Thermal isomerization of (+)-cis- and (-)-trans-pinane leading to (-)-β-citronellene and (+)-isocitronellene

Article abstract of DOI:10.1002/chem.200800298

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- (la) and (-)-trans-pinane (1b) in a flow-type reactor reveals significant differences in both reactivity and selectivity concerning the formation of (-)-β-citronellene (2) and (+)-isocitronellene (3) as main products. Possible explanations for these results are discussed on the basis of reaction mechanism and groundstate geometries for 1a and 1b. 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 ₁₀H₁₈-hydrocarbon products formed from 1.

Full text of DOI:10.1002/chem.200800298

FULL PAPER
DOI: 10.1002/chem.200800298
Thermal Isomerization of (+)-cis- and (ꢀ)-trans-Pinane Leading to
(ꢀ)-b-Citronellene and (+)-Isocitronellene**
Achim Stolle,[a] Bernd Ondruschka,*[a] Werner Bonrath,[a, b] Thomas Netscher,[b]
Matthias Findeisen,[c] and Markus M. Hoffmann[d]
Abstract: Catalyzed and uncatalyzed
rearrangement reactions of terpenoids
play a major role in laboratory and in-
dustrial-scale synthesis of fine chemi-
cals. Herein, we present our results on
the thermally induced isomerization of
pinane (1). Investigation of the thermal
behavior of (+)-cis- (1a) and (ꢀ)-
trans-pinane (1b) in a flow-type reactor
reveals significant differences in both
reactivity and selectivity concerning
the formation of (ꢀ)-b-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 1a and 1b. To
identify side reactions caused from ene
cyclizations of 2 and 3, additional py-
rolysis experiments were conducted
that enabled the identification of
almost all compounds in the network
Keywords: ene reaction
· NMR
of
C10H18-hydrocarbon
products
spectroscopy · pericyclic reaction ·
rearrangement · terpenoids
formed from 1.
Introduction
(1; Scheme 1) yields acyclic isomers and p-menthane-type
monocyclic terpenes.[4–7] The rearrangement of a-pinene
leads to the formation of limonene and alloocimene.[4,8] Li-
nalool, an important building block in the synthesis of vita-
min E and flavor compounds, is formed by thermal isomeri-
zation of 2-pinanol.[9] All of these isomerization reactions of
bicyclic pinane-type compounds yield at least one acyclic
Monoterpenes from renewables (e.g. limonene, b-citronel-
lene (2), a-pinene, b-pinene) and terpenoids, such as 2-pina-
nol or citral, play an important role in the industrial synthe-
sis of flavors and fragrances.[1–3] They are valuable chiral
building blocks for the synthesis of ligands and natural prod-
ucts, and they are important chiral auxiliaries. The thermal
isomerization of bicyclic terpenes with the pinane skeleton
[a] Dipl.-Chem. A. Stolle, Prof. Dr. B. Ondruschka, Dr. W. Bonrath
Institute for Technical Chemistry and Environmental Chemistry
Friedrich-Schiller University Jena
Lessingstr. 12; 07743Jena (Germany)
Fax : (+49)364-194-8402
E-mail: Bernd.Ondruschka@uni-jena.de
[b] Dr. W. Bonrath, Dr. T. Netscher
R&D Chemical Process Technology, DSM Nutritional Products
P.O. Box 2676; 4002 Basel (Switzerland)
[c] Dr. M. Findeisen
Institute for Analytical Chemistry
University Leipzig, LinnØstr. 3
04103Leipzig (Germany)
[d] Dr. M. M. Hoffmann
Department of Chemistry, State University of New York College
at Brockport, 350 New Campus Drive
Brockport, NY 14420 (USA)
[**] For a report on the second part of this study, see: A. Stolle, W. Bon-
rath, B. Ondruschka, D. Kinzel, L. Gonzµlez, J. Phys. Chem. A 2008,
112, 5885–5892.
Scheme 1. Reaction network for the thermally induced rearrangement of
(+)-cis- (1a) and (ꢀ)-trans-pinane (1b) including the main reaction path-
ways.
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isomer. If the rearrangement includes biradical intermedi-
ates wherein at least one radical is resonance stabilized (e.g.
a-pinene), the formation of p-menthane-type compounds is
observed (e.g. limonene from a- or b-pinene).[4,5,8–11] The use
of compounds without p-bonds in the reactive part of the bi-
cyclic molecule (carbon atoms C1, C2 of 1; Scheme 1)[12]
leads to the formation of acyclic products only (e.g. 2 from
1 or linalool from 2-pinanol) by opening of the cyclobutane
ring.[5,9] In most cases, the acyclic products undergo consecu-
tive reactions leading to substituted cyclopentanes if 1,6-
dienes are initially formed (e.g. 4, 5 in Scheme 1).[4,5,9,13–15]
The products from the reaction network for the thermal
rearrangement based on 1 are shown in Scheme 1. Accord-
ing to their general structure, they can be classified into
three groups assigned by three reaction pathways. Two
major reaction pathways (I and II) lead to the formation of
b-citronellene (2)[4,5,13,16–20] and isocitronellene (3)[5,20] as the
main products. Whereas the formation of 2 from 1 (path I)
is thoroughly investigated, the formation of 3 (path III) is
mentioned in only two references.[5,9] Evidently, most studies
covering the pyrolysis of 1 were conducted by using the cis-
isomer 1a only as the starting material. The pyrolysis of 1a
leads to the formation of 2 and 3 with selectivities of 90 and
10%, respectively. trans-Pinane (1b) yields 2 and 3 also, but
with a lower selectivity for 2 (60%) relative to 1a.[5] Based
on this knowledge, there seems to be an apparent oversight
in not finding 3 in the product mixtures of most undertaken
studies, despite its low but significant concentration. Addi-
tionally, as shown in Scheme 1, there is also a minor reaction
pathway (path III) that yields monocyclic terpenes of the p-
menthane-type (6–8).[4,19] Furthermore, consecutive reac-
tions of 2 and 3 lead to the formation of substituted cyclo-
pentanes 4 and 5, respectively. To the best of our knowledge,
the consecutive reaction after formation of 3 leading to 5
has not been investigated yet. Finally, a holistic approach of
studying the reaction sketched in Scheme 1 in its entirety is
much needed in light of the
study of the thermal behavior of the title compounds (1–3)
allows for the detailed description of their reaction behav-
ior. Specifically, the stereochemical influence of the relative
configuration of the methyl-group at C2 on the reaction be-
havior of 1a and 1b in Scheme 1 is thoroughly investigated.
Based on the composition of the liquid mixture of products
from pyrolyses, conclusions on a number of details concern-
ing the reaction mechanism are drawn.
Results and Discussion
Identification of the thermal isomerization products: To
identify the products of pinane pyrolysis, comparative pyrol-
ysis experiments of (+)-cis- (1a), (ꢀ)-trans-pinane (1b),
(ꢀ)-b-citronellene (2), and (+)-isocitronellene (3) were con-
ducted and the chromatograms are shown in Figure 1. Ex-
periments were carried out in an electrically heated quartz
reactor described previously by using oxygen-free nitrogen
as the carrier gas.[10] Chromatograms depicted in Figure 1 re-
sulted from experiments with the pure compounds listed on
the right-hand side. Figure 1 reveals that 1a and 1b differ
from each other with respect to the selectivity of the prod-
ucts formed. As described later, almost all compounds (4–8)
were identified. Multiple assignments in the case of 4 and 5
refer to different diastereomers formed during the reaction
and will be discussed later in this section. In each chromato-
gram of 1a and 1b there is one peak (marked as X) that has
not been identified. These unidentified products occurred at
temperatures (T) above 5708C in amounts smaller than 3%
and are, therefore, not relevant for the description of the
thermal behavior of the investigated compounds that react
at temperatures starting at 4258C.
As can be seen from the chromatograms in Figure 1, the
comparative pyrolysis experiments of 1a and 1b indicate
that both pinane isomers yield 2 and 3 as the main products,
fact that differences in experi-
mental setups, residence times,
and analytical methods are
most likely responsible for the
oftentimes,
contradicting
yields, and compositions of the
products reported in the litera-
ture.[4,5,13,19–21]
Herein, results concerning
the thermal behavior of (+)-1a
and (ꢀ)-1b and of their acyclic
main products (ꢀ)-2 and (+)-3
in the gas-phase within a tem-
perature range of 400–6008C
are presented. By using a com-
bination of GC and NMR
spectroscopic analytical tech-
niques, the formation of side
products and their structural
elucidation is reported. The
Figure 1. Chromatograms of the mixtures of pyrolysis products of the thermal isomerization of (+)-cis-pinane
(1a), (ꢀ)-trans-pinane (1b), (ꢀ)-b-citronellene (2), and (+)-isocitronellene (3; T: 6008C, 15 mL starting materi-
al, carrier gas: N2, flow rate: 1.0 Lminꢀ1, t: 0.49 s; GC-FID; assigned compounds 4–8 are described in the text;
X: unidentified compounds).
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Isomerization of (+)-cis- and (ꢀ)-trans-Pinane
FULL PAPER
and the thermal treatment of these leads to the formation of
further consecutive products (4, 5; Scheme 1), arising from
rearrangement reactions of the acyclic main products by
ene-type cyclizations.[5,13,14,18,22] The chromatogram of the
mixture of pyrolysis products obtained from 2 in Figure 1
shows that product 4 was formed exclusively from 2, where-
as 5 resulted from the thermal rearrangement of 3. In addi-
tion, monocyclic products (trans-D8-p-menthene, 6; cis-D8-
p-menthene, 7, and carvomenthene, 8; Scheme 1) were
found in the mixture of pyrolysis products of both pinane
isomers. Structures were confirmed by using GCMS analysis
upon comparison of their residence time and mass spectra
with those from reference compounds. Compounds 6–8 were
not present in either of the experiments carried out with 2
or 3 as the substrate, which indicated that they were pro-
duced directly from the rearrangement of 1a and 1b.
As shown in Figure 1, the thermal isomerization of 2
yields four different products of 4 which turned out to be
diastereomers. According to their retention time (t) listed in
Table 1 the diastereomers are named 4a–d, starting with 4a
eluting at 6.9 min. Besides 4c, all products were isolated by
preparative GC. The determination of their structures and
relative configurations was performed by a combination of
different NMR spectroscopic techniques (1H and 13C NMR,
HMBC, HSQC). Table 1 lists the most important data ena-
bling structural elucidation of the four isomers (Scheme 2).
Due to their nature as unsaturated cyclic hydrocarbons, de-
termination of their structure is difficult because of overlap-
ping chemical shifts (d) in their proton NMR spectra. The
signal sets for the substituents of the cyclopentane skeleton
are suitable for structural interpretations, whereas the sig-
nals corresponding to the protons of cyclic CH or CH2 ma-
jorities are not. The most conspicuous differences are found
for the signal sets representing the =C7H2 protons, listed in
Table 1. Depending on the relative orientation of the sub-
stituents at C1 and C2 (Scheme 2) either one or two signals
occur. As found for 4b and 4d and indicated in Table 1,
when both groups are placed on the same side of the ring
(cis configuration) the proton spectrum shows two singlets,
each singlet representing one proton.[22–26] Otherwise (trans
1
configuration) the H NMR spectrum shows only one signal
representing both =C7H2 protons (4a, 4c). Determination of
the complete configuration by comparing d and the coupling
constants (J) for the CH protons of the cyclopentane skele-
ton was not possible due to severe signal overlapping. Nev-
ertheless, considerations of the signal sets for the protons of
the methyl substituents enable for complete elucidation.
Data compiled in Table 1 (column 4) offers no significant
3
change in d and J for the protons at C9, but the differences
in the chemical shifts of the protons at C10 are significant
(column 5). The differences in
d for 4a and 4b
(d=0.16 ppm) is lower than for the differences towards 4d
(d=0.38 and 0.22 ppm compared to 4a and 4b, respectively).
Obviously, 4a and 4d differ extremely with regard to their
stereochemistry, leading to the conclusion that 4a has to
have a trans,trans-configuration, whereas 4d has to be
cis,cis-configured (Table 1).[13,22] Determination in the case
of 4b an 4c was possible upon consideration that only a
total of four combinations of cis or trans are possible. Thus
with 4b containing a cis,trans-configuration, this leaves 4c
with the exact opposite configuration. Absolute configura-
tions were assigned by comparing the enantiomeric excess
(ee) of the products of 4 with the ee of 2 used herein, assum-
ing that their formations are enantiospecific.[13,22,26]
Isomerization of 3 leads to the formation of three differ-
ent products: 5a, 5b, and 5c with residence times of 6.4, 7.4,
and 8.0 min, respectively (Figure 1), which were separated
by using preparative GC. Because of their low concentra-
tions it was not possible to identify the structures for 5a and
5c, whereas the configuration of 5b was elucidated based on
13C and 1H NMR spectroscopic data. The 13C NMR spec-
trum reveals seven resonances, allowing for the conclusion
that the molecule has a highly symmetrical structure (all-cis-
or all-trans-5; Scheme 2). The carbon atoms C1/C3, C4/C5,
and C6/C10 showed identical d values, which was proven by
Scheme 2. Configuration of the rearrangement products (4a–d, 5a–c) ob-
tained from the thermal isomerization of (ꢀ)-b-citronellene (2; Table 1)
and (+)-isocitronellene (3), respectively.
Table 1. NMR spectroscopic data for structural elucidation of the rearrangement products 4a–d (Scheme 2).
1H NMR spectroscopic signal for protons at CiHx [ppm][b]
Product
t [min][a]
=C7H2
C9H3
C10H3
Relative orientations[c]
Absolute configuration[d]
3
3
4a
4b
4c
4d
6.9
7.8
8.0
9.1
4.62 (s, 2H)
0.90 (d, J=5.0 Hz)
0.81 (d, J=4.8 Hz)
trans,trans
cis,trans
trans,cis
cis,cis
1R,2R,3S
1S,2R,3S
1S,2S,3S
1R,2S,3S
3
3
4.70 (s, 1H), 4.57 (s, 1H)
0.92 (d, J=5.0 Hz)
0.65 (d, J=5.2 Hz)
[e]
[e]
[e]
–
–
–
3
4.74 (s, 1H), 4.58 (s, 1H)
0.87 (d, J=5.1 Hz)
0.43(d, 3J=5.3Hz)
[a] Retention time from Figure 1. [b] c.f. Scheme 2. [c] Relative orientations of the isopropenyl group at C1 (1st designation) and the methyl-group at C3
(2nd designation) towards the methyl group at C2 (Scheme 2). [d] Based on the assumption that the formation occurs enantiospecifically from (ꢀ)-(3S)-
b-citronellene (2; cf. following section). [e] Not enough material for NMR spectroscopic measurement.
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B. Ondruschka et al.
1
the H NMR spectroscopic data, which showed identical d
3
and J for the protons at C6 and C10. In analogy to 4b and
4d (Table 1) the proton NMR spectrum showed two singlets
for the =CH2 group in 5, which enabled the conclusion that
the isopropenyl group is located cis to either the methyl
group at C1 or C2. Combination of these results leads to the
conclusion that the all-cis configuration is the correct one
for 5b. The 13C NMR spectra of 5a and 5c showed ten sig-
nals and both compounds have a similar fragmentation pat-
tern (EIMS). Therefore, their configuration had to be either
cis,trans or trans,cis, whereby an assignment was not possi-
ble.
Thermal behavior of cis- and trans-pinane: Studies of the
thermal behavior of 1a and 1b in pyrolysis experiments
were carried out within a temperature range of 400–6008C
for the pure compounds and for an equal mixture of 1a and
1b. In Figure 2, the dependency of pinane conversion (X)
Figure 3. a) Ratio of the formation of acyclic to monocyclic products (o;
[Eq. (1)]) and ratio of (ꢀ)-b-citronellene (2) to (+)-isocitronellene (3; g;
[Eq. (2)]) depending on reaction temperature (T) for the pyrolysis of
&
^
(+)-cis- (1a, ) and (ꢀ)-trans-pinane (1b, ; 15 mL starting material, car-
rier gas: N2, flow rate: 1.0 Lminꢀ1, t: 0.47–0.61 s). b) Yield of (ꢀ)-b-citro-
nellene (2; Y2) depending on reaction temperature (T) for the pyrolysis
^
Figure 2. Conversion (X) of (+)-cis-pinane (1a; ), (ꢀ)-trans-pinane (1b;
~
&
), and a mixture of 1a and 1b (1a/1b=0.94; ) depending on the reac-
tion temperature (15 mL starting material, carrier gas: N2, flow rate:
&
of (+)-cis- (1a, &) and (ꢀ)-trans-pinane (1b, ; 15 mL starting material,
1.0 Lminꢀ1, t: 0.47–0.61 s).
carrier gas: N2, flow rate: 1.0 Lminꢀ1, t: 0.47–0.61 s).
versus the reaction temperature (T) is depicted, revealing
that 1a is more reactive than 1b. The T for equal conver-
sions of 1a and 1b is shifted about 408 to higher values in
the case of 1b relative to the cis isomer.[5] The thermal treat-
ment of the equal mixture of 1a and 1b also shown in
Figure 2 confirms this trend because the curve of conversion
corresponds with the behavior of the pure compounds. The
conversion of the mixture is mainly affected by 1a at lower
temperature, whereas in experiments above 5258C, the in-
fluence of 1b on the curve is significantly higher. At reac-
tion temperatures lower than 4008C, neither for 1a nor 1b
are detectable conversions observed, whereas at tempera-
tures higher than 6008C, the conversions for both are com-
plete, and besides isomerization further decomposition of
the C10H18 hydrocarbons to lower weight compounds takes
place.
temperature T, classified according to the reaction pathways
depicted in Scheme 1. Three different classes of major prod-
ucts are formed from 1a and 1b: (ꢀ)-b-citronellene (2; path
I), (+)-isocitronellene (3; path II), and monocyclic products
(6–8; path III). The ratio, o, of paths I plus II (acyclic prod-
ucts) and path III (6–8) is expressed by Equation (1), where-
by o considers only the initial reactions from 1. Due to this
fact, side products (4, 5) have to be considered also. Despite
the fact that 6–8 are formed in low amounts only (overall
<6%), o is higher than 1.0 for both 1a and 1b increasing
with rising temperatures from 5.2 up to 14.1 (425–6008C)
and from 11 to 17.7 (475–6008C), respectively (Figure 3a).
After reaching a maximum at 5128C (15.9) and at 5508C
(20.5), o drops slightly in favor of the formation of 6–8. The
maxima correspond to the maximal overall yields of the acy-
clic isomers (Figure 3b). Exemplary for 2, Figure 3b express-
es that the yields of 2 pass through maxima at 512 and
5508C for isomerization of 1a and 1b, respectively. With in-
Besides their behavior concerning the overall conversion,
1a and 1b differ in selectivity (S). Figure 3a and b illustrate
the dependence of overall yielded products on pyrolysis
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Chem. Eur. J. 2008, 14, 6805 – 6814

Isomerization of (+)-cis- and (ꢀ)-trans-Pinane
FULL PAPER
creasing temperature, the amount of rearranged products
formed from the acyclic primary products increases.
ocyclic products (6–8) formed from 1 seem to arise from dif-
ferent reaction pathways. Whereas 2 and 3 seem to arise
from a fragmentation of the cyclobutane ring in 1 (path I
and II; Scheme 3), the formation of p-menthenes 6–8 can be
explained by using a biradical pathway combined with H-
shift reactions[1,5] according to the formation of limonene
from a- or b-pinene (path III; Scheme 5).[4,5,8–11]
path I þ path II ½2Š þ ½3Š þ ½4Š þ ½5Š
ð1Þ
ð2Þ
s ¼
g ¼
¼
path III
½6Š þ ½7Š þ ½8Š
path I
path II
½2Š þ ½4Š
¼
½3Š þ ½5Š
Additionally, data recorded in Figure 3a illustrate the ratio
of reaction pathways I and II (g) expressed by Equation (2),
again including side reactions leading to 4 and 5. The differ-
ences in g between 1a and 1b are significant. It is evident
from Figure 3a that g1a decreases linearly from 18.2 at 425 to
5.5 at 6008C, whereas g1b remains flat between 1.0–1.2 over
the investigated temperature range, which is a clear indica-
tor for the similarities of the transition states leading to 2 or
3 (c.f. following section). Pyrolyses of the trans-isomer 1b
leads to lower selectivities of 2 in favor of 3.
Scheme 3. Formation of (ꢀ)-b-citronellene (2; path I) and of (+)-isocitro-
nellene (3; path II) from (+)-cis- (1a) and (ꢀ)-trans-pinane (1b).
The formation of 2 and 3 from 1 proceeds as a highly
enantioselective reaction. Table 2 lists the enantiomeric
Two possible mechanisms for the fragmentations of four-
membered rings (cyclobutane, oxetane) are discussed in the
literature.[27,28] A concerted mechanism of cyclobutane frag-
mentation has to proceed by a 4-p-electron-containing anti-
aromatic transition state. According to the Woodward–Hoff-
mann rules, those (retro)-[2+2]-cycloadditions are thermally
forbidden reactions.[29] Nevertheless, some reports in the lit-
erature show some examples of rigid cyclobutanes, the ther-
mal induced fragmentations of which seem to proceed as a
thermally forbidden retro-[2+2]-cycloaddition.[28] The
second and more plausible explanation for the formation of
2 and 3 from 1 involves a stepwise mechanism via biradical
Table 2. Enantiomeric excess (ee; [Eq. (3)]) for (+)-cis- (1a) and (ꢀ)-
trans-pinane (1b) and of the acyclic products ((ꢀ)-2, (+)-3) formed by
thermal rearrangement of 1.[a]
Entry Substrate[b]
T
[8C]
X
eesubstrate
[%]
ee(ꢀ)-2
[%]
ee(+)-3
[%]
[%][c]
1
2
3
4
5
6
(+)-1a
(+)-1a
(+)-1a
(ꢀ)-1b
(ꢀ)-1b
(ꢀ)-1b
475
500
525
475
500
525
31
68
95
5
15
41
94
92
93
9395
94
97
94
98
100[d]
9396
94
95
93
94
92
94
[a] ee values were determined by enantioselective GC analysis by using a
Supelco Beta-DEXTM 120 column from pyrolysis experiments with the
substrates listed. [b] Initial ee values of (+)-1a and (ꢀ)-1b are 94 and
93%, respectively. [c] Conversion of substrate. [d] Due to high conver-
sion, the signal for (ꢀ)-1a was below the detection limit.
1
6
5
6
ꢀ
ꢀ
intermediates. Scission of C
C or C C carbon–carbon
bonds in either 1a or 1b yielded biradicals BR1 and BR2,
respectively (Scheme 3). Consecutive ring opening in these
reaction intermediates leads to the formation of 2 and 3.
Nevertheless, there is one argument counting against a bir-
adical route: the absence of a double bond at C2 in 1 does
not allow for stabilization of a formed 1,4-biradical. Without
the presence of functional groups that can stabilize a free
radical by conjugation (e.g. C=C in a- or b-pinene[4,8,10] or
C=O in nopinone,[4,30]) the lifetime of the formed biradical
is very short and the barrier of activation is high. Therefore,
a decision as to which of the two routes (concerted or step-
wise) leads to the detected acyclic main products requires
further investigations focused on both kinetic experiments[31]
and ab initio calculations[32] of the presumptive transition
states.
excess (ee) expressed by Equation (3) for the product mix-
tures from pyrolysis of both 1a and 1b at different tempera-
tures. The ee remains constant for both starting materials
within the temperature range investigated (conversion
range, respectively). In contrast to a-pinene, no racemiza-
tion occurred.[5,8] The configuration of the optically active
acyclic reaction products (ꢀ)-2 and (+)-3 is determined by
the absolute configuration of the starting material
(Table 2).[5] Both almost optically pure diastereomers (+)-1a
and (ꢀ)-1b used herein were S-configured at C2 and the
same configurations are found for the chiral centers of the
formed products ((ꢀ)-2, (+)-3) with nearly identical ee
(Table 2, Scheme 2).[5,17]
Results reveal that 1a and 1b differ in both reactivity
(Figure 2) and selectivity concerning the formation of 2 and
3 (Figure 3). A previous study suggests that the interaction
of the C2 methyl group with one of the two methyl groups
ðꢀÞꢀðþÞ
ð3Þ
eeðꢀÞ
¼
ðꢀÞ þ ðþÞ
6
1
6
ꢀ
at C results in a weakening of the C C bond for the cis-
isomer (1a), which leads to a higher reactivity in the active
part of the molecule.[5] This behavior would allow for the ex-
Reaction mechanisms for the primary pyrolysis reactions:
Due to their different structures, the acyclic (2, 3) and mon-
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6809

B. Ondruschka et al.
planation of the higher reactivity of 1a and the higher selec-
tivity for the formation of 2 relative to g1b (Figure 3a). Ap-
parently, the activation energies for the two possible transi-
tion states (Scheme 3) are influenced by the relative config-
uration of the C2 methyl group. NMR NOE spectroscopic
experiments were conducted to answer this question by de-
tecting magnetic dipole interactions between protons
through space if the distance between the protons is lower
than approximately 5 Š. Scheme 4 illustrates the ground-
Scheme 5. Formation of cis-/trans-D8-p-menthene (7/6) and D1(2)-p-men-
thene (8) from pinane (1; path III).
of 6 (Scheme 5, Figure 1). Because of the fact that the birad-
ical formed from 1 did not have any influence on the config-
uration at C4, the stereochemistry of the products is con-
trolled by the configuration of the starting material, which
was almost enantiomerically pure (+)-1a (ee: 94%) and
(ꢀ)-1b (ee: 93 %).
Scheme 4. (+)-cis- (1a) and (ꢀ)-trans-pinane (1b; superscript numbers
1
refer to the numbering of nH1–10 listed in the H NMR spectroscopic data
(see the Experimental Section).
state conformation of both pinane isomers. If the conjec-
tures would be correct, a NOE effect is to be expected be-
It has to be pointed out that 6–8 are formed with an accu-
mulated overall selectivity of 6% only. Therefore, the addi-
tional reaction channel leading to 6–8 from BR1 is a minor
reaction pathway. Similar products from the o-menthene
type (e.g. 9) that might yield from a respective reaction
channel of BR2 (Scheme 5) have not been identified. With
respect to these findings, pyrolysis of 1a and 1b initially
passes through two different biradicals, in which subsequent
fragmentation leads to 2 and 3. The formation of monocyclic
primarily formed products 6–8 is due to a minor reaction
channel based on one of the two biradicals.
10
9
2
9
ꢀ
ꢀ
ꢀ
ꢀ
tween H C and H C or H C and H C for 1a and 1b,
respectively.
1D NOE experiments with pure 1a showed no interaction
between the protons at C10 and C9 (Scheme 4), but a NOE
7
2
ꢀ
ꢀ
contact between the proton H C and H C was observed.
This contact was not detected in the case of 1b, whereby an
7
7’
10
ꢀ
ꢀ
ꢀ
interaction of H C and H C with H C was observed.
Therefore, the conjectures concerning an interaction be-
tween H10 and H9 in case of 1a could not be confirmed.
Nevertheless, the results allow for the conclusion that the
trans geometry is advantageous for stabilization of the mole-
7
ꢀ
cule due to attractive interactions with other protons (H C ,
Thermal behavior of b-citronellene and isocitronellene:
After passing through a maximum, the overall yields of the
acyclic products drop with increasing temperature and the
formation of consecutive products is observed (4, 5; Fig-
ures 1, 3a, and 3b). To study the formation of these products,
pyrolysis experiments with 2 and 3 were carried out in a
temperature range from 400–6008C. The comparison of the
conversions of 2 (X2; pointed line and empty symbols) is
shown in Figure 4, that is, resulting from pyrolysis of the
pure compound with those resulting from the thermal iso-
merization of 1a and 1b, while changing the reaction tem-
perature T. The high accordance for X2 of pure 2 and those
from the reaction mixtures reveals the similarities between
the three independent pyrolysis experiments of 1 (Figure 4).
Additionally, Figure 4 illustrates the dependency of selectivi-
ty for the rearranged products (S4) and T, revealing 4b (cis,
trans-4, Table 1, Scheme 2) as the main product from rear-
rangement of 2. It is shown that the selectivities for the for-
mation of 4b and for 4d drop in favor of the formation of
the two other minor isomers (4a, 4c). Whereas the decrease
in the case of 4b is linear, the selectivity profile for the
cis,cis-isomer (4d) passes through a maximum.
7’
10
ꢀ
ꢀ
H C , H C ), similar to the diaxial attraction in cis-1,3-dis-
ubstituted cyclohexanes. Therefore, 1b is thermally the
more stable molecule.
As shown in Figures 1 and 3b, monocyclic products 6–8
were found in the reaction mixtures of both pinanes in low
amounts besides the acyclic main products (2, 3). There are
reports in the literature on the formation of similar p-men-
thane-type products in which a- or b-pinene is pyrolyzed.
Both yield limonene and y-limonene, the latter as an addi-
tional monocyclic reaction product from the thermal isomer-
ization of b-pinene.[4,8,10,33] According to the formation of 2
from 1a and 1b, the initial formation of a 1,4-carbon-cen-
tered biradical (BR1; cf. Scheme 3) by bond breakage be-
tween C1 and C6 is necessary for the side-reaction pathway
leading to 6–8. p-Menthenes 6–8 are formed by [1,5]H-
shifts, in which the hydrogen migration from C9 to C2 yields
those isomers with the exocyclic double bond. The reaction
route by a hydrogen shift from C1 to C8 leading to 8 is sup-
pressed in favor of the formation of 6 and 7 due to the
higher selectivity (cf. Figure 1). Compound 1a yields pref-
erably the cis-isomer (7), whereas 1b leads to the formation
6810
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Chem. Eur. J. 2008, 14, 6805 – 6814

Isomerization of (+)-cis- and (ꢀ)-trans-Pinane
FULL PAPER
tive configurations) are attributed to the different presump-
tive transition states (Scheme 6). Due to the absence of re-
pulsive 1,3-diaxial interactions of methyl substituents, inter-
mediates leading to 4b and 4d are the most stable ones and
4b and 4d are, therefore, the major products obtained from
the isomerization of 2 (Figure 4).[13,37]
In Figure 5, the dependency of the conversion of 3 (X3)
with the pyrolysis temperature, T, for the pure compound
and for 3 from pyrolysis of 1a and 1b is illustrated. With in-
Figure 4. Conversion of (ꢀ)-b-citronellene (2; X2) after pyrolysis of 1a,
1b, and direct conversion of 2 as a function of temperature. Selectivities
(S4) of rearranged products (4a–c) are shown on the right y axis. (15 mL
starting material, carrier gas: N2, flow rate: 1.0 Lminꢀ1, t: 0.47–0.66 s).
~
&
^
^
a: X; : X from 1a; &: X from 1b; : S4a
;
: S4b
;
: S4c; ”: S4d.
Many examples of 1,6-dienes forming cyclization products
as observed for 2 and 3 are reported in the litera-
ture.[5,13–16,34,35] For instance, the thermal treatment of lina-
lool yields 1,2-dimethyl-3-isopropenylcyclopentanol,[9,14,24]
and consecutive reactions of the acyclic main product from
the pyrolysis of nopinone lead to the formation of 2-methyl-
3-isopropenylcyclopentanone.[4,30] Generally, 1,6-dienes with
hydrogen in the a position of any of the two double bonds
are able to form substituted cyclopentanes. The cyclization
of 1,7-dienes with a hydrogen lead to the formation of sub-
stituted cyclohexenes, whereas the ring formation of systems
with fused rings failed.[34,35] The cyclization proceeds by a
concerted ene-type reaction (Scheme 6).[36] It is obvious, that
the configuration of the chiral center in 2 remains un-
changed during the reaction. Therefore, the ee of the desired
products is the same as for the initial 2. Almost enantiomer-
ically pure (ꢀ)-2 used herein yielded cyclization products
4a–d with a 3S configuration (Table 1, Scheme 2). The con-
figurations of C1 and C2 in 4 (and, therefore, also their rela-
Figure 5. Conversion of (+)-isocitronellene (3, X3) after pyrolysis of 1a,
1b, and direct conversion of 3 as a function of temperature. Selectivities
(S5) of rearranged products (5a–c) are shown on the right y axis (15 mL
starting material, carrier gas: N2, flow rate: 1.0 Lminꢀ1, t: 0.47–0.61 s).
~
&
^
a: X; : X from 1a; &: X from 1b; : S5a
; : S5b; ”: S5c.
creased temperatures, the increased X3 raises the yields of
the rearranged products (5a–c). The accordance between
pyrolysis of pure 3 and 3 obtained after pyrolysis of the two
pinane isomers (1a, 1b) is not as high as for 2 shown in
Figure 4. In contrast to the formation of rearrangement
products yielded from 2, data shown in Figure 5 suggest that
one major rearrangement product (5b) is formed from 3 the
selectivity of which remains constant when increasing the
temperature. At reaction temperatures above 5508C, the se-
lectivity drops slightly in favor of the other rearrangement
products. The rate of 5a/5b and of 5c/5b is about 0.08 and
0.085, respectively. According to other 1,6-dienes (2, myr-
cene, linalool) the formation of 5 also proceeds by ene-type
cyclization (Scheme 7).[4,5,9,13,15,34,35] The presumptive transi-
tion state leading to the formation of 5b reveals no repul-
sive diaxial interactions of methyl groups, explaining the
high selectivity.
Scheme 6. Formation of rearrangement products (4) obtained from the
thermal isomerization of (ꢀ)-b-citronellene (2) and presumptive inter-
mediates for the formation of 4b and 4d.
Scheme 7. Formation of the main rearrangement product 5b obtained
from the thermal isomerization of (+)-isocitronellene (3) by ene cycliza-
tion.
Chem. Eur. J. 2008, 14, 6805 – 6814
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6811

B. Ondruschka et al.
150.9, 1H: 600.1 MHz) equipped with a 5 mm TBI probe head in 5 mm
tubes at 278C. The referring numbering of the protons in both 1a and 1b
is described in Scheme 8, wherein the superscript numbers refer to the
superscript numbers of nH1–10 listed below. The assignments of the chemi-
cal shifts to the desired protons were accomplished by using HSQC and
Conclusion
Gas-phase pyrolysis experiments of (+)-cis- (1a), (ꢀ)-trans-
pinane (1b), (ꢀ)-b-citronellene (2), and (+)-isocitronellene
(3) in an electrically heated flow-type reactor were carried
out to study the thermal rearrangement of the title com-
pounds (Scheme 1). It was shown that 1a and 1b differ tre-
mendously in both reactivity and product selectivity, reveal-
ing 1a as the more reactive compound. Whereas the cis-
isomer of 1 yields 2 as the main component (selectivity:
90%), thermal isomerization of 1b leads to the formation of
2 and 3 with a ratio of 55:45. It was shown that the acyclic
main products are presumably formed by a highly enantiose-
lective fragmentation of the cyclobutane ring in 1. The abso-
lute configuration of the products is determined by the con-
figuration on C2 of the starting material. In addition to 2
and 3, monocyclic p-menthene-type products (6–8) are
formed from both 1a and 1b with an overall selectivity of
6%. Apparently, the reactions responsible for the formation
of products 6–8 pass through the same biradical transition
states that are also responsible for the formation of 2.
Stand-alone pyrolysis experiments of 2 led to the identifi-
cation of consecutive reaction products (4a–d, Schemes 1
and 2), the relative configurations of which were determined
by using NMR spectroscopic and GCMS techniques. Ene
cyclization leads to the formation of 4, whereas the desired
transition states control the isomer distribution, yielding 4b
(cis,trans-isomer) as the main product. For the first time, the
thermal isomerization of 3 was investigated, revealing that
according to the formation of 4 from 2 products 5a–c are
formed by ene cyclization.
Scheme 8. (+)-cis- (1a) and (ꢀ)-trans-pinane (1b; superscript numbers
1
refer to the numbering of nH1–10 listed in the H NMR spectroscopic data
(see the Experimental Section).
HMBC techniques (reference 13C: [D]CHCl3 =77.0 ppm; 1H: TMS
intern=d=0 ppm). 1D NOESY experiments (double gradient spin echo
with selective pulse) were carried out by using the following parameters:
mixing time: 800 ms, repetition time: 3.7 s, selective 1808 pulse (“sinc”-
profile), duration: 180 ms, excitation band width: ca. 20 Hz.
A
G
((+)-cis-pinane;
(+)-1a): ee=94%; 1H NMR (600.1 MHz, [D]CHCl3, 278C, TMS): d=
2.31 (m, 1H7), 2.13(m, 1H 2), 1.95 (m, 1H3), 1.93(m, 1H 4), 1.87 (m,
1H5), 1.82 (m, 1H4’), 1.76 (td, 1H1, 3J1!7 =6.8, J=2.2 Hz), 1.42 (m, 1H3’),
1.18 (s, 3H8), 1.02 (s, 3H9), 1.00 (d, 3H10 3J10!2 =7.3Hz), 0.86 ppm (d,
,
7 2 2 1
1H7’, 2J7’!7 =9.4 Hz); NOE-contacts: H C !H C , H C !H C , no
ꢀ
ꢀ
ꢀ
contact H C !H C
[D]CHCl3, 278C, TMS): d=48.1 (CH), 41.4 (CH), 38.8 (C), 35.9 (CH),
33.9 (CH2), 28.3(CH 3), 26.5 (CH2), 23.8 (CH2), 23.3 (CH3), 22.8 ppm
(CH3); MS (70 eV): m/z (%): 138 (2) [M]+, 123(32) [C 9H15]+, 96 (33)
[C7H12]+, 95 (100) [C7H11]+, 83(46 [C 6H11]+, 82 (61) [C6H10]+, 81 (57)
[C6H9]+, 67 (59) [C5H7]+, 55 (58) [C4H7]+.
A
G
((ꢀ)-trans-pinane;
(ꢀ)-1b): ee=93%; 1H NMR (600.1 MHz, [D]CHCl3, 278C, TMS): d=
2.06 (m, 1H2, 3J=7.4 Hz), 2.00 (dt, 1H7, 2J7!7’ =9.9, 3J7!1 =5.9 Hz), 1.84
(m, 1H5), 1.75 (m, 1H4’), 1.72 (m, 1H4), 1.65 (m, 1H3), 1.61 (t, 1H1, 3J=
6.0 Hz), 1.33 (d, 3H7’, 2J7’!7 =10.0 Hz), 1.21 (m, 1H3’), 1.19 (s, 3H8), 0.85
Experimental Section
(d, 1H10, 3J10!2 =6.7 Hz), 0.82 ppm (s, 3H ); NOE contacts: H C !H
7’
ꢀ
ꢀ
General remarks: (1R,2S,5R)-cis-pinane ((+)-1a; purity: 99%, ee: 94%),
(1S,2S,5S)-trans-pinane ((ꢀ)-1b; purity: 99%, ee: 93%), (3S)-b-citronel-
lene ((ꢀ)-2; purity 95%, ee: 95%), and (5S)-isocitronellene ((+)-3;
purity: 99%, ee: 98%) were purchased from Fluka and were used with-
out further purification. Purity was determined by capillary gas chroma-
tography. Ethyl acetate was used for the dilution of liquid pyrolysis prod-
ucts. All reported yields, selectivities, conversions, or any other parameter
(ee, o, g) and point data in the figures are based on mean values of at
least two independent experiments.
C10, H C !H C , no contact H C !H C ; 13C NMR (150.9 MHz,
[D]CHCl3, 278C, TMS): d=47.6 (CH), 40.9 (CH), 39.5 (C), 29.3 (CH),
26.8 (CH3), 24.6 (CH2), 23.9 (CH2), 23.0 (CH2), 21.6 (CH3), 20.9 ppm
(CH3); MS (70 eV): m/z (%): 138 (2) [M]+, 123(35) [C 9H15]+, 96 (50)
[C7H12]+, 95 (100) [C7H11]+, 83(73) [C 6H11]+, 82 (62) [C6H10]+, 81 (67)
[C6H9]+, 67 (62) [C5H7]+, 55 (67) [C4H7]+.
2
Pyrolysis experiments: The investigated terpenes were pyrolyzed at a
temperature range of 400 to 6008C. Dilution gas pyrolysis experiments
were carried out in an electrically heated quartz tube of 50 cm length and
with a pyrolysis zone of approximately 20 cm, by using apparatus report-
ed previously.[10] In all experiments, oxygen-free nitrogen with a purity of
99.999% was used as the carrier gas. The starting material (15 mL) was
introduced into a quartz ladle at the top part of the pyrolysis apparatus
by using a glass syringe (50 mL). The starting material was carried along
with the nitrogen stream into the reactor. Vaporization of the starting
material was supported by heating the ladle to 2508C with a hot blast.
Pyrolysis products were collected in a cold trap (liquid nitrogen) and
were dissolved in 1.5 mL of ethyl acetate. The liquid products obtained
were analyzed by GC-FID and GCMS.
Analyses were carried out in a 6890 Series GC and 5890 Series II/5972
Series MSD GC from Agilent Technologies. Products were identified by
comparing either retention time and/or mass spectra of pure reference
compounds. GC-FID: (HP 5, 30 m”0.32 mm”0.25 mm, H2–5 psi, pro-
gram): 358C (hold: 1 min), 4 Kminꢀ1 up to 808C, 4.5 Kminꢀ1 up to 908C,
35 Kminꢀ1 up to 2808C (hold: 3min), injector temperature: 250 8C, de-
tector temperature: 2808C. GC-MSD: (HP 5, 30 m”0.32 mm”0.25 mm,
He ꢀ7 psi, program): 558C (hold 1 min), 5 Kminꢀ1 up to 1508C,
20 Kminꢀ1 up to 2808C (hold: 5 min), injector temperature: 2808C, EI.
NMR spectra were recorded by using a Bruker AV-400 spectrometer.
Enantiomeric excess values were determined with a permethylated b-cy-
clodextrine column (Supelco Beta-Dex 120, 30 m”0.25 mm”0.25 mm).
Analyses were carried out with a HP 6890: H2–12 psi, program: 688C
(hold: 13.12 min), 25 Kminꢀ1 up to 2408C (hold: 10 min), split ratio: 3.4,
injector temperature: 2408C, detector temperature: 2808C.
Structural elucidation of pyrolysis products: For structural elucidation of
the reaction products from pyrolysis of b-citronellene (2) and isocitronel-
lene (3), 1.5 mL of the desired substrates (30”50 mL) were pyrolyzed at a
temperature of 5508C with a flow rate of 0.8 Lminꢀ1 (t=0.62 s). The col-
lected products were dissolved in 3mL of ethyl acetate. These product
mixtures were separated by using preparative GC (packed column,
Structural elucidation of cis- (1a) and trans-pinane (1b): NMR spectra of
1a and 1b in CDCl3 were recorded on a Bruker DRX-600 system (13C:
6812
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Chem. Eur. J. 2008, 14, 6805 – 6814

Isomerization of (+)-cis- and (ꢀ)-trans-Pinane
FULL PAPER
10 m”14 mm, coverage: 20% SE-54, support: Vol. A4 60–80 mesh, inlet
temperature: 2808C, detector temperature: 2808C, carrier gas: N2, flow
rate: 618 mLminꢀ1, injection volume: 50–57 mL, number of cycles: 20)
and the isolated fractions were analyzed and identified by using 1H and
13C NMR spectroscopy.
[M]+, 123(24) [C 9H15]+, 96 (34) [C7H12]+, 95 (21) [C7H11]+, 83(21)
[C6H11]+, 82 (39) [C6H10]+, 81 (100) [C6H9]+, 67 (46) [C5H7]+, 55 (18)
[C4H7]+.
AHCTRE(GNU 1S,2S,3R)-2-Isopropenyl-1,3-dimethylcyclopentane (2,3-cis,cis-5; 5b):
1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=4.81 (s, 1H), 4.73(s,
1H), 1.87 (m, 4H), 1.66 (s, 3H), 1.49 (t, 1H, 3J=9.9 Hz), 1.28 (m, 2H),
0.94 ppm (d, 6H, 3J=6.3Hz); 13C NMR (100 MHz, [D]CHCl3, 258C,
TMS, underlined values represent two C atoms with identical d): d=
(S)-3,7-Dimethyl-1,6-octadiene ((ꢀ)-b-citronellene; (ꢀ)-2): ee=94%;
1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=5.61 (m, 1H), 5.03(t,
1H, 3J=1.4 Hz), 4.86 (m, 2H), 2.06 (m, 1H), 1,89 (m, 2H), 1.61 (s, 3H),
1,52 (s, 3H), 1.23 (m, 2H), 0.92 ppm (d, 3H, 3J=6.7 Hz); 13C NMR
(100 MHz, [D]CHCl3, 258C, TMS): d=143.8 (=CH), 130.3 (=C), 123.6 (=
145.5 (C), 111.5 (=CH2), 64.4 (cycl. CH), 37.9 (cycl. CH), 32.2 (cycl.
+
ꢀ
CH2), 18.7 (CH3), 17.6 (=C CH3); MS (70 eV): m/z (%): 138 (2) [M] ,
CH), 111.4 (=CH2), 36.3 (CH), 35.7 (CH2), 24.7 (=C E-CH3, =C CH2);
123(12) [C 9H15]+, 96 (29) [C7H12]+, 95 (29) [C7H11]+, 83(100) [C 6H11]+,
82 (17) [C6H10]+, 81 (33) [C6H9]+, 69 (26) [C5H9]+, 67 (27) [C5H7]+, 55
(75) [C4H7]+.
ꢀ
ꢀ
+
ꢀ
19.1 (CH3), 16.7 (=C Z-CH3); MS (70 eV): m/z (%): 138 (20) [M] , 123
(24) [C9H15]+, 95 (83) [C7H11]+, 82 (91) [C6H10]+, 81 (52) [C6H9]+, 69
(84) [C5H9]+, 67 (100) [C5H7]+.
2,3-cis,trans- or 2,3-trans,cis-2-Isopropenyl-1,3-dimethylcyclopentane
(5c): The concentration of the isolated product was too low to perform
NMR spectroscopic studies. As a result of this fact, it was not possible to
determine the relative structure (cf. text). MS (70 eV): m/z (%): 138 (5)
[M]+, 123(21) [C 9H15]+, 96 (22) [C7H12]+, 95 (21) [C7H11]+, 83(62)
[C6H11+], 82 (25) [C6H10]+, 81 (100) [C6H9]+, 67 (40) [C5H7]+, 55 (21)
[C4H7]+.
(S)-5,7-Dimethyl-1,6-octadiene ((+)-isocitronellene; (+)-3): ee=95%;
1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=5.79 (m, 1H, 3J=10.0,
3J=16.8, 3J=6.8 Hz), 4.97 (dd, 1H, 3J=17.2, 2J=1.6 Hz), 4.91 (d, 1H,
3J=10.0 Hz), 4.86 (d, 1H, 3J=9.6 Hz), 2.33 (m, 1H), 2.00 (m, 2H), 1.67
(s, 3H), 1.59 (s, 3H), 1.28 (m, 2H), 0.90 ppm (d, 3H, 3J=6.6 Hz);
13C NMR (100 MHz, [D]CHCl3, 258C, TMS): d=139.0 (=CH), 131.2
(=C), 130.0 (=CH), 114.7 (=CH2), 37.0 (CH2), 32.0 (CH), 31.8 (CH2), 25.8
ꢀ
ꢀ
(=C E-CH3), 21.2 (CH3), 18.0 ppm (=C Z-CH3); MS (70 eV): m/z (%):
139 (1) [M+H]+, 138 (6) [M]+, 123(13) [C 9H15]+, 96 (25) [C7H12]+, 95
(19) [C7H11]+, 83(44) [C 6H11]+, 82 (31) [C6H10]+, 81 (100) [C6H9]+, 67
(39) [C5H7]+, 55 (16) [C4H7]+.
Acknowledgements
A
(2,3-trans,trans-4;
4a): 1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=4.62 (s, 2H), 1.98
(q, 1H, 3J=10.2 Hz), 1.70 (m, 2H), 1.52 (s, 3H), 1.46 (s, 1H), 1.12 (m,
3H), 0.90 (d, 3H, 3J=4.9 Hz), 0.81 ppm (d, 3H, 3J=4.8 Hz); 13C NMR
(100 MHz, [D]CHCl3, 258C, TMS): d=146.1 (=C), 108.7 (=CH2), 55.0
(cycl. CH), 44.6 (cycl. CH), 41.1 (cycl. CH), 31.9 (cycl. CH2), 28.2 (cycl.
A.S. and B.O. are grateful to Dr. N. Theyssen (Max-Planck-Institute of
Coal Research, Mühlheim, Germany) for arranging the possibility for
performing the preparative GC runs useful for product isolation and
identification in Mühlheim. M.M.H. acknowledges support from the
American Chemical Society (PRF 46578-UFS) and SUNY Brockport for
a sabbatical leave at FSU Jena, Germany.
ꢀ
CH2), 18.1 (=C CH3), 17.9 (CH3), 15.5 ppm (CH3); MS (70 eV): m/z
(%): 138 (10) [M]+, 123(15) [C H15]+, 96 (87) [C7H12]+, 95 (60) [C7H11]+
9
, 81 (100) [C6H9]+, 69 (53) [C5H9]+, 67 (58) [C5H7]+.
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2002, 58, 6943–6950.
ACHTREUNG(1S,2R,3S)-1-Isopropenyl-2,3-dimethylcyclopentane (2,3-cis,trans-4; 4b):
1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=4.70 (s, 1H), 4.57 (s,
1H), 2.46 (q, 1H, 3J=8.5, 3J=7.8 Hz), 1.85 (m, 2H), 1.64 (s, 3H), 1.57
(m, 4H), 0.92 (d, 3H, 3J=5.0 Hz), 0.65 ppm (d, 3H, 3J=5.2 Hz);
13C NMR (100 MHz, [D]CHCl3, 258C, TMS): d=146.2 (=C), 108.6
(=CH2), 52.2 (cycl. CH), 48.3(cycl. CH), 42.0 (cycl. CH), 23.1 (cycl.
CH2), 27.0 (cycl. CH2), 22.3( =C-CH3), 20.4 (CH3), 14.6 ppm (CH3); MS
(70 eV): m/z (%): 138 (5) [M]+, 123(45) [C 9H15]+, 96 (43) [C7H12]+, 95
(100) [C7H11]+, 81 (90) [C6H9]+, 69 (39) [C5H9]+, 68 (56) [C5H8]+, 67 (69)
[C5H7]+.
ACHTREUNG(1S,2S,3S)-1-Isopropenyl-2,3-dimethylcyclopentane (2,3-trans,cis-4; 4c):
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Apparently, the concentration of the isolated product was too low to per-
form NMR spectroscopic studies. It was still possible to determine the
correct structure by comparing the data of the other isomers (cf. text).
MS (70 eV): m/z (%): 138 (6) [M]+, 123(49) [C 9H15]+, 96 (37) [C7H12]+,
95 (100) [C7H11]+, 81 (79) [C6H9]+, 69 (57) [C5H9]+, 68 (82) [C5H8]+, 67
(73) [C5H7]+.
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ACHTREUNG(1R,2S,3S)-1-Isopropenyl-2,3-dimethylcyclopentane (2,3-cis,cis-4; 4d):
1H NMR (400 MHz, [D]CHCl3, 258C, TMS): d=4.74 (s, 1H), 4.58 (s,
1H), 2.37 (m, 1H), 1.90 (m, 2H), 1.67 (s, 3H), 1.57 (m, 4H), 0.87 (d, 3H,
3J=5.1 Hz), 0.43ppm (d, 3H,
3J=5.3Hz); 13C NMR (100 MHz,
[D]CHCl3, 258C, TMS): d=145.6 (=C), 108.3( =CH2), 50.5 (cycl. CH),
38.4 (cycl. CH), 37.1 (cycl. CH), 28.7 (cycl. CH2), 23.7 (cycl. CH2), 22.5
ꢀ
(=C CH3), 15.4 (CH3), 6.7 ppm (CH3); MS (70 eV): m/z (%): 138 (9)
[M]+, 123(20) [C 9H15]+, 96 (92) [C7H12]+, 95 (47) [C7H11]+, 81 (88)
[C6H9]+, 69 (27) [C5H9]+, 68 (100) [C5H8]+, 67 (78) [C5H7]+.
2,3-cis,trans- or 2,3-trans,cis-2-Isopropenyl-1,3-dimethylcyclopentane
(5a): The concentration of the isolated product was too low to calculate
coupling constants and to assign the relative configuration. 1H NMR
(400 MHz, [D]CHCl3, 258C, TMS): d=4.83(m, 2H), 2.27 (m, 4H), 1,75
(s, 1H), 1.33 (s, 3H), 1.30 (s, 3H), 1.22 (m, 2H), 0.83 ppm (m, 6H);
13C NMR (100 MHz, [D]CHCl3, 258C, TMS): d=138.3 (C), 110.8
(=CH2), 36.0 (cycl. CH), 31.0 (cycl. CH2), 30.8 (cycl. CH2), 24.8 (cycl.
ꢀ
CH), 18.1 (CH3), 17.0 ppm (=C CH3); MS (70 eV): m/z (%): 138 (9)
Chem. Eur. J. 2008, 14, 6805 – 6814
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6813

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[36] Thermal radical sources (tert-butylperoxide, ethyleneoxide) showed
no tremendous effect on selectivity and the amount of polymeric
products when b-citronellene (2) is isomerized in the gas-phase (c.f.
reference [14]). Therefore, and because of the absence of any poly-
merization product, a radical chain mechanism can be ruled out.
[37] According to reference [13], the other transition states/intermedi-
ates are destabilized due to ecliptic arrangements of the methyl and
terminal vinyl group of b-citronellene (2), which contributes to the
cyclization process.
Received: February 18, 2008
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Published online: June 18, 2008
6814
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Chem. Eur. J. 2008, 14, 6805 – 6814