The reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene: Monoepoxidation kinetics and computational modeling

Article abstract of DOI:10.1515/hc-2014-0211

The reaction of dimethyldioxirane (1) with excess 1,3-cyclohexadiene (2a) and 1,3-cyclooctadiene (2b) in dried acetone yielded the corresponding monoepoxides in excellent yield. Second-order rate constants for monoepoxidation were determined using UV meth

Full text of DOI:10.1515/hc-2014-0211

Heterocycl. Commun. 2015; 21(1): 47–53
Davita McTush-Camp, Pedro C. Vasquez and Alfons L. Baumstark*
The reaction of dimethyldioxirane with
1,3-cyclohexadiene and 1,3-cyclooctadiene:
monoepoxidation kinetics and computational
modeling
Abstract: The reaction of dimethyldioxirane (1) with
excess 1,3-cyclohexadiene (2a) and 1,3-cyclooctadi-
Introduction
ene (2b) in dried acetone yielded the corresponding
Studies have shown dioxiranes to be powerful oxidants
monoepoxides in excellent yield. Second-order rate
[1–4]. Dimethyldioxirane (1), in particular, has been
constants for monoepoxidation were determined using
useful in carrying out many synthetic transformations,
UV methodology. The k2 value for the monoepoxidation
adding an oxygen to a variety of substrates [1–4]. One
of 1,3-cyclohexadiene was found to be 1.14 0.06 m-1 s-1,
of the most studied reactions of 1 is the epoxidation of
whereas that for the monoepoxidation of 1,3-cycloctadi-
olefins. Dimethyldioxirane epoxidations have been shown
ene was 0.31 0.03 m-1 s-1. Basic density functional calcu-
to be sensitive toward steric and substituent effects [1–4].
lations at the B3LYP/6-31G level were employed to model
Kinetic studies have shown the relative reactivities for
the monoepoxidations. As expected, the calculations
cis vs. trans epoxidation by 1 of simple alkyl substituted
were consistent with a concerted, electrophilic process
alkenes to be approximately 8:1 or greater [5]. The reverse
with a spiro-transition state. As found for the epoxidation
reactivity trend for the epoxidation of phenyl-substituted
of simple alkenes, the calculated transition-state geom-
alkenes was noted as the major exception [6]. However,
etry showed a slight asynchronous tilt of the dioxirane
all epoxidations by 1 have been shown to be quantita-
plane relative to that of the remaining alkene portion of
tive and stereospecific. Mechanistically, the epoxidations
the diene and a slight tilt back from the face of the diene.
have been postulated to occur via a spiro-transition state
Relative reactivities (relative k2 values) were determined
[1–7]. Semiempirical (AM1) calculations have been shown
using the difference in the calculated electronic activa-
to be useful in modeling dimethyldioxirane epoxidation
tion energies and were consistent with the experimen-
of simple alkenes within a structurally homologous series
tal relative k2 values without the need to correct for the
[8]. Density functional theory (DFT) calculations were
medium (solvent). Reactivity differences for epoxidation
used to model the epoxidation of ethylene, propene, and
can be quickly predicted by this approach as long as
cis- and trans-2-butene by both the parent unsubstituted
there are reasonable structural similarities between the
dioxirane and 1 and predicted a spiro-transition state
substrates.
in agreement with the mechanism postulated based on
experimental data [9–12]. Ab initio calculations, employed
Keywords: density functional theory (DFT) calculations;
to model cis and trans epoxidations by 1 also predicted
dimethyldioxirane; epoxidation; kinetics.
a spiro-transition state and the correct relative reactivity
of cis vs. trans simple alkenes [13]. Semiempirical (AM1)
DOI 10.1515/hc-2014-0211
Received December 29, 2014; accepted January 2, 2015
calculations failed to predict the relative reactivities of the
epoxidation of alkenes by 1 within a series in which there
were only minor reactivity differences as well as across
two series for which there are small reactivity differences
[14]. Basic gas phase DFT calculations proved to be effec-
*Corresponding author: Alfons L. Baumstark, Department of
tive for modeling the epoxidation of α,β-unsaturated
Chemistry, Center for Biotechnology and Drug Design, Georgia State
esters by 1 as well as for the epoxidation of several
University, Atlanta, GA 30303, USA, e-mail: abaumstark@gsu.edu
acyclic conjugated dienes (s-trans systems) by dioxirane,
Davita McTush-Camp and Pedro C. Vasquez: Department of
Chemistry, Center for Biotechnology and Drug Design, Georgia State
University, Atlanta, GA 30303, USA
systems for which steric effects are minimized [15, 16]. No
studies have been reported for the epoxidation of cyclic
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48ꢀ ꢀD. McTush-Camp et al.: Reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene
conjugated s-cis dienes. We report here the results of a
kinetic study with computational modeling of the epoxi-
dation of 1,3-cyclohexadiene (2a) and 1,3-cycloctadiene
(2b) by dimethyldioxirane (1).
Results and discussion
The reaction of 1 with excess 1,3-cyclohexadiene (2a) and
1,3-cyclooctadiene (2b) were carried out in dried acetone
(Scheme 1). Under these conditions, the monoepoxide was
the major product in either case. The monoepoxides were
Figure 1ꢂRepresentative pseudo-first-order plot for the
monoepoxidation of 1,3-cyclooctadiene (2b) by 1 in acetone at 23°C.
1
isolated in low yield and characterized by H NMR and
GC-MS data. The spectroscopic data were in agreement
with the literature values [17].
1,3-cyclohexadiene and 1,3-cyclooctadiene as shown
in Figure 3. The calculated transition-state geometries
for these cases are consistent with previous results for
the epoxidation of simple alkenes by 1, which also show
similar asymmetry and tilts for the spiro-transition
states [9–14].
Kinetic studies with 2a and 2b were carried out using
UV methodology in dried acetone at 23°C. The epoxida-
tions by 1 were shown to be of the first order in dioxirane
and in diene; second-order overall. 1,3-Cyclohexadiene
was found to be more reactive toward monoepoxidation
than 1,3-cyclooctadiene.
Pseudo-first-order conditions with a 10:1 diene/diox-
irane ratio were employed to determine the k2 value for
the monoepoxidation of 1,3-cyclooctadiene. Owing to the
greater reactivity, second-order conditions were employed
to determine the k2 value for the monoepoxidation of
1,3-cyclohexadiene. Representative plots of the kinetic
data are shown in Figures 1 and 2. The k2 values for the
monoepoxidation of 1,3-cyclohexadiene and 1,3-cyclooc-
tadiene are 1.14 0.06 and 0.31 0.03 m-1 s-1, respectively. The
k2 value for the monoepoxidation of 1,3-cyclohexadiene is
comparable to those for the monoepoxidation of disubsti-
tuted acyclic conjugated dienes (1.29–1.42 m-1 s-1) by 1 [14].
The k2 value for the monoepoxidation of 1,3-cyclooctadi-
ene is comparable to that for the epoxidation of simple
cis-alkenes by 1 [8].
The basic DFT approach was employed to quickly
predict relative reactivities. These calculations were
carried out in the gas phase because solvent corrections
have been shown to lower the individual values but not
affect the difference between values [18]. The predicted
activation energies were determined by taking the dif-
ference between the calculated energies of the transi-
tion state and those of the ground states of the reactants
while controlling for conformational effects. The pre-
dicted activation energies for 1,3-cyclohexadiene and
1,3-cyclooctadiene were found to be 13.37 and 14.14 kcal,
respectively. The krel predicted values were calculated
from the electronic activation energy difference for the
two cyclic dienes. The ground-state geometry calcula-
tions of 1,3-cyclohexadiene and 1,3-cyclooctadiene show
that 1,3-cyclohexadiene resembles a relatively flat s-cis
The monoepoxidations of the cyclic dienes were
modeled by applying a basic DFT approach using the
B3LYP/6-31G basis set. Regardless of the initial ori-
entation of the reactants, minimized transition-state
calculations yielded a spiro-transition state with a
slight asynchrony tilt for the monoepoxidation of
H
O
dry acetone
23°C
O
O
H
n( )
n( )
Me
Me
k2
1
2a : n = 1
2b : n = 3
3a : n = 1
3b : n = 3
Figure 2ꢂRepresentative second-order plot for the
monoepoxidation of 1,3-cyclohexadiene (2a) by 1 in acetone at 23°C.
Scheme 1ꢂ
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D. McTush-Camp et al.: Reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadieneꢂ
ꢂ49
Figure 3ꢂCalculated density functional B3LYP 6-31G* transition-state structures for the monoepoxidation of 1,3-cyclohexadiene (2a) and
1,3-cyclooctadiene (2b) by 1.
system. The ground-state structure of the 1,3-cyclooctadi- data are consistent with the DFT results predicting that
ene shows the two double bonds to have a torsion angle the 1,3-cyclohexadiene would be more reactive than the
of 51°, reducing conjugation. The predicted relative rates 1,3-cyclooctadiene.
for 2a/2b of 3.9: 1 are consistent with the experimental
On the basis of the successful modeling of the cyclic
kinetic data for 2a/2b of 3.7:1. The kinetic and predicted dienes, the relative rate constants for cis-/trans-phe-
data are shown in Table 1. The HOMO-LUMO energies nyl-substituted alkenes 4a,b were evaluated using this
were taken from the DFT B3LYP 6-31G optimized geome- quick, gas-phase calculation method. The transition-state
tries of 1,3-cyclohexadiene, 1,3-cyclooctadiene, and dime- geometries of the epoxidation of cis-1-phenylpropene and
thyldioxirane. The HOMO energies of 1,3-cyclohexadiene trans-1-phenylpropene by 1 were determined (Figure 4).
and 1,3-cyclooctadiene are -5.59 and -6.05 eV, respectively. The second-order rate of the epoxidation of cis-1-phenyl-
The LUMO energy of dimethyldioxirane is -0.47 eV. These propene and trans-1-phenylpropene by 1 (Scheme 2) have
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50ꢀ ꢀD. McTush-Camp et al.: Reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene
Table 1ꢂExperimental and predicted (DFT) data for the monoepoxidation of 1,3-cyclohexadiene (2a) and 1,3-cyclooctadiene (2b) by
compound 1.
Molecule
k2expt (m-1 s-1)
Total ground-
state energiesa
(kcal/mol)
Transition-
state energya
(kcal/mol)
Activation
energya
ΔEelect
k2rel
Predicted
Experimental
(E elect
)
2a
2b
1.14 0.06
0.31 0.03
314 809.20
-364 143.10
-314 795.83
-364 128.96
13.37
14.14
0.80
3.9 0.3
≡1b
3.7 0.6
≡1b
a Calculated.
b Normalized.
Figure 4ꢂCalculated density functional B3LYP 6-31G* transition-state structures for the epoxidation of cis-1-phenylpropene (4a) and
trans-1-phenylpropene (4b) by 1.
been previously reported as 0.18 0.01 and 0.29 0.02,
respectively, and noted as an exception of the overall
trend of the reactivity of epoxidation by 1 of cis vs. the
corresponding trans-alkene [6]. As expected, the calcula-
tion predicted that the epoxidation occurs via a concerted,
electrophilic process with a spiro-transition state includ-
ing a slight asynchrony tilt in the oxygen insertion bond
formation.
dry acetone
23 °C
O
H
H
O
O
H
H
[3]
k2
Me
Me
Me
Ph
Me Ph
1
4a
5a
Scheme 2ꢂ
The electronic activation energies for the epoxidation experimental relative reactivity was 1.6:1, thus success-
of cis-1-phenylpropene and trans-1-phenylpropene were fully predicting the greater reactivity of the trans isomer.
determined to be 14.00 and 13.80 kcal, respectively. The Table 2 summarizes the electronic and experimental data.
calculated relative reactivity was calculated to be 1.2:1, The DFT results show that the exception in relative reac-
trans-1-phenylpropene/cis-1-phenylpropene, whereas the tivity for 4a and 4b is actually consistent with the steric
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D. McTush-Camp et al.: Reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadieneꢂ
ꢂ51
Table 2ꢂExperimental and predicted (DFT) data for the epoxidation of cis-1-phenylpropene (4a) and trans-1-phenylpropene (4b) by
compound 1.
Molecule
k2expt (m-1 s-1)
Total ground-
state energiesa
(kcal/mol)
Transition-
state energya
(kcal/mol)
Activation
energya
ΔEelect
k2rel
Predicted
Experimental
(Eelect
)
4a (cis)
4b (trans)
0.18 0.01
0.29 0.02
-387 313.44
-387 316.19
-387 299.44
-387 302.39
14.00
13.80
0.20
≡1b
1.2 0.2
≡1b
1.6 0.2
a Calculated.
b Normalized.
[5, 7]. Stock solutions (ꢀꢀ≤ꢀꢀ0.1 m in acetone) were collected by redis-
tillation of the initial azeotrope [1–4]. The dimethyldioxirane con-
centration in acetone was determined using UV methodology (λ
330 nm and molar absorptivity of 12.9 cm-1 m-1) [6]. Dimethyldioxirane
stock solutions were stored at -20°C over anhydrous Na2SO4 until
use. Stock solutions were stable for months under these conditions.
1,3-Cyclohexadiene and 1,3-cyclooctadiene were commercially avail-
able and were distilled before use. Stock solutions of the dienes in
dried acetone were prepared under anhydrous Na2SO4 at room tem-
perature and used the same day. For kinetic runs, the temperature of
the jacketed sample cell (23.0 0.3°C) was monitored via an YSI probe.
HPLC-grade acetone from Aldrich was stored under Na2SO4 until use
for all kinetic runs. Care needs to be taken to avoid advantageous
water since the k2 values will increase substantially depending on the
amount of water present [6].
effect argument, but in this case, the trans isomer has the
least steric interactions leading to the apparent reversal.
Conclusions
1,3-Cyclohexadiene (2a) was found to be more reactive
than 1,3-cyclooctadiene toward monoepoxidation by 1.
Kinetic studies showed that the overall reactivity for the
monoepoxidation of the essentially s-cis 1,3-cyclohexadi-
ene is comparable to that of the s-trans acyclic conjugated
dienes with similar number of substituents. The results of
the study of the less reactive substrate, 1,3-cyclooctadi-
ene (2b), are similar to the second-order rate constants of
simple cis-alkenes. It appears that the two double bonds
behave as if they are isolated alkenes for this case. Basic
DFT calculations (gas phase) are consistent with the con-
certed (spiro) mechanism for the monoepoxidation of the
dienes and predicted the relative reactivity with reasona-
ble agreement with experimental data. Application of this
approach to epoxidation of cis-/trans-1-phenylpropene
successfully predicted the previously noted unexpected
reactivity for this pair of alkenes. The method appears to
be able to detect the effect of subtle steric interactions in
these closely related systems. The basic DFT method is a
reliable quick approach to model dioxirane epoxidation
reactions and for predicting relative reactivities as long as
the reactants have reasonable structural similarities and
conformational factors are controlled without the need
for time consuming solvent corrections. The challenge
of predicting the relative reactivities for substrates with
vast structural differences toward epoxidation by 1 still
remains to be addressed.
Product studies
The monoepoxides, 3a and 3b, were identified by GC-MS (Shimadzu
GCMS-QP5000, Column: Supelco 2-4001 fused silica capillary SE-54
30 m length, 0.25 mm diameter) and 1H NMR (Bruker-400 MHz) data.
To 0.21 mmol of diene (2a or 2b), 0.20 mmol of 0.088 m dimethyl-
dioxirane in acetone was added at room temperature. Afer several
minutes, GC-MS analysis showed that the monoepoxide was formed
in excellent yield with slight amounts of unreacted diene present
as well as trace amounts of diepoxides for each case. The acetone
was carefully removed at 0°C under reduced pressure, yielding the
monoepoxide in roughly 50% yield in each case. The 1H NMR spectra
were in good agreement with the published data [17].
3,4-Epoxycyclohexene (3a):ꢁ1H NMR (400 MHz,CDCl3): δ 5.94 (m,
2H), 3.51 (m, 1H), 3.24 (m, 1H), 2.04 (m, 2H), 1.64 (m, 2H).
3,4-Epoxycyclooctene (3b):ꢁ1H NMR (400 MHz, CDCl3): δ 5.77 (m, 1H),
5.60 (m, 1H), 3.46 (m, 1H), 3.12 (m, 1H), 2.07 (m, 2H), 1.3–1.8 (m, 6H).
Kinetics
Kinetic studies were carried out on a Shimadzu UV-3101 PC spec-
trometer with jacketed cells. A solution of dimethyldioxirane in dried
acetone (known concentration) was placed in the jacketed sample
cell and was allowed to thermally equilibrate at 23°C. The appropri-
ate amount of cyclic diene stock solution in dried acetone was added
Experimental
Dimethyldioxirane was prepared by the general procedure developed
by Murray, for the reaction of oxone and acetone, with modifications
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52ꢀ ꢀD. McTush-Camp et al.: Reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene
conformations were checked and corrected for conformational dif-
ferences, if present. For 2b, this was a problem that was solved
by constraining the conformation of the starting material during
the transition-state calculation. All calculations were completed
in the gas phase at 23°C for this study. The electronic activation
energies were calculated as follows: Eelect ꢀ=ꢀ energy of transition
state–(ground-state energy of dioxirane+ground-state energy of
substrate). To calculate the relative reactivities, the difference in the
rapidly and mixed (∼4 s) via pipette. The disappearance of dimethyl-
dioxirane was monitored vs. time at 330 nm for at least two half-lives.
For 1,3-cyclooctadiene, pseudo-first-order conditions were employed
with the diene concentration at least tenfold greater than that of 1.
For the more reactive 1,3-cyclohexadiene, second-order conditions
were employed. The k2 values were the average of at least three exper-
iments. All kinetic runs had correlation coefficients of 0.99 or greater.
Afer completion of each kinetic run, formation of the monoepoxide
was confirmed by GC-MS analysis.
electronic activation energies was used (Arrhenius equation) in the
-∆Eelect
following manner: k2rel =( e(
/RT ) )( k2normalized). The substrate
with the lower reactivity is normalized to 1. Inclusion of a solvent
in calculations, while affecting the individual values, did not affect
the electronic activation energy differences, in agreement with the
literature [18]. Therefore, all calculations were completed in the gas
phase at 23°C for this study.
Computational methodology
All calculations (gas phase) reported in this paper were per-
formed with the Spartan’10 molecular modeling program. The DFT
approach using Becke3-LYP and the 6-31 G basis set was utilized for
all calculations. Energies for the minimized ground-state structures
were determined for all the reactants [dimethyldioxirane (1), cyclic
dienes (2a and 2b), and the phenyl-substituted alkenes (4a and
4b)] and the products [acetone, corresponding monoepoxides (3a
and 3b), and epoxides (5a and 5b)]. As expected, the computational
data predicted that the reactions are exothermic in all cases. The
optimized ground-state energy structure of 1,3-cyclohexadiene was
predicted to be a relatively flat s-cis system. The optimized ground-
state energy structure of 1,3-cyclooctadiene is consistent with the
two double bonds not in the same plane with a torsion angle of
51°. The HOMO-LUMO energies were obtained from the optimized
geometries using the DFT approach using Becke3-LYP and the 6-31 G
basis set. As expected, the optimized transition state for each diene
resulted in a spiro-orientation with a slight asynchronous tilt away
from the other double bond of the cyclic system. Regardless of the
initial approach of the dioxirane, all valid transition states resulted
in the spiro-orientation. To save time, the calculations were carried
out with the methyl groups of the dioxirane oriented away from the
most hindered side of the diene, which always yielded the lowest
energy spiro-transition state. Each transition-state calculation was
checked for validity. A valid transition state had only one negative
eigenvalue, which also corresponded to the eigenvector in a sepa-
rate intrinsic reaction coordinate (IRC) calculation. When animated,
the IRC linked the starting materials to the desired products through
the spiro-oriented transition state. For the cyclic dienes, two reac-
tion sites are present and can be approached from the top or bot-
tom, yielding different outcomes due to conformational factors.
Because 1,3-cyclohexadiene is relatively flat, the transition states for
all approaches for monoepoxidation are of similar reactivity. Alter-
natively, for 1,3-cyclooctadiene, conformation is a major factor. All
approaches to the reaction sites were calculated. Due to significant
energy differences, one of the approaches was found to have a sub-
stantially lower electronic activation energy and therefore was the
major contributor to the overall reactivity for that case. Weighted
averages of the electronic activation energies for all orientations of
attack of 1 on the other substrates were not necessary. For the other
cases, the lowest electronic activation energy was taken as repre-
sentative. As shown for simple cis-alkenes [5], the approach to cis-
1-phenylpropene is with the methyl groups of the dioxirane away
from the hindered side. For the trans-1-phenylpropene, the transi-
tion state with the lowest energy is with the methyl groups of the
Acknowledgments: Acknowledgment is made to the US
Department of Education (GAANN Fellowship to Davita
McTush-Camp) and to the Georgia State University Depart-
ment of Chemistry. We would like to thank Dr. Navarro-
Einsenstein for help with the product studies.
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