Initiation of radical chain reactions of thiol compounds and alkenes without any added initiator: Thiol-catalyzed cis/trans isomerization of methyl oleate

Article abstract of DOI:10.1002/chem.201103252

A kinetic study of the dodecanethiol-catalyzed cis/trans isomerization of methyl oleate (cis-2) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis-2, respectively, and an overall kinetic isotope effect k_H/k_D of 2.8 was found. The initiation was shown to be a complex reaction. The electron-donor/-acceptor (EDA) complex of dodecanethiol (1) and cis-2 formed in a pre-equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule-assisted homolysis (MAH) of the sulfur-hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E_A=82 kJ M^-1. Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well-known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf-life stability of cis-unsaturated thiol compounds and of mixtures of alkenes and thiol compounds. Copyright

Full text of DOI:10.1002/chem.201103252

FULL PAPER
DOI: 10.1002/chem.201103252
Initiation of Radical Chain Reactions of Thiol Compounds and Alkenes
without any Added Initiator: Thiol-Catalyzed cis/trans Isomerization of
Methyl Oleate
Ursula Biermann,[a] Werner Butte,[a] Rainer Koch,[a] Patrice A. Fokou,[b] Ogˇuz TꢀrꢀnÅ,[c]
Michael A. R. Meier,[c] and Jꢀrgen O. Metzger*[a]
Abstract: A kinetic study of the dodec-
anethiol-catalyzed cis/trans isomeriza-
tion of methyl oleate (cis-2) without
added initiator was performed by fo-
cusing on the initiation of the radical
chain reaction. The reaction orders of
the rate of isomerization were 2 and
0.5 for 1 and cis-2, respectively, and an
overall kinetic isotope effect kH/kD of
2.8 was found. The initiation was
shown to be a complex reaction. The
electron-donor/-acceptor (EDA) com-
plex of dodecanethiol (1) and cis-2
formed in a pre-equilibrium reacts with
thiol 1 to give a stearyl and a sulfuranyl
radical through molecule-assisted ho-
molysis (MAH) of the sulfur–hydrogen
bond. Fragmentation of the latter gives
the thiyl radical, which catalyzes the
cis/trans isomerization. A computation-
al study of the EDA complex, MAH
reaction, and the sulfuranyl radical cal-
culated that the activation energy of
the isomerization was in good agree-
ment with the experimental result of
EA =82 kJmꢀ1. Overall, the results may
explain that the thermal generation of
thiyl radicals without any initiator is re-
sponsible for many well-known ther-
mally initiated addition reactions of
thiol compounds to alkenes and their
respective polymerizations and for the
low shelf-life stability of cis-unsaturat-
ed thiol compounds and of mixtures of
alkenes and thiol compounds.
Keywords: isomerization · methyl
oleate · molecule-assisted homolysis
(MAH) · radicals · renewable re-
sources
Introduction
thiyl radicals.[4] These studies support the mechanism in
Scheme 1; however, side reactions are also at work and give
the same products but change the rate of reaction. Thus, for
The cis/trans isomerization of unsaturated fatty acids, that is,
oleic acid of lipids, in biological systems is of great impor-
tance.[1] Chatgilialoglu et al. showed that thiyl radicals are
among the most efficient isomerizing agents and studied in
detail the kinetics of the cis/trans isomerization of oleic acid
of phospholipids and methyl oleate with thiol compounds
occurring in a radical chain reaction.[2–4] The reaction was in-
itiated by either the thermal decomposition of azo com-
pounds,[2] g-irradiation,[2,3] or photochemically generated
Scheme 1. Thiyl radical-catalyzed isomerization of mono-unsaturated
fatty acid methyl esters.[2–4]
[a] Dr. U. Biermann, Prof. Dr. W. Butte, Dr. R. Koch,
Prof. Dr. J. O. Metzger
the rather high thiol concentrations of 0.30 and 0.93m, blank
experiments on nonirradiated samples showed some isomer-
ization. This outcome indicates “an unknown additional
thermal generation of thiyl radicals”, which could be of im-
portance in biological systems.[3]
Institute of Pure and Applied Chemistry
University of Oldenburg, Carl-von-Ossietzky Str. 9–11
26129 Oldenburg (Germany)
Fax : (+49)441-798-193718
E-mail: juergen.metzger@uni-oldenburg.de
The radical addition of thiol compounds to alkenes initiat-
ed photochemically or thermally with radical initiators is a
well-known and intensively studied reaction.[5] It is well
known that certain thiol/ene mixtures are quite thermally
unstable at room temperature, and many studies on the re-
lated shelf-life stability are available.[6] However, only very
few investigations on the initiation of this dark reaction
have been reported. In the 1980s, Nuyken et al. studied the
very efficient thermally initiated radical polymerization of 3-
[b] Dr. P. A. Fokou
University of Applied Sciences Emden/Leer
Fachbereich Technik, NaWaRo
Constantiaplatz 4, 26723 Emden (Germany)
[c] O. TꢀrꢀnÅ, M. A. R. Meier
Karlsruhe Institute of Technology (KIT)
Institute of Organic Chemistry
Fritz-Haber Weg 6, 76131 Karlsruhe (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103252.
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and 4-vinylbenzenethiol and dithiol compounds with dienes
and diynes without the addition of any special initiator.[7] Ki-
netic evidence was found for the molecule-assisted homoly-
Because a molecule that has an alkyl chain with a cis-con-
figured double bond and a thiol functionality should not be
stereochemically stable, we wondered about cis-octadec-9-
enethiol, which is commercially available and is “assigned to
prepare disordered self-assembled monolayers that may
have a significant influence on the fluidity of supported
liquid membranes.”[22]
The question is open as to how the dark reaction of radi-
cal addition reactions of thiol compounds to alkenes and the
thiol-catalyzed cis/trans isomerization of cis-alkenes ob-
served by Chatgilialoglu et al.[3] may be initiated without
any added initiator. To answer this question unambiguously,
we performed a kinetic and computational study of the
thiol-catalyzed thermal isomerization of methyl oleate be-
cause the isomerization reaction is much simpler than the
addition reaction and has been studied kinetically in
detail.[2–4] Most importantly, there is no hydrogen transfer in-
volved in the isomerization reaction. Thus, we could have a
chance to verify or exclude unambiguously initiation
through MAH[9] by performing an appropriate experiment
on the kinetic isotope effect kH/kD.
sis (MAH)[8] of the S H bond as an initiation reaction. The
ꢀ
activation energy of the addition of thiophenol to styrene as
a model reaction was EA =65 kJmꢀ1, which was in good
agreement with the calculated activation energy of the bi-
molecular reaction of thiophenol and styrene to give a thiyl
and
a 1-phenylethyl radical as an initiation reaction
(Scheme 2).[9] Klemm and Sensfuß observed the formation
Scheme 2. Radical formation by a bimolecular reaction of a thiol and an
alkene, a molecule assisted homolysis (MAH).[9]
of an electron-donor/-acceptor (EDA) complex of a thiol
and alkene in solution by UV spectroscopic analysis and
postulated the formation of a thiyl and an alkyl radical from
this EDA complex (Scheme 3).[10] This result is equivalent to
the MAH reaction postulated by Nuyken et al.[9] EDA com-
plexes of thiol compounds and alkenes have also been stud-
1
ied by H NMR spectroscopic analysis.[11]
Results
In preliminary experiments, we treated dodecanethiol (1)
with methyl oleate (cis-2) in a ratio of 1.1:1 without a sol-
vent at 1188C (Scheme 4). Analysis of the reaction solution
Scheme 3. Radical formation via a EDA complex of a thiol and an
alkene.[10]
Initiation of radical chain reactions by MAH or molecule-
induced homolysis[12] have been reported previously, for in-
stance, the thermal initiation of styrene polymerization
through the de Mayo mechanism,[13] the thermally initiated
addition of alkanes to alkenes[14] and alkynes[15] (i.e., the ane
reaction), uncatalyzed transfer hydrogenation and transfer
hydrogenolysis,[16] and the thermally induced redox reactions
of carbonyl compounds and alcohols[17] fall into this catego-
ry. It may be mentioned that this simple elementary reaction
of two closed-shell molecules to give two radicals, which is
the reverse reaction of the well-known disproportionation
reaction of two radicals, has not found an entrance into text-
books and has been remained rather unknown despite being
suggested about 50 years ago.[18] Recently, Mayer introduced
a generalized concept for hydrogen-atom transfer (HAT)
that included MAH reactions with a Marcus theory ap-
proach.[19]
The radical addition of thiol compounds to alkenes has at-
tracted renewed interest as a “thiol/ene click reaction” be-
cause of quantitative yields, rapid reaction rates, mild reac-
tion conditions, and compatibility with water and oxygen.[20]
It has been demonstrated recently that mixing thiol and
methyl 10-undecenoate, a renewable 1-alkene, gave the ad-
dition product quantitatively without the use of an initiator
and under solvent-free conditions.[21]
Scheme 4. Dodecanethiol-catalyzed isomerization of methyl oleate (cis-2)
to methyl elaidate (trans-2).
after 22 hours showed a [trans-2]/ACTHNUGTRNEUNG[cis-2] ratio of 2.89:1. The
concentration of thiol 1 and the total concentration of cis-2
and trans-2 was approximately constant over the reaction
time. Only very small amounts (i.e., <5%) of the addition
products methyl 9- and 10-dodecanethiylstearate were
formed. Traces of didodecyldisulfide (7), the expected termi-
nation product of the radical chain reaction, were also de-
tected. The ratio of [trans-2]/
24 hours. The addition of AIBN as an initiator showed a
[trans-2]/[cis-2] ratio of 4.28:1 after 15 minutes.
ACHTUNGTNER[NUNG cis-2] at 898C was 0.83:1 after
AHCTUNGTRENNUNG
Moreover, we studied the stereochemical stability of cis-
octadec-9-enethiol. A commercial sample[22] was used, and
the ratio of the trans and cis isomers, determined by GC
after arrival of the sample, was 0.98:1. At room tempera-
ture, the isomerization reaction continued (Scheme 5). The
[trans]/
5.9:1 was obtained after about 38 days (see Figure S1 in the
Supporting Information).[23] Chatgilialoglu et al. gave
ACHTUNGTREN[NGNU cis] ratio was 4:1 after 10 days and the final ratio of
a
rather similar equilibrium ratio of 5.15:1 at room tempera-
ture for methyl oleate.[4]
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Radical Initiation of a Thermal Thiol-Catalyzed Isomerization Reaction
FULL PAPER
Scheme 5. Isomerization of cis- to trans-octadec-9-enethiol.
The kinetic investigation of the isomerization reaction
was performed by mixing the substrates thiol 1 and cis-2,
both freshly distilled under nitrogen, at room temperature
under nitrogen without any solvent. A homogeneous solu-
tion was obtained. The isomerization reaction was studied at
89, 98, 108, and 1188C by using a mixture of 1 and cis-2 with
concentrations of 1.6 and 1.5m, respectively, at room tem-
perature in which the temperature dependence of the con-
centration of 1 and of cis-2 was taken into account (see Fig-
ure S2 in the Supporting Information).[24] Two experiments
at each temperature were performed in two reactors in par-
Figure 2. Rate of isomerization of cis-2 at 1188C: Dependence on the
concentration of 1 and cis-2.
allel. The ratio of [trans-2]/ACTHNUGRTNEUNG[cis-2] was determined by GC
analysis. The rate of isomerization ris was constant over the
studied range of conversion (Figure 1). Additionally, the de-
Figure 3. Arrhenius presentation of the temperature dependence of the
rate constant kexp. gives EA =82 kJmꢀ1 and A=2.0ꢂ106 mꢀ1.5 sꢀ1
.
Figure 1. Rate of isomerization of methyl oleate (cis-2) to trans-2 cata-
lyzed by dodecanethiol ([cis-2]/[1]=1:1.1) at 89, 98, 108, and 1188C.
pendence of the rate of isomerization on the concentration
of the reactants was studied at 1188C by using mixtures of 1
and cis-2 with concentrations of 3.1 and 0.49m and 0.4 and
2.28m, respectively (see Figure 2 and Table S1 in the Sup-
porting Information for the results).[23]
Evaluation of the dependence of the rate of isomerization
on the concentration of 1 and 2 (Figure 2) gives rate Equa-
tion (1) with rate constant kexp. The second reaction order
for thiol 1 and 0.5 reaction order for cis-2 seems to be re-
markable. Furthermore, monodeuterated dodecanethiol
C12H25SD (D-1) was reacted at 1188C by using a mixture of
D-1 (3.1m) and cis-2 (0.49m) to give ris,D-1 =5.5ꢂ10ꢀ5 sꢀ1.
The experiment was performed in parallel with 1 to give
Figure 4. Kinetic isotope effect: Rate of isomerization of cis-2 (0.96m)
catalyzed by thiol 1 (3.1m) and D-1 (3.1m), respectively (ris,1 =1.56ꢂ
10ꢀ4 sꢀ1; ris,d-1 =5.5ꢂ10ꢀ5 sꢀ1).
r
is,1 =1.56ꢂ10ꢀ4 sꢀ1 (Figure 4), thus revealing an overall ki-
The temperature dependence of kexp (Figure 3) gives the
Arrhenius equation [Eq. (2)] (where EA =82 kJmolꢀ1 and
A=2.0ꢂ106 mꢀ1.5 sꢀ1).
netic isotope effect kH/kD of 2.8.
dð½t-2ꢁ=½c-2ꢁÞ
2
1=2
ð1Þ
ris ¼
¼ kexp½1ꢁ ½2ꢁ
log kexp ¼ 6:3ꢀ4285=T
ð2Þ
dt
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J. O. Metzger et al.
Computational results: We performed DFT calculations
with the new dispersion-corrected functional B97D. This ap-
proach and several others have been developed with an im-
proved description of weak van der Waals interactions in
mind.[25] The suitability of B97D for the given problem has
been evaluated extensively.[26,27]
The structures for all the compounds shown in Scheme 6
have been optimized and their relative enthalpies deter-
mined. Spin contamination does not play a significant role
in all the structures with unpaired electrons.[28] (The com-
plete results are compiled in Table S2 in the Supporting In-
formation).
Figure 5. Energy profile of the initiation reaction.
However, all the calculated energies of 9-4 and10-4 differ by
as little as 3 kJmꢀ1, so that all the reported numbers refer to
the radical at the 10 position. The fragmentation of sulfuran-
yl radical 5 to thiyl radical 3 and thiol 1 is calculated to be
an endothermic but exergonic reaction (DHR =19.4 kJmꢀ1
and DGR =ꢀ31.1 kJmꢀ1 at 1008C).
Scheme 6. Radical chain mechanism of the thermal isomerization of cis-2
catalyzed by thiol 1. Initiation by MAH of thiol 1 and the EDA complex
of 1 and cis-2 formed in pre-equilibrium (reaction 0) to give alkyl radical
4 and sulfuranyl radical 5 in reaction 1.
Discussion
Chatgilialoglu et al. showed that the thiol-catalyzed rate of
isomerization of cis-2 to trans-2 is directly proportional to
the steady-state concentration of thiyl radical 3 [Scheme 1;
Eq. (3)].[2–4]
The enthalpic difference between cis-2 and trans-2 is cal-
culated to be 3.8 kJmꢀ1, which is about 1 kJ lower than the
experimental results[3] and calculations by Chatgilialoglu
et al.[4] The EDA complex of 1 and cis-2 is formed through a
hydrogen bond from thiol 1 to the electron-rich double
bond of cis-2 (see Figure S3 in the Supporting Information)
and possesses a reaction enthalpy of DHR =ꢀ39 kJmꢀ1, but
is endergonic because of entropic reasons. The reaction en-
thalpy of the MAH reaction of thiol 1 and cis-2 is calculated
to be 191 kJmꢀ1. In comparison, the reaction enthalpy of the
MAH reaction of 1 and the EDA complex is determined to
be 212 kJmꢀ1. With respect to the substrates, this finding
gives a remarkably small enthalpy of only 173 kJmꢀ1 consid-
ering the exothermic formation of the EDA complex
(Figure 5). The activation energy of the MAH reaction is
approximately equal to the reaction enthalpy because the
activation energy of the reverse disproportionation reaction
is approximately EA =0 kJmꢀ1.[14,16,18] The energy profile of
the initiation reaction is given in Figure 5. In principle, two
regioisomeric alkyl radicals 4 with a single electron in the 9
or 10 position (9-4 and 10-4, respectively) can be expected.
dð½trans-2ꢁ=½cis-2ꢁÞ kZa kfE þ kaEkfZ
ð3Þ
ris ¼
¼
½3ꢁ ¼ kis½3ꢁ
kEf kZf
dt
The rate of the isomerization of the thermal reaction has
a reaction order of two and 0.5 for thiol 1 and cis-2, respec-
tively, thus giving the first evidence that both substrates are
involved in the initiation reaction of the radical chain and
the formation of thiyl radical 3. Moreover, the kinetic iso-
tope effect kH/kD gives ample evidence that a hydrogen-
transfer reaction is involved in the rate-determining step of
the initiation reaction.
To obtain more detailed information on the initiation re-
action, we checked the reaction order for both substrates ex-
pected for the initiation reactions suggested in previous re-
ports.[9,10] The initiation by alkene-assisted homolysis of the
ꢀ
S H bond in a bimolecular reaction of thiol 1 and cis-2 to
give thiyl radical 3 and alkyl radical 4, a MAH reaction as
suggested by Nuyken et al. (Scheme 2),[9] would give a reac-
tion order of 0.5 for 1 and cis-2, which is not in agreement
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Radical Initiation of a Thermal Thiol-Catalyzed Isomerization Reaction
FULL PAPER
with the experimental results (see Scheme S1 in the Sup-
porting Information). Radical formation through a mono-
molecular fission of the EDA complex of thiol 1 and alkene
cis-2 formed in a pre-equilibrium (Scheme 3)[10] gives the
same reaction order for both substrates and can be excluded
as well (see Scheme S2 in the Supporting Information).
The experimentally determined reaction order of two for
thiol 1 gives an indication that possibly more than one thiol
molecule may be involved in the initiation reaction. It could
be possible that the EDA complex of thiol 1 and alkene cis-
2 may react with another equivalent of thiol 1 in a MAH re-
action to give a thiyl and an alkyl radical and thiol 1. The re-
rate-determining reaction step 1 (Scheme 6). Interestingly
and as further confirmation of the proposed mechanism, the
activation energy of the initiation reaction through pre-equi-
librium 0 and reaction of EDA and 1 in reaction step 1
(Scheme 6) is calculated to be EA =173 kJmꢀ1, which is
about 19 kJmꢀ1 lower than the activation energy of
192 kJmꢀ1 of the direct MAH reaction of 1 and cis-2
(Figure 5). The late transition state of the MAH reaction
gives evidence that no or only a very minor steric effect on
the rate of reaction 1 (Scheme 6) can be expected.
ꢀ
ꢁ
1=2
k1KEDA
2kt
1
K3
2
1=2
ris ¼ kis½5ꢁ ¼ kis
½1ꢁ ½2ꢁ
sulting steady-state concentration of radical 3 and the reac-
tion order of one for 1 was again not in agreement with the
experimental results (see Scheme S3 in the Supporting Infor-
mation).
ð4Þ
dð½t ꢀ 2ꢁ=½c ꢀ 2ꢁÞ
2
1=2
ris ¼
¼ kexp½1ꢁ ½2ꢁ
dt
ꢀ
ꢁ
À
1=2
k1KEDA
2kt
1
K3
However, a closer look at the steps in this initiation reac-
tion showed that radical 3 is not obtained directly in the
MAH step. In the first step, a sulfuranyl radical 5 may be
formed, as shown in reaction step 1, which is the rate-deter-
mining step of Scheme 6. The rate of isomerization obtained
for a radical chain reaction (as depicted in Scheme 6) is
given in Equation (4) and is in perfect agreement with the
experimental rate given in Equation (1) (see Scheme S4 in
the Supporting Information).[29,30] The rate constant kexp is
given in Equation (5).
Intramolecular addition reactions of an alkylthiyl radical
with an additional SH group to give a cyclic sulfuranyl radi-
cal have been detected by laser flash photolysis.[31a,b] In con-
trast, an intermolecular addition of simple alkylthiyl radicals
to thiol or thioether compounds has not been directly ob-
served in solution, which can be explained by the unfavora-
ble equilibrium constant K3 (Scheme 6). In contrast, the sta-
bilization of alkylthiyl radicals by the formation of sulfuran-
yl radicals RSS(H)R (R=CH3, C2H5, i-C3H7, tert-C4H9)
could be shown by photolysis of thiol compounds in glassy
matrices at 77 K.[31a,c] Moreover, the reaction of an octadeca-
nethiyl radical with octadecanethiol via a sulfuranyl-type
radical to give the perthiyl radical was studied within the
channel of a thiol/thiourea clathrate.[32] Remarkably, in the
example discussed herein, the sulfuranyl radical is directly
formed in the initiation reaction 1 (Scheme 6).[33] Thus, the
rate of isomerization will be proportional to the steady-state
concentration of sulfuranyl radical 5 and the concentration
ð5Þ
ð6Þ
kexp ¼ kis
Á
EA;exp ¼ EA;is þ 0:5 EA;1 þ DHꢂ ꢀ EA;t ꢀ DH3ꢂ
0
To further confirm the validity of this mechanism, the
overall activation energy was determined by applying Equa-
tion (6). By using the calculated data in Table S2 (see the
Supporting Information; EA,1 =212.5; DHꢂ =ꢀ39.2; EA,t =0;
0
DHꢂ =19.4 kJmꢀ1) and EA,is ꢃ10 kJmꢀ1,[35] an activation
3
energy EA,exp of about 77 kJmꢀ1 is estimated, which is in
good agreement with the experimentally derived activation
energy of EA =82 kJmꢀ1 [Figure 3, Eq. (2)].
The kinetic isotope effect kH/kD of the rate of isomeriza-
ꢀ
tion gives clear evidence that the cleavage of the S H bond
is involved in the rate-determining reaction step. Equa-
tions (4) and (5) show that preferentially reaction
1
(Scheme 6), the MAH reaction of the EDA complex and
thiol 1, and, possibly to a small extent, equilibria 0 and 3
(Scheme 6) can contribute to the isotope effect of the over-
all reaction (i.e., kH/kD =2.8). This result gives a kinetic iso-
tope effect k1,H/k1,D of approximately 7.8 at 1188C. The ki-
netic isotope effect of hydrogen abstraction by carbon radi-
cals from thiol compounds has been reported to be kH/kD =
1.9–6.6.[36]
Conclusion
of thiyl radical
3 will be steered by equilibrium 3
(Scheme 6), which is strongly shifted to the right, as known
from a previous report[31] (see Schemes S4 and S5 in the
Supporting Information) and shown by our DFT calcula-
tions (see Table S2 in the Supporting Information).
The results of our study of the thermal thiol-catalyzed iso-
merization of cis-2 are in agreement with an initiation of the
radical chain reaction by the formation of an EDA complex
of a thiol and an alkene compound in a pre-equilibrium re-
action and a MAH reaction of an EDA complex and thiol
compound to give an alkyl and a sulfuranyl radical. The
latter is in equilibrium with the thiyl radical, which catalyzes
the isomerization reaction. It can be assumed that this initia-
tion reaction can explain the “unknown additional thermal
generation of thiyl radicals” reported by Chatgilialoglu
et al.[3] Moreover, it can be assumed that this reaction is the
initiation reaction of the many well-known thermally initiat-
ꢀ
The bond-dissociation energy of the S H bond of about
366 kJmꢀ1[34] will be decreased by the MAH reaction of 1
and cis-2 to about 192 kJmꢀ1 (see Table S2 in the Supporting
ꢀ
Information) because of the simultaneous formation of a C
H bond in alkyl radical 4 with the homolytic dissociation of
ꢀ
the S H bond (Scheme 2). Additionally, the activation
energy of the initiation reaction will be decreased by the pri-
mary formation of the stabilized sulfuranyl radical 5 in the
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J. O. Metzger et al.
sions, and Stefanie Baldauf for technical assistance. Generous allocation
of computer time at the CSC (University of Oldenburg) is gratefully ac-
knowledged.
ed addition reactions of thiol compounds to alkenes and
their respective polymerizations without any initiator. Fur-
thermore, this reaction is the reason for the low shelf-life
stability of cis-unsaturated thiol compounds and mixtures of
alkenes and thiol compounds, thus combining the sugges-
tions of Nuyken et al.[9] and Klemm and Sensfuß.[10]
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[3] C. Chatgilialoglu, A. Altieri, H. Fischer, J. Am. Chem. Soc. 2002,
124, 12816–12823.
Experimental Section
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General: Dodecanethiol (1) and cis-octadec-9-enethiol were purchased
from Aldrich (Steinheim, Germany). Methyl oleate (cis-2; 91.2% oleic
acid, 3.0% palmitic acid, 2.8% linoleic acid, 1.9% stearic acid) was pur-
chased from T+T Oleochemie GmbH (Alzenau, Germany). Compounds
1 and cis-2 were distilled under nitrogen by means of a Kugelrohr oven
and were stored under nitrogen. cis-Octadec-9-enethiol was used as re-
ceived.
Analytical equipment: Analytical GC was performed on a Varian 3400
chromatograph with an flame ionization detector (FID) and fused-silica
capillary column SP 2380-FAME (60 m; I.D.=0.25 mm; dF =0.20 mm; Su-
pelco). Mass spectra were recorded on a Finnigan MAT 95 spectrometer.
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker DRX 500
spectrometer at 300 K with a residual nondeuterated solvent (1H NMR)
or CDCl3 (13C NMR) as internal standards.
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nical University of Munich, Munich, 1986.
Dodecanethiol-D1 (D-1): Compound 1 (5 g) and ethanol-D1 (12 mL)
were heated to reflux for 20 min. Ethanol-D1 was removed in vacuo. This
procedure was repeated two times. The product was distilled in vacuo
(4.3 g, 86%). H NMR spectroscopic analysis showed a degree of deuter-
ation of >98%.
1
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Kinetic measurements: A Schlenk-type reactor (I.D.=12 mm, height=
45 mm, volume=5 mL) with a septum, stirring bar, and heating mantle
connected to a circulation thermostat was used. Appropriate amounts of
1 and cis-2 were charged under nitrogen and mixed at room temperature
by stirring (1000 rpm). The mixture was heated in <2 min to the prese-
lected reaction temperature and stirring was stopped. Two experiments
at each temperature and concentration were performed in two reactors
in parallel. Samples (1 mL) for GC analysis were removed by using a sy-
ringe through the septum at preselected reaction times and were diluted
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immediately with CH2Cl2. The [trans-2]/ACHTNUTGRNEUNG[cis-2] ratio was determined by
GC analysis.
Isomerization of cis-octadec-9-enethiol: cis-Octadec-9-enethiol (100 mg)
was placed in a vial with a septum under nitrogen. Samples (1 mL) for
GC analysis were removed by using a syringe through the septum at pre-
selected reaction times.
Computational details: Most of the calculations were performed by using
the program package Gaussian09.[37] DFT with the dispersion-corrected
ACTHNUTRGNEUNG
B97D functional[38] and the triple-zeta basis set 6–311+G(d,p)[39] was
used for the optimization of the ground-state geometries in the gas phase
and thermochemical calculations. Additional single-point calculations
employed the M06–2X functional.[40] The necessary URCCSD(T)/6–
31G(d) calculations for the G3ACTHNURTGNEU(GN MP2)-RAD level of theory employed the
program package MOLPRO.[41] The default convergence criteria and in-
tegration grid of the program were used. Stationary points were verified
as ground states by calculating the number of imaginary harmonic vibra-
tional frequencies at the same level of theory.
[22] J. Porwoll, Material Matters 2010, 5, 55.
[23] See the Supporting Information.
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[25] a) M. Korth, S. Grimme, J. Chem. Theory Comput. 2009, 5, 993;
b) Review: S. Grimme, WIREs Comput. Mol. Sci. 2011, 1, 211; c) L.
Goerigk, S. Grimme, Phys. Chem. Chem. Phys. 2011, 13, 6670.
[26] It is documented that reactions of organic radicals are well descri-
bed by the new functionals, see: V. Tognetti, P. Cortona, C. Adamo,
Int. J. Quantum Chem. 2010, 110, 2320; another new, but well-estab-
lished functional, M06–2X was employed to recalculate all the reac-
tions in Scheme 6 by using the B97D-based geometries, thus giving
Acknowledgements
We kindly acknowledge financial support from the German Federal Min-
istry of Food, Agriculture and Consumer Protection (represented by the
Fachagentur Nachwachsende Rohstoffe; FKZ 2026905). We thank Pro-
fessor Dr. Jꢀrgen Gmehling (University of Oldenburg) for providing the
data on the density of our substrates at reaction temperatures, Professor
Dr. Oskar Nuyken (Technical University of Mꢀnchen) for helpful discus-
8206
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8201 – 8207

Radical Initiation of a Thermal Thiol-Catalyzed Isomerization Reaction
FULL PAPER
reaction energies usually within 5 kJmolꢀ1 (see the Supporting Infor-
mation for full details).
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[35] kis =1.7ꢂ105 mꢀ1 sꢀ1 (room temperature)[4] gives EA =10 kJmꢀ1 as-
suming logA=7.
[27] By using a model system (MeS+MeSH!MeSSHMe), additional
benchmarking was performed at the G3
N
[36] W. A. Pryor, K. G. Kneipp, J. Am. Chem. Soc. 1971, 93, 5584–5586.
[37] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
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Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
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Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
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Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢄ.
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2003, 118, 4849), which allows a highly accurate calculation of open-
shell systems, such as the sulfuranyl radical 5; all the methods pre-
dict the existence of this radical, which is stabilized by 6–
21 kJmolꢀ1 (8 kJmolꢀ1 at G3
ACTHUNTGRENNG(U MP2)-RAD); although the dispersion-
corrected functionals slightly overestimate the stability (i.e., 19–
21 kJmolꢀ1) of the sulfuranyl radical, all the optimized geometries
are very similar (see the Supporting Information for full details).
[28] The influence of spin contamination was additionally probed for at
the UHF/6–311+GACHTUNGTRENNUNG(d,p) level of theory as DFT methods sometimes
inaccurately calculate this property; again, no significant contamina-
tion of the wavefunction with higher spin states has been found.
[29] Interestingly, an analogous rate equation that is in agreement with
the experimental rate given in Equation (1) is obtained by assuming
a MAH reaction of a hydrogen-bonded thiol dimer 12 and 2 (see
Scheme S5 in the Supporting Information); it is not possible to dif-
ferentiate both possibilities kinetically; dimerization constants of
primary alkylthiol compounds Kdimer =0.01 have been reported;[30]
the association constant of cyclohexene and 4-chlorothiophenol was
determined to be KEDA =0.1.[11]
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[40] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215.
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Werner, P. J. Knowles, R. Lindh, F. R. Manby, M. Schꢀtz, P. Celani,
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[33] By using a dithiol that is a 1,3-propanedithiol instead of a mono-
thiol, such as 1, the overall reaction (Scheme 6) was significantly
faster, thus giving evidence that the initiation reaction 1 may be an
intra- and monomolecular MAH reaction; we thank a referee for
this valuable suggestion.
Received: October 14, 2011
Revised: February 15, 2012
Published online: May 16, 2012
Chem. Eur. J. 2012, 18, 8201 – 8207
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8207