Thermal Isomerization of (+)‐cis‐ and (−)‐trans‐Pinane Leading to (−)‐β‐Citronellene and (+)‐Isocitronellene

Abstract

Catalyzed and uncatalyzed rearrangement reactions of terpenoids play a major role in laboratory and industrial-scale synthesis of fine chemicals. Herein, we present our results on the thermally induced isomerization of pinane (1). Investigation of the thermal behavior of (+)-cis- (1 a) and (-)-trans-pinane (1 b) in a flow-type reactor reveals significant differences in both reactivity and selectivity concerning the formation of (-)-beta-citronellene (2) and (+)-isocitronellene (3) as main products. Possible explanations for these results are discussed on the basis of reaction mechanism and ground-state geometries for 1 a and 1 b. To identify side reactions caused from ene cyclizations of 2 and 3, additional pyrolysis experiments were conducted that enabled the identification of almost all compounds in the network of C(10)H(18)-hydrocarbon products formed from 1.

An effort to generalize the thermal isomerization of 6,6-dimethylbicyclo[3.1.1]heptanes and 6,6-dimethylbicyclo[3.1.1]heptenes: Comparative pyrolysis of pinane, α-pinene, and β-pinene

“…Raise of pyrolysis temperature disfavors the formation of 4 expressed by the decreasing ratio of both acyclic products. This is in clear contrast to the isomerization of 1b, whereas the reaction routes affording 4 and 5 are at the same level over the whole temperature range [22,26]. Whereas the ratio of acyclic ring-opening products 6 + 7 to monocyclic 9 resulting Scheme 3. .…”

“…1a and 2 have a semi-chair or ''Y'' conformation, characterized by a planar C 3 -bridge. In case of 1b and 3 this C 3 -bridge is bent down leading to a less strained chair conformation [22,25,26]. In case of the latter configuration the carbon-carbon bond angles of the cyclohexane subunit are less distorted than for structures 1a and 2.…”

“…Additionally, the presumed type of reaction is given. It is conspicuous that those compounds that are thermally more stable (4, 5 and 8), form substituted cyclopentanes via intramolecular (3,4)ene-reactions [17,26,35,41]. The respective cyclization products are pictured in Scheme 4.…”

“…b-Citronellene (4) and isocitronellene (5) are the main products from pyrolysis of both 1a and 1b [22,26,35,36]. The thermal rearrangement of 2 primarily yields 3Z-ocimene (6) [12,17,21,24,37], which rapidly undergoes [1,5]sigmatropic hydrogen shift forming alloocimene (7) [38].…”

Kinetic and mechanistic aspects of myrcene production via thermal-induced β-pinene rearrangement

“…Whereas products with structures similar to 5 are known from various analogous compounds of 2 (b-citronellene, isocitronellene, linalool; Table 2), substituted products with cycloheptene skeleton (6) yield from the rearrangement of 2 only [11,23,28]. Studying the thermal rearrangement of various 1,6-dienes with hydrogen in aposition, the generation of cyclopentane-type products is known to occur [12][13][14]28,[32][33][34][35][36][37][38][39][40][41]. For instance, the thermal treatment of linalool yields 1,2-dimethyl-3-isopropenylcyclopentanol (Table 2) [28,38], and consecutive reactions of the acyclic main product from the pyrolysis of nopinone lead to the formation of 2-methyl-3-isopropenylcyclopentanone (Table 2) [12,13,35,36].…”