monoterpenes and monoterpenoids in SCFs; whereas the other types of reactions remain undiscussed. This review covers all types of transformations of monoterpenes and monoterpenoids in SCFs, which have been published up to early 2012, including the results obtained by the authors and literature data.…”

Reactivity of verbenol and verbenone epoxides in supercritical solvents

“…Since the formation of compounds with cyclopentane skeleton 7 and 8 by thermal isomerization seems unlikely, 11 even residual acidity of supercritical media in the presence of triethylamine appears sufficient for initiation of verbenol epoxide 5 transformations. In the presence of ZnBr 2 verbenol epoxide 5 transforms into compound 7, 12 while in the presence of Montmorillonite clay the main product is compound 6 (44% yield); compounds with cyclopentane skeleton 7 and 8 are formed in smaller quantities (16 and 10%, respectively).…”