Activation parameters for the epoxidation of substituted cis/trans pairs of 1,2-dialkylalkenes by dimethyldioxirane

Article abstract of DOI:10.1002/ejoc.200600427

The first activation parameter data for the reaction of dimethyldioxirane (1) with five cis/trans pairs of alkenes are reported. The epoxidation of cis-1,2-dialkylalkenes (2cis: R¹ = Me, R² = iPr; 3cis: R¹ = Me, R² = tBu; 4cis: R¹ = R² = Et; 5cis: R¹ = Et, R² = iPr; 6cis: R¹ = Et, R² = tBu) and trans-1,2-dialkylalkenes (2trans: R¹ = Me, R² = iPr; 3 trans: R¹ = Me, R² = tBu; 4trans: R¹ = R² = Et; 5trans: R¹ = Et, R² = iPr; 6trans: R¹ = Et, R² = tBu) by 1 produced the corresponding epoxides, quantitatively and stereospecifically, as the sole observable products. Activation parameters of the epoxidation of the five pairs of alkenes, 2cis-6cis and 2trans-6trans, by 1 were determined using the Arrhenius method. Enhanced selectivity for cis- vs. trans-alkene epoxidation was observed at lower temperatures. In general, the ΔG^? terms were larger and showed more variability for the reaction of 1 with trans-alkenes as compared to those for the corresponding cis isomers. The ΔH^? terms mirrored trends observed in ΔG ^? because ΔS^? terms for all ten of the compounds were roughly identical. The ΔΔG^? values, a comparison of the trans to the cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol for the five sets of data and appeared to be dependent on relative steric interactions. The experimental activation parameter data, consistent with predictions from ab initio calculations based on a spiro transition-state model, showed that the lower reactivity of trans-alkenes is due to enthalpy factors. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600427

FULL PAPER
DOI: 10.1002/ejoc.200600427
Activation Parameters for the Epoxidation of Substituted cis/trans Pairs of
1,2-Dialkylalkenes by Dimethyldioxirane
Brian S. Crow,[a] W. Rucks Winkeljohn,[a] Angela Navarro-Eisenstein,[a]
Elba Michelena-Baez,[a] Paul J. Franklin,[a] Pedro C. Vasquez,[a] and Al Baumstark*[a]
Keywords: Epoxidation / Activation parameters / Kinetics / Dimethyldioxirane
The first activation parameter data for the reaction of di-
methyldioxirane (1) with five cis/trans pairs of alkenes are
reported. The epoxidation of cis-1,2-dialkylalkenes (2cis: R1
= Me, R2 = iPr; 3cis: R1 = Me, R2 = tBu; 4cis: R1 = R2 = Et;
5cis: R1 = Et, R2 = iPr; 6cis: R1 = Et, R2 = tBu) and trans-1,2-
dialkylalkenes (2trans: R1 = Me, R2 = iPr; 3trans: R1 = Me, R2
= tBu; 4trans: R1 = R2 = Et; 5trans: R1 = Et, R2 = iPr; 6trans:
R1 = Et, R2 = tBu) by 1 produced the corresponding epoxides,
quantitatively and stereospecifically, as the sole observable
products. Activation parameters of the epoxidation of the five
pairs of alkenes, 2cis–6cis and 2trans–6trans, by 1 were de-
termined using the Arrhenius method. Enhanced selectivity
for cis- vs. trans-alkene epoxidation was observed at lower
temperatures. In general, the ∆G‡ terms were larger and
showed more variability for the reaction of 1 with trans-al-
kenes as compared to those for the corresponding cis iso-
mers. The ∆H‡ terms mirrored trends observed in ∆G‡ be-
cause ∆S‡ terms for all ten of the compounds were roughly
identical. The ∆∆G‡ values, a comparison of the trans to the
cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol
for the five sets of data and appeared to be dependent on
relative steric interactions. The experimental activation pa-
rameter data, consistent with predictions from ab initio calcu-
lations based on a spiro transition-state model, showed that
the lower reactivity of trans-alkenes is due to enthalpy fac-
tors.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
Dioxiranes are important reagents for the derivatization
of a wide variety of substrates.[1] Whether generated in
situ[2] or isolated as a solution,[3] dioxiranes have been
shown to be a powerful class of oxidizing agents. In particu-
lar, dimethyldioxirane (1) has been widely used due to the
ease of preparation, efficiency of oxygen atom transfer and
environmental factors. Oxidations by 1 continue to be a
topic of great interest in part because of the high chemo-,
regio- and stereoselectivity observed in these reactions. For
example, there has been a recent report of the kinetic study
Figure 1. Representation of the spiro transition states for epoxid-
ation of cis- and trans-alkenes by dimethyldioxirane (1).
The observed reactivity differences between cis- and
of the N-oxidation of substituted pyridines with 1;[4] how- trans-alkenes led, in part, to our postulation that the elec-
ever, the epoxidation of alkenes remains the most exten- trophilic epoxidation of alkenes by 1 proceeds through an
sively studied process. Historically, kinetic studies at 23 °C oxygen atom transfer process with a “spiro” transition
have shown that the reaction of alkenes with 1 is sensitive state.[5] The synthesis of epoxides by reaction with asym-
to steric factors, with cis-alkenes having a greater relative metric dioxiranes generated in situ in biphasic systems, of
reactivity as compared to that of the corresponding trans value for the preparation of chiral compounds, also has
isomers[5] (see Figure 1).
been shown to be mechanistically consistent with a spiro
transition-state model.[6] Recent kinetic experiments on the
epoxidation of geraniol and model olefins have further in-
vestigated medium effects on the reaction with dimethyldi-
oxirane (1).[7] The relative reactivity of epoxidation of al-
kenes by 1 has been modeled using various theoretical
methods, including ab initio calculations.[3c,8a–8c] Other than
rate-constant data at one temperature, there is little experi-
[a] Department of Chemistry, Center for Biotechnology and Drug
Design, Georgia State University,
Atlanta, Georgia 30303-3083, USA
Fax: +1-404-651-1416
E-mail: chealb@langate.gsu.edu
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Epoxidation of Substituted cis/trans Pairs of 1,2-Dialkylalkenes
FULL PAPER
mental evidence to evaluate the factors determining relative conditions, produced k2 values that were within experimen-
reactivity of alkene epoxidation by dioxirane. In this study, tal error ( 5%) regardless of the compound used in excess.
we report the first set of experimental activation parameter The second-order rate constants for the reactions (1) and
data for the oxidation of five cis/trans-dialkylalkene pairs (2) were determined over approximately a 25–30 degree
by dimethyldioxirane (1).
temperature range. The second-order rate constants at
23 °C for the epoxidation of alkenes 3cis–6cis and 3trans–
6trans have been previously reported and are in agreement
with the current data.[3b,3c,5,9,13] The rate constant data
show that selectivity for the epoxidation of the cis vs. trans
Results and Discussion
Solutions of dimethyldioxirane (1) in dried acetone were compounds increases at lower temperatures. For example,
prepared from the reaction of “Oxone” with acetone. The the reaction of 2cis and 2trans with 1 shows a kcis/ktrans
reaction of 1 with five cis-1,2-dialkylalkenes [cis-4-methyl- ratio of 9.4 at 15 °C which decreases to 7.1 at 40 °C. Earlier
2-pentene (2cis), cis-4,4-dimethyl-2-pentene (3cis), cis-3- studies[3b] have investigated epoxidation selectivity by 1 as
hexene (4cis), cis-2-methyl-3-hexene (5cis), cis-2,2-dimethyl- a function of steric interactions.[3b] The present results show
3-hexene (6cis)] and the five corresponding trans-1,2-dialk- that selectivity is quite sensitive to temperature changes as
ylalkenes [trans-4-methyl-2-pentene (2trans), trans-4,4-di- well. Clearly, to achieve maximum selectivity, epoxidation
methyl-2-pentene (3trans), trans-3-hexene (4trans), trans-2- should be carried out at the lowest temperature possible.
methyl-3-hexene
(5trans),
trans-2,2-dimethyl-3-hexene For each temperature, the k2 values listed in Tables 1 and 2
(6trans)] was found to be stereospecific and produced the are the average of at least three separate, kinetic experi-
corresponding epoxides exclusively [reactions (1 and 2)], as ments.
expected.
The activation parameters for each of the five pairs of
Product studies for the epoxidation of 2cis–6cis/2trans– cis/trans-alkenes were calculated from the data in Tables 1
6trans were carried out with at least 1.1 equiv. of 1. In all and 2 by the Arrhenius method. Excellent correlations were
cases, the epoxides were obtained in essentially quantitative obtained for all ten data sets. This is the first report of acti-
yields (Ն 98%). Epoxides generated from alkenes 3cis–6cis/ vation-parameter data for the reaction of 1 with cis/trans
3trans–6trans have been previously reported as products of pairs of alkenes. Activation parameters have been pre-
oxidation by 1.[3b–3c,5,9] The data for epoxides from 2cis and viously reported for the epoxidation of several isolated al-
2trans have been published.[10–12] Physical and spectroscopic kenes by 1.[14,15] Representative Arrhenius plots for alkene
data for the ten epoxides [reactions (1) and (2)] are consis- pairs 2 and 6 are shown in Figure 2 (r = 0.998 for 2cis, r =
tent with those of authentic samples and with the published 0.998 for 2trans; r = 0.993 for 6cis, r = 0.996 for 6trans). The
data.[3b,9–12]
Arrhenius plots for the cis-alkenes are very closely grouped,
The kinetic study of the epoxidation of the ten alkenes indicative of essentially similar reactivity of all five cis-al-
by 1 was carried out employing UV methodology. As ex- kenes with dioxirane 1, regardless of the R group. The
pected,[5] each reaction was found to be of the first order in Arrhenius plots for the trans-alkenes show a larger spread
both alkene and dioxirane, of the second order overall. The between the linear correlations, with those with the larger
kinetic experiments, conducted under pseudo-first-order steric interactions showing lower reactivity towards 1. The
(1)
(2)
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A. Baumstark et al.
FULL PAPER
Table 1. Second-order rate constants for the epoxidation of cis-al-
kenes, 2cis–6cis, by 1 in dried acetone.
Table 2. Second-order rate constants for the epoxidation of trans-
alkenes, 2trans–6trans, by 1 in dried acetone.
[a] 0.3 °C. [b] Errors are standard deviation of three experiments.
[c] Ref.[5] [d] Ref.[3c]
[a] 0.3 °C. [b] Errors are standard deviation of three experiments.
[c] Ref.[5] [d] Ref.[3c]
findings are consistent with previous studies, at 23 °C,
which showed that the k2 values for epoxidation of cis-al- trans- and cis-alkenes toward 1 can be readily evaluated by
kenes by 1 are essentially independent of the relative sub- examination of a ∆∆G‡ term (obtained by subtracting the
stituent size while those for trans-alkenes are highly depend- ∆G‡ value for the cis-alkene from that for the corresponding
ent on the relative alkyl substituent size.[5] The calculated trans isomer). All five ∆∆G‡ terms are consistent with
activation-parameter data for epoxidation of the five pairs one another, yielding positive values of 1.2 0.3 to
of cis/trans-alkenes are summarized in Table 3.
1.8 0.4 kcal/mol. This clearly shows that the cis-alkenes
Interestingly, the ∆G‡ values for the cis-alkene epoxid- are more reactive toward 1 than the corresponding trans
ation in this study are all roughly of the same magnitude isomers. In addition, it appears that the increasing relative
(within 0.2 kcal/mol) with an average value of 18.6 kcal/ alkyl size correlates with the increase in the magnitude of
mol, indicative of essentially little or no sensitivity to alkyl the ∆∆G‡ values.
group variation and the resulting steric interactions. In con-
For example, alkenes studied with the least steric size
trast, the ∆G‡ values for the trans-alkenes are of greater (2trans, 2cis and 4trans, 4cis) have a ∆∆G‡ value of 1.2–
magnitude than those for the cis-alkenes. The variability of 1.3 kcal/mol. Furthermore, pairs 3trans/3cis and 5trans/5cis
the ∆G‡ values for the trans-alkenes, ranging from 19.6 to have a ∆∆G‡ value of 1.6–1.4 kcal/mol, while the most hin-
20.5 kcal/mol, appear to correlate directly with increasing dered pair 6trans/6cis has the highest net steric bulk and
steric interactions (vide infra). Normally, interpretation of exhibits a ∆∆G‡ value of 1.8 kcal/mol. However, the magni-
activation-parameter data is focused on the ∆G‡ values due tude of the maximum difference (0.6 kcal/mol) is less than
to the inherent problem of compensating errors between the error. Because the ∆G‡ values for the cis-alkenes are
∆H‡ and ∆S‡ terms. However, the ∆S‡ values determined essentially identical, the observed apparent increase in mag-
in this study are essentially constant across both series with nitude of the ∆∆G‡ values appears to be due to the changes
the magnitude of the difference generally close to the error in ∆G‡ values for the epoxidation of the trans-alkenes.
( 0.8 eu). Thus, the ∆H‡ terms in the present study should Clearly, the ∆G‡ values of the trans-alkenes are consistently
be less prone to systematic error. A comparison of the ∆H‡ 1 kcal/mol or more larger than those for the corresponding
terms shows essentially the same trends as observed in com- cis isomers. Since the ∆S‡ values for all ten of the alkenes
parison of the ∆G‡ value, which verifies the consistency of are essentially identical, the increase in ∆G‡ for the trans
the data. The inherent reactivity difference between the compounds is solely due to enthalpy factors.
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Epoxidation of Substituted cis/trans Pairs of 1,2-Dialkylalkenes
FULL PAPER
Figure 2. Arrhenius plots for the epoxidation of cis-4-methyl-2-pentene (2cis, ᭿)/trans-4-methyl-2-pentene (2trans, ᮀ) and cis-2,2-dimethyl-
3-hexene (6cis, ᭡)/trans-2,2-dimethyl-3-hexene (6trans, ∆) by 1 in dried acetone.
Table 3. Activation parameters[a] for the epoxidation of 2cis/2trans–6cis/6trans by 1 in acetone.
Alkene
k2 [–1 s–1][a]
∆H‡ [kcal/mol][b]
∆S‡ [eu][b]
∆G‡ [kcal/mol][b]
∆∆G‡ [kcal/mol][c]
2cis
0.43 0.01
7.28 0.15
–37.7 0.3
18.45 0.16
1.3 0.3
2trans
3cis
0.0488 0.0015
0.261 0.002
9.42 0.25
7.37 0.48
–34.8 0.4
–38.3 0.4
19.73 0.17
18.70 0.21
1.6 0.4
1.2 0.3
1.5 0.4
1.8 0.4
3trans
4cis
0.0182 0.0007
0.46 0.01
9.27 0.49
7.14 0.54
–37.2 0.4
–37.9 0.3
20.28 0.17
18.38 0.17
4trans
5cis
0.0570 0.0017
0.322 0.009
8.25 0.68
7.92 0.20
–38.3 0.4
–36.0 0.4
19.61 0.17
18.60 0.17
5trans
6cis
0.0295 0.0012
0.263 0.005
9.92 0.21
7.51 0.35
–34.3 0.4
–37.8 0.4
20.08 0.18
18.71 0.17
6trans
0.0119 0.0004
9.47 0.46
–37.3 0.4
20.52 0.21
[a] 23 °C. [b] Errors listed at 60% confidence limits. [c] ∆G‡(trans) – ∆G‡(cis).
Epoxidation of alkenes by 1 have been extensively mod- on the “spiro transition state” model.[8b] Subsequent calcu-
eled using several different computational approaches. Se- lations[8c] also predicted ∆∆G‡ values and that the acti-
veral ab initio studies successfully modeled the epoxidation vation entropies would be very similar for epoxidations of
of ethylene by dioxirane.[8] In general, this approach has cis- and trans-alkenes by dioxirane.[16] Less time-consuming
strongly supported[8a] a one-step reaction with a “spiro” semiemperical (AM1) calculations have been performed, in-
transition state.[5] High-level calculations on the epoxid- dependently, on the epoxidation of a series of cis- and a
ation of cis/trans-2-butene by dimethyldioxirane have been series of trans-alkenes with dimethyldioxirane.[3c,9] As ex-
carried out by Jorgensen and Houk.[8b] This extensive study pected, the “spiro” model was again found to be the only
predicted that the epoxidation for cis-2-butene by dioxirane acceptable transition state geometry. Using this semiemper-
would be more reactive than that for the trans isomer by ical approach, relative transition state energies were calcu-
1.7 kcal/mol with essentially equal entropy of activation lated for the epoxidation of a series of configurationally
values for epoxidation for the cis and trans isomers based similar alkenes which allowed for relative rates to be esti-
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The dioxirane/acetone solution (0.200 mL, 0.070 ) was added
through a syringe and the solution was mixed for 2–3 s. Upon com-
pletion of the kinetic run, the solution was analyzed by GC-MS
and the expected epoxide was observed in the appropriate amount
in all cases. The pseudo-first-order constants (kobsd.) were calcu-
lated from a linear plot of the natural logarithm of the absorbance
of the dioxirane against time which showed excellent linearity (r Ն
0.99) for all ten sets of kinetic experiments. Data were collected for
at least two half-lives for all kinetic experiments. The values for the
second-order rate constants were computed using the pseudo-first-
mated. Interestingly, the resulting calculated relative rates
of epoxidation were in good agreement with relative rates
based on the actual k2 values for each series.[3c,9] Since the
AM1 modeling approach does not take entropy into ac-
count, the excellent correlation would be consistent with the
entropy of activation values being essentially constant for
cis- or trans-alkene series.
In the current study, the entropy of activation values
were found to be essentially identical for the epoxidation of
all ten alkenes (average is equal to –37.0 with an average order relationship: k2 = kobsd./[substrate in excess]. Pseudo-first-or-
der reaction conditions with dioxirane in excess produced k2 values
of the same magnitude within experimental error ( 5%).
deviation of 1.1 eu). The entropy of activation terms, in this
study, are in excellent agreement with the reported value for
the oxidation of cyclohexene (–35.5 eu) by 1 in acetone.[14]
Interestingly, the steric size of the various substituents does
not appear to be reflected in the ∆S‡ values. This suggests
that the transition-state organization is similar for both cis-
and trans-alkenes and enthalpy-controlled, in excellent
agreement with the original predicted results of Jorgensen
and Houk[8b] and those of Sarzi-Amadé et al.[8c] based on
a “spiro” epoxidation mechanism.
Preparation of Dimethyldioxirane: Dimethyldioxirane (1) was pre-
pared according to work done by Murray[3a] with minor alter-
ations.[3d,5] A 2-L three-necked round-bottomed flask was equipped
with a pressure-equalizing addition funnel, a solid addition funnel,
and an air condenser attached to a receiving trap in a Dewer flask
at –78 °C. A solution of 60.0 mL deionized water and 45.0 mL of
acetone was added to the flask containing 72.0 g of NaHCO3 and
1.00 g of Na2EDTA. “Oxone” (150 g) and a solution containing
40.0 mL of acetone and 60.0 mL of water were added, simulta-
neously over 30 min, while the mixture was stirred vigorously at
room temperature at 110 Torr. The dioxirane was distilled from the
generating flask and collected in a distillation receiving trap. The
dioxirane/acetone solution was redistilled at 5 °C and 8 Torr, yield-
ing approximately 75 mL of an acetone solution of 1. This solution
was stored in an amber bottle containing anhydrous Na2SO4 at
–22 °C for weeks without appreciable decomposition. Typical con-
Conclusions
The activation-parameter data have been determined for
the epoxidation of five cis/trans-dialkylalkene pairs by 1.
The entropy of activation values for all ten epoxidations are
essentially identical and are independent of alkene struc- centrations of 1, calculated from UV absorption at 330 nm and/or
reaction methods, were determined to be 0.08–0.10 .
ture. Oxidation of trans-alkenes with 1 exhibit larger and
variable ∆H‡ and ∆G‡ terms when compared to the reaction
with the corresponding cis isomers and show a direct de-
pendence on increasing steric interactions. The enthalpy of
activation and ∆G‡ values for oxidation of the cis-alkenes
are essentially constant and appear independent of the R-
group relative size. The experimental ∆∆G‡ value for the
five sets of cis/trans isomers are in excellent agreement with
the predictions based on ab initio calculations performed
on cis- and trans-alkene epoxidation by 1.[8b,c]
Acknowledgments
Acknowledgments are made to the U.S. Army ERDEC (DAAA 15-
94-K-0004) SEAS subcontract and to the Georgia State University
Research Fund for support of this work.
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Experimental Section
Product Studies: A solution of dimethyldioxirane (1, 1.1 equiv.) in
acetone was added to the neat alkenes 2cis–6cis/2trans–6trans (ca.
50 mg). The reaction mixture was stirred at room temperature and
progress monitored by GC-MS. Upon completion of the reaction,
GC-MS analysis showed that the epoxide was the sole product
formed in quantitative yield. The solvent and remaining excess 1
were removed under reduced pressure at 0 °C. The yields of the
corresponding epoxides were determined by weight and isolated in
Ն85% yield. The physical and spectroscopic data for all ten com-
pounds are in excellent agreement with published data.[3b,9–12]
Kinetic Studies: All solutions for kinetic experiments were prepared
with dried HPLC-grade acetone. In general, for convenience, the
reactions were studied at a 10:1 substrate to dioxirane ratio. The
reactions were carried out at a constant temperature ( 0.3 °C) and
were followed by determining the decrease in concentration of 1 by
monitoring the absorbance at 330 nm. For example, 0.400 mL of a
0.500  solution of alkene 2trans was placed in a 1-cm UV quartz
cuvette with 2.400 mL dried acetone, and allowed to equilibrate.
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[5] A. Baumstark, C. J. McCloskey, Tetrahedron Lett. 1987, 28,
3311–3314.
[6] Z.-X. Wang, Y. Tu, M. Frohn, J.-R. Zhang, Y. Shi, J. Am.
Chem. Soc. 1997, 119, 11224–11235.
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[7] A. Baumstark, P. J. Franklin, P. C. Vasquez, B. S. Crow, Mole-
cules 2004, 9, 117–124.
3cis[3b] and 3trans[5] are slightly lower than previously reported,
presumably reflecting a systematic error in previous tempera-
ture measurement.
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[13] The k2 values at 23 °C for eight of the epoxidations are in excel-
lent agreement with previously reported values. The values for
[14] R. W. Murray, D. Gu, J. Chem. Soc., Perkin Trans. 2 1993,
2203–2207.
[15] R. W. Murray, Chem. Rev. 1989, 89, 1187–1201.
[16] Relative gas-phase activation enthalpies for the epoxidation of
alkenes by dioxirane are reported by Sarzi-Amadé et al.[8c] as
theoretical ∆∆G‡ values. The reported relative experimental
∆∆G‡ values appear to be calculated partially based on rate-
constant data. The experimental ∆∆G‡ values (vide infra) from
the Arrhenius approach agree with the calculated ∆∆G‡ values
reported by Sarzi-Amadé et al.
Received: May 15, 2006
Published Online: August 10, 2006
Eur. J. Org. Chem. 2006, 4642–4647
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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