Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide

Article abstract of DOI:10.1016/j.tet.2018.05.022

The application of nitrous oxide as an alternative oxidant provides new opportunities for selective oxidation of olefins. Here, we studied for the first time the thermal oxidation of isobutene with N₂O in the liquid phase. The study revealed that the oxidation proceeds via 1,3-dipolar cycloaddition of N₂O to the C[dbnd]C bond by two routes forming unstable 4,5-dihydro-[1,2,3]-oxadiazole intermediates. The main route (the contribution of 91%) includes the addition of the N₂O oxygen to the second carbon atom in olefin. In this case, the oxadiazole decomposes with the C–C bond cleavage yielding acetone, methylene (:CH₂), and N₂. The methylene then readily reacts with isobutene and benzene (solvent). The minor route involves the addition of the N₂O oxygen to the first carbon atom and the oxadiazole decomposition with a hydrogen shift leading to isobutanal and N₂. The main distinctive feature of the studied reaction is the formation of methylene in high yield.

Full text of DOI:10.1016/j.tet.2018.05.022

Tetrahedron xxx (2018) 1e7
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Tetrahedron
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Generation of methylene by the liquid phase oxidation of isobutene
with nitrous oxide
Sergey Semikolenov, Dmitry Ivanov, Dmitry Babushkin, Sergey Malykhin,
Alexander Kharitonov, Konstantin Dubkov*
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, 630090, Novosibirsk, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The application of nitrous oxide as an alternative oxidant provides new opportunities for selective
oxidation of olefins. Here, we studied for the first time the thermal oxidation of isobutene with N2O in
the liquid phase. The study revealed that the oxidation proceeds via 1,3-dipolar cycloaddition of N2O to
the C]C bond by two routes forming unstable 4,5-dihydro-[1,2,3]-oxadiazole intermediates. The main
route (the contribution of 91%) includes the addition of the N2O oxygen to the second carbon atom in
olefin. In this case, the oxadiazole decomposes with the CeC bond cleavage yielding acetone, methylene
(:CH2), and N2. The methylene then readily reacts with isobutene and benzene (solvent). The minor route
involves the addition of the N2O oxygen to the first carbon atom and the oxadiazole decomposition with
a hydrogen shift leading to isobutanal and N2.
Received 17 January 2018
Received in revised form
6 May 2018
Accepted 7 May 2018
Available online xxx
Keywords:
Olefins
Nitrous oxide
1,3-Dipolar cycloaddition
Oxidation
The main distinctive feature of the studied reaction is the formation of methylene in high yield.
© 2018 Elsevier Ltd. All rights reserved.
Methylene
1. Introduction
energetically more profitable (DrG0 ¼ 30.5 kcal/mol) than from
298
О2
(
DrG0 ¼ 55.3 kcal/mol).8,9 However, the N2O molecule is
298
Selective oxidation of olefins to ketones or aldehydes remains
one of the most challenging problems. For example, the well-
known Wacker process is used for the production of only acetal-
dehyde or acetone via the liquid phase oxidation of ethylene or
propylene, respectively, with oxygen in the presence of a homo-
geneous PdCl2/CuCl2 catalyst in the HCl medium.1,2 The disadvan-
tages of the process include the formation of chlorinated by-
products and acid wastes, corrosion activity of the catalyst, and
difficult recovery and purification of the target products. A similar
oxidation of other olefins proceeds less selectively.
The application of nitrous oxide (N2O) as an alternative oxidant
instead of molecular oxygen opens new opportunities for selective
oxidation of olefins. In recent decades, an increased interest in this
field was additionally stimulated by the necessity to utilize N2O
anthropogenic emissions, since nitrous oxide is considered as a
strong greenhouse gas and ozone-depleting compound.3e7
kinetically rather stable to decomposition up to ~1000 ꢀC due to the
high activation barrier of this reaction (50e60 kcal/mol).10 So, the
involvement of N2O in oxidations commonly requires its activation
by catalysts. Such heterogeneous catalysts have been found, for
example, for the gas phase oxidation of benzene and other aromatic
compounds to phenols,11e14 oxidation of methane15e19 and
ethane20e22 to alcohols and aldehydes, and epoxidation of pro-
pylene and 1-butene.23e27 Complexes of metals are also able to
catalyze, for example, the epoxidation of olefins28,29 and oxidation
of alcohols to aldehydes and ketones with nitrous oxide.30e35
Meanwhile, rates of these liquid phase reactions are not suffi-
ciently high.
Another possibility to involve N2O in oxidations is based on its
ability to react as a 1,3-dipole with various dipolarophils such as
olefins and alkynes.36e38 Reactions of this type with olefins, which
proceed thermally without a catalyst and give ketones and alde-
hydes, have been discovered more than 60 years ago.39,40 However,
the new interest in such oxidations emerged only in early 2000s,
when they were performed in the liquid phase at 423e553 K with a
higher selectivity that often exceeded 90%.41e43
Generally, the oxidation of organic compounds with N2O is
thermodynamically favorable.8 In addition, according to thermo-
dynamic calculation, a release of the O atom from N2O is
The very high selectivity of such reactions is due to the fact that
N2O reacts only with the double bonds of olefins and is inert toward
other bonds. According to experimental data39e44 and quantum-
* Corresponding author.
E-mail address: dubkov@catalysis.ru (K. Dubkov).
https://doi.org/10.1016/j.tet.2018.05.022
0040-4020/© 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Semikolenov S, et al., Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide,
Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.05.022

2
S. Semikolenov et al. / Tetrahedron xxx (2018) 1e7
chemical modeling,44e46 the reaction proceeds by the non-radical
mechanism including 1,3-dipolar cycloaddition of N2O to the
olefin double bond with the formation of unstable 4,5-dihydro-
[1,2,3]-oxadiazole intermediate (Scheme 1). Its subsequent
decomposition with the release of N2 leads to ketone or aldehyde.
Since in many cases the main oxidation products are ketones, re-
actions of this type were called the ketonization reactions.44,47
Depending on orientation of N]NeO molecule relative to the
double bond upon cycloaddition, the reaction can proceed by
different routes. Besides, the route involving the cleavage of the
CeC bond in the oxadiazole cycle (the cleavage route) is also an
important feature of the reaction.39,43,44 Carbonyl product formed
by this route contains fewer carbon atoms than the parent olefin.
We demonstrated earlier that the contribution of different re-
action routes and, accordingly, the composition of the resulting
products depend on the olefin structure.43,47,48 For example, for 1,2-
disubstituted internal olefins, the contribution of the cleavage route
to the total oxidation rate is much smaller as compared to mono-
substituted terminal olefins.43,48 So, in the first case the main
oxidation products are ketones, while 1-olefins give a mixture of
ketone, aldehydes, and cyclopropane derivatives. Besides, in our
previous work,49 the effect of cis/trans isomerism on the mecha-
nism of 1,2-disubstituted olefins oxidation with N2O was revealed
using experimental and theoretical approaches.
and results in the formation of methyl ethyl ketone (MEK) as the
main product (~84 mol %) and a small amount of isobutanal
(~3.3 mol. %).48 The reaction with the cleavage yields acetaldehyde
(~7.4 mol. %) and ethylidene (:CHeCH3). Ethylidene then reacts
with 2-butene and benzene to form the oxygen-free products:
1,2,3-trimethylcyclopropane (~3.9 mol. %) and 7-methylcyclohepta-
1,3,5-triene (~0.2 mol. %), respectively. The total fraction of carbonyl
products is ca. 95 mol. %.
Upon oxidation of 1-butene having the terminal mono-
substituted double bond, the reaction without cleavage yields MEK
(~33.8 mol. %) and butanal (~12.8 mol. %).48 The reaction with the
cleavage gives propanal (~30 mol %) and methylene (:CH2), which
reacts with 1-butene and benzene to produce ethylcyclopropane
(~13 mol %) and cycloheptatriene (~9 mol %). The total fraction of
carbonyl products in this case is ca. 76 mol. %.
Table 1 illustrates the results of isobutene oxidation under the
same conditions. Similar to 1-butene, isobutene contains the ter-
minal double bond. However, this bond is 1,1-disubstituted owing
to the presence of two СН3 substituents at the second carbon atom.
One can see that the oxidation of isobutene yields only two
carbonyl compounds: isobutanal (1) (~5 mol %) and acetone (2)
(~50 mol %); their total fraction among all the products is ca. 55 mol.
%. In addition, 1,1-dimethylcyclopropane (3) (~26 mol %) and
cycloheptatriene (4) (~16 mol %) are among the reaction products.
As in the case of 1- and 2-butenes,48 the ratio of products is
virtually independent of temperature.
The composition of products formed upon oxidation of iso-
butene (Table 1) agrees well with the mechanism of 1,3-dipolar
cycloaddition of N2O to the double bond of olefins.39,44e46
Scheme 2 shows that, depending on the regioselectivity of the
cycloaddition, two isomers of oxadiazole intermediate (I and II) can
form in this case. Respectively, two main reaction routes (RI and RII)
can be implemented.
At the same time, the liquid phase oxidation of terminal olefins
having the 1,1-disubstituted double bond by nitrous oxide has not
been studied earlier. Therefore, the goal of our study was to clarify
the reaction mechanism for this important class of olefins by the
example of isobutene oxidation in terms of regioselectivity of N2O
cycloaddition to the double bond, the contribution of different re-
action routes, and the composition of products by applying both
experimental and quantum-chemical methods.
The formation of isobutanal 1 in the course of reaction shows
that intermediate I decomposes only without cleavage of the CeC
bond in the oxadiazole cycle (pathway 1). This pathway is accom-
panied by the release of N2 molecule and transfer of H atom to the
adjacent carbon atom (the 1,2-hydrogen shift or H-shift). Similar to
monosubstituted terminal olefins,39,43,44 the oxidation of isobutene
does not produce formaldehyde 5, which could be expected in the
case of cleavage of the CeC bond in intermediate I (pathway 2).
Quantum-chemical simulation of the reaction between 1-hexene
and N2O also indicates that the contribution of such pathway to
the total oxidation rate for terminal olefins should be very small
(~0.25%).44
In intermediate II, oxygen adds to the second carbon atom,
which has two methyl substituents. Hence, in this case, the C]O
group can be formed only via the transfer of the CH3 group to the
adjacent C atom in the CeC bond (the 1,2-CH3 shift or CH3-shift)
(pathway 3) or via the cleavage of this bond (cleavage pathway 4).
The presence of acetone 2 as the main carbonyl product shows that
intermediate II decomposes mostly with the cleavage of the CeC
bond (pathway 4). A possible reason is that the CH3-shift is quite a
slow process. According to experimental data on the oxidation of 2-
butene,48 the rate of CH3-shift is more than an order of magnitude
lower as compared to the Н-shift. Thus, in the case of isobutene, the
contribution of the pathway with the CH3-shift (pathway 3), which
should give methyl ethyl ketone 6, is very small. This agrees with
the absence of MEK among the products.
2. Results and discussion
2.1. Liquid phase oxidation of isobutene with N2O
Liquid phase oxidation of olefins with nitrous oxide is carried
out in the batch reaction system that includes liquid and gas pha-
ses. In the case of olefins having different physical properties
(boiling point, saturated vapor pressure, solubility, etc.), their
amounts in the liquid phase, where the reaction proceeds,41,42 will
differ even under similar conditions (temperature, pressure, etc.).
Thus, it is usually impossible to compare quantitatively the data on
oxidation of different olefins, particularly, their reactivity toward
N2O.
However, the oxidation of butene isomers, which differ in the
double bond position and substitution degree but have close
physical properties, can be performed under similar reaction con-
ditions. This makes it possible to make a correct comparison of the
results and reveal the effect of olefin structure on the reaction
mechanism and composition of products.
In our earlier work, the liquid phase oxidation of 1-butene and
2-butene with nitrous oxide in a benzene solution at 453e513 K
was studied in detail.48 In the case of 2-butene, which contains the
internal 1,2-disubstituted double bond, the reaction runs mostly
without cleavage of the CeC bond in the oxadiazole intermediate
As seen from Scheme 2, the cleavage pathway 4 should yield
methylene (:CH2) together with acetone. Owing to the high reac-
tivity typical of carbenes,50,51 methylene then reacts with isobutene
(reaction 5) and benzene (reaction 6) to produce 1,1-
dimethylcyclopropane 3 and cycloheptatriene 4, respectively.
Quantum-chemical calculations (see Section 2.2) predict that
Scheme 1. The formation and decomposition of 4,5-dihydro-[1,2,3]-oxadiazole inter-
mediate upon N2O interaction with olefins.
Please cite this article in press as: Semikolenov S, et al., Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide,
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S. Semikolenov et al. / Tetrahedron xxx (2018) 1e7
3
Table 1
Effect of reaction time and temperature on isobutene oxidation with N2O.a
Run
T, K
t, h
Isobutene conversion,
XR, %
N2O conversion,
Reaction rate, r,
mmol/L,h
Distribution of products in the liquid phase, ni, mol %
XN O, %
2
1
2
3
4
Other
1
2
3
4
5
6
453
473
493
493
493
513
12
12
3
6
12
2
1.5
7.3
5.6
11.8
22.6
10.6
1.0
4.8
3.7
7.9
15.1
7.3
2.0
9.7
31
32
30
85
3.8
5.0
5.4
5.1
5.1
5.9
5.1
47.3
50.8
49.2
50.5
52.0
51.6
50.2
30.1
28.5
27.0
25.2
23.1
23.0
26.2
16.4
14.2
16.8
16.7
17.1
16.3
16.3
2.4
1.5
1.6
2.5
2.7
3.2
2.3
Averaged concentrations
a
Reaction conditions: 100 ml reactor, NR0 ¼ 0.08 mol, NN0 ¼ 0.12 mol, 50 ml benzene.
2O
Scheme 2. Mechanistic scheme for isobutene oxidation with N2O.
the pathway with the cleavage leads to the primary formation of
carbene toward olefin.
diazomethane (CH2N2) instead of methylene. A similar diazo-
mechanism of the cleavage was proposed earlier for the oxidation
of monosubstituted terminal olefins.39,44 Since diazomethane is
very unstable and should rapidly decompose into methylene and
nitrogen under the reaction conditions,44 it is difficult to identify
experimentally an active fragment that is formed upon cleavage.
Note that, according to Scheme 2, the total fraction of products 3
and 4, which are formed upon addition of methylene, should be
equal to the fraction of acetone 2 among the reaction products
(~50 mol %, Table 1). However, the experimental fraction of the
addition products is somewhat lower (~42.5 mol. %). A similar
discrepancy was observed earlier for the oxidation of 1-butene.48
This can be related to additional consumption of methylene in
side reactions, the products of which are not detected by the
analysis.
Data of Table 1 allow a more detailed analysis of the reaction
mechanism. Thus, the ratio of rate constants k3 and k4 for methy-
lene interaction with isobutene and benzene (Scheme 2) can be
estimated from the fractions of 1,1-dimethylcyclopropane 3 (n3,
Table 1) and cycloheptatriene 4 (n4) among the products and from
the initial amounts of olefin (N0i-But, mol) and benzene (N0B) using
the following equation:
One can see from Scheme 2 that the contribution of route RII
ðFR Þ, resulting in the formation of intermediate II, to the total
II
oxidation rate can be calculated from the fractions of isobutanal 1
and acetone 2 formed by routes RI and RII, respectively:
n2
FR
¼
(2)
II
n1 þ n2
The calculation using the averaged data of Table 1 gives
FR ¼ 91%. Upon oxidation of monosubstituted terminal olefins,
II
quite a high contribution of a similar route is also observed. For
propylene it is equal to 70%, while for 1-butene, 1-hexene and 1-
octene, FR is 83, 79 and 81%, respectively.48 Hence, the addition
II
of N2O oxygen to the second carbon atom in the double bond of
terminal olefins is energetically more favorable. This agrees with
the results of quantum-chemical simulation.44,46 In particular, a
theoretical contribution of route RII for 1-hexene is 75%,44 which is
close to the experimental value.
Such regioselectivity of N2O cycloaddition may be related to the
orienting effect of electron-donor alkyl substituents at the double
bond in terminal olefin, which should shift electron density to the
first carbon atom.44,52 So, the oxygen atom in N2O molecule, on
which the negative charge is mostly located,36,44,46 should prefer-
entially be added to position 2. For isobutene, which has two
methyl substituents at the second carbon atom, such effect should
be more pronounced as compared to monosubstituted olefins,
which agrees with experimental data.
n3$NB0
k3
k4
¼
(1)
n4$Ni0
-
But
Equation (1) gives the averaged ratio k3/k4 ~ 11. A close ratio
In the case of isobutene, route RII yields intermediate II, which
decomposes only with the cleavage of the CeC bond (cleavage
pathway 4, Scheme 2). Therefore, the contribution of the cleavage
(~10) was obtained earlier for the interaction of methylene with 1-
butene and benzene.48 This indicates a much higher activity of
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S. Semikolenov et al. / Tetrahedron xxx (2018) 1e7
route (Fcleav) i-C4H8 þ N2O / [ II ] / 2 þ:CH2 þ N2 to the total
double bond.
oxidation rate is equal to that of route RII (F , Eq. (2)), i.e. is also
As the ground state of olefins and N2O is singlet, methylene that
is formed upon oxidation of terminal olefins with the cleavage is
likely to be in the singlet state, too. Thus, these reactions may be of
interest for in situ generation of singlet methylene from simple and
stable compounds. In the case of isobutene oxidation, this method
gives an especially high yield of the active carbene.
Evidently, the use of the indicated method for methylene gen-
eration has some limitations. Organic compounds suitable for the
addition of methylene generated by this method should be ther-
mally stable under the reaction conditions and inert to N2O. Such
substrates can be represented, for example, by alkanes and com-
pounds with aromatic rings. If olefins are used for the addition, they
should preferably have a lower reactivity toward N2O in compari-
son with isobutene.
Nevertheless, we believe that in some applications the iso-
butene oxidation by N2O can be used as a source of methylene. In
this case, selectivity for the addition products will depend on the
concentration ratio of organic substrate and isobutene and also on
their reactivity toward methylene. Thus, to increase selectivity for
such products, the oxidation of isobutene with N2O should be
carried out in a solution of organic substrate used for methylene
addition.
To check the applicability of this method for the insertion of
methylene into CeH bonds of alkanes, we performed an additional
experiment on the oxidation of isobutene with N2O in a solution of
n-hexadecane at 523 K for 24 h. Under these conditions, the iso-
butene conversion was 95%. The liquid phase products were
acetone and isobutanal obtained in a ratio of 10:1 (the isobutene
oxidation products), and also the products of methylene reactions
with isobutene (1,1-dimethylcyclopropane) and n-hexadecane
(methylhexadecanes and n-heptadecane).
RII
91%. This value is much higher than Fcleav for linear internal olefins
and monosubstituted terminal olefins, where a similar intermedi-
ate can decompose both with and without the cleavage of the CeC
bond. For example, the contribution of the cleavage route is 4.2% for
cis-3-heptene,49 13.5% for trans-3-heptene,49 8% for 2-butene and
2-pentene (mixtures of cis/trans isomers),48 and 38% for 1-
butene.48 In distinction to these olefins, in the oxidation of iso-
butene the major part of reacting molecules is cleaved. Evidently,
this difference is also related with specific features of isobutene
structure and reaction mechanism, namely, with the presence of
the 1,1-disubstituted C]C bond in its composition and a hindered
CH3-shift.
A comparison of data on isobutene oxidation (Table 1) with the
data reported in48 shows that the structure of butene isomers also
affects their reactivity toward N2O. The reaction rate (r) under
similar conditions and, accordingly, the reactivity decrease in the
following order: 1-butene > 2-butene > isobutene. For example, at
473 K the oxidation rates for the listed isomers are, respectively,
26.4, 17.9 and 9.7 mmol/L,h.
The lower reactivity of isobutene may also be attributed to the
presence of the 1,1-disubstituted double bond, namely to the
electronic effects of two electron-donor methyl substituents at the
double bond on the N2O cycloaddition. The apparent activation
energy for isobutene oxidation (28 kcal/mol) calculated from the
Arrhenius plot (Fig. 1) is substantially higher than that for 1-butene
and 2-butene (21 and 22 kcal/mol, respectively).48
For the synthesis of carbonyl compounds, the route with the
cleavage during the oxidation of olefins with N2O is an undesirable
process that decreases the reaction selectivity. Nevertheless, in the
case of isobutene, where this process is most intensive, it can be of
special interest for methylene generation. Methylene, similar to
other highly active carbenes, is employed in organic synthe-
sis.50,51,53 In particular, the addition of carbenes to the double bonds
of olefins is widely used for obtaining cyclopropane derivatives. The
most used and simple method for methylene generation is based on
thermal, photo- or catalytic decomposition of explosive and toxic
diazomethane (CH2N2). However, depending on conditions, its
decomposition can yield triplet or singlet methylene. At the same
time, exactly the singlet methylene is the most suitable species for
cyclopropanation due to stereospecificity of its addition to the
Selectivity of methylene addition to n-hexadecane and iso-
butene was ~60 and ~40%, respectively. After distillation of volatile
products, n-hexadecane contained ~20 mol
adecanes and ~4 mol % of n-heptadecane (See the Supplementary
file). Such mixture of isomers can be used as a retention time
standard for chromatography.
In order to rationalize the experimental results and elucidate
details of the reaction mechanism, quantum-chemical calculations
of the reaction routes for isobutene interaction with N2O were
performed. The obtained results are presented in the next section.
% of methylhex-
2.2. Quantum-chemical study of the reaction mechanism
The elementary steps of the N2O reaction with isobutene were
simulated using the CBS-QB3 method. This method was success-
fully applied for studying the N2O interactions with cis- and trans-
isomers of 3-heptene and other olefins reported in our previous
paper.49 It was shown that the distribution of products observed
experimentally for the gas phase oxidation of cyclohexene with
nitrous oxide at 598e723 K quantitatively agrees with theoretical
calculation. For the liquid phase reactions, a good qualitative
agreement was observed between theory and experiment.
The theoretical method applied here also assumes that all the
reactions occurred in the gas phase. This strongly simplifies the
simulation while retaining good agreement with experimental
data. The Ea and
DHr values (Table 2) were calculated at 298.15 K
and pressure 1 atm. Quantum-chemical simulation revealed that,
similar to other olefins,44e46 in the first step of the reaction 4,5-
dihydro-[1,2,3]-oxadiazole (OD) is formed via 1,3-dipolar cycload-
dition of N2O to the isobutene double bond. Two ways of cycload-
dition and, respectively, two reaction paths are possible. The
oxygen atom of N2O can add either to the first carbon atom in the
double bond of isobutene (path I) or to the second one (path II)
Fig. 1. The Arrhenius plot for isobutene oxidation with N2O (according to data of
Table 1).
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S. Semikolenov et al. / Tetrahedron xxx (2018) 1e7
5
Table 2
be involved in decomposition via the 1,2-hydrogen shift. Accord-
ingly, this intermediate can decompose only via the cleavage of CeC
Calculated activation energies and enthalpies for the gas phase oxidation of iso-
butene with N2O (T ¼ 298.15 K, pressure 1 atm; 1, 2, 5e8 e the reaction products, see
Scheme 2, Fig. 2, and the text).
bond yielding acetone
2
and diazomethane
8
(path 3,
Ea ¼ 22.8 kcal/mol) or via the transfer of methyl group over the CeC
bond yielding methyl ethyl ketone 6 and nitrogen (path 4,
Ea ¼ 27.7 kcal/mol). However, the reaction with the 1,2-alkyl shift is
also hindered by a higher activation barrier. As a result, a theoretical
ratio of the fractions of products 6 and 2 (n6: n2) is only 0.03.
Thus, theory predicts the most favorable reaction route to be the
formation of OD-II intermediate and its decomposition with the
cleavage of the CeC bond. A theoretical contribution of this route,
which is determined primarily by the OD-I/OD-II ratio, is ~90%, as in
experiment. This provides a clear explanation of the products dis-
tribution found experimentally.
According to calculations, the cleavage should liberate a very
unstable diazomethane 8. The preferable pathway of its further
transformation includes decomposition under the reaction condi-
tions with the formation of singlet methylene and nitrogen.
Data of Table 2 indicate that exactly the first reaction step, the
formation of 1,2,3-oxadiazole intermediate, should be the rate-
limiting one due to its higher activation barrier. This agrees with
the quantum-chemical simulation of N2O reactions with other
olefins.45,46 Thus, for the main route of isobutene oxidation (i-
C4H8 þ N2O / [OD-II] / 2 þ 8), the activation energy of the first
step is 25.8 kcal/mol, which is higher as compared to the second
step (22.8 kcal/mol). Note that the activation energy of this route
(25.8 kcal/mol) calculated for the gas phase reaction cannot be
compared directly with the experimental value for the liquid phase
reaction (28 kcal/mol). Nevertheless, in this case the indicated
values are quite close.
Path number
Reaction
Ea, kcal/mol
DHr, kcal/mol
(I)
i-C4H8 þ N2O / OD-I
i-C4H8 þ N2O / OD-II
OD-I / 1 þ N2
27.1
25.8
17.5
22.9
22.8
27.7
ꢁ7.7
(II)
(1)
(2)
(3)
(4)
ꢁ9.1
ꢁ58.3
þ17.1
þ5.9
OD-I / 5 þ 7
OD-II / 2 þ 8
OD-II / 6 þ N2
ꢁ62.8
giving, respectively, OD-I or OD-II intermediates (see Fig. 2). In the
second step the oxadiazoles decompose, yielding the reaction
products.
A comparison of the calculated activation energies for two ways
of N2O cycloaddition (27.1 and 25.8 kcal/mol, Table 2) shows that
path II is preferential. Assuming that the ratio of paths I and II is
determined by the reaction kinetics, i.e. by the probability of
passing the activation barrier, the calculation predicts the following
ratio of OD-I and OD-II concentrations under standard conditions:
ODꢁI=ODꢁII ¼ kI=kII ¼ expðꢁ
D
Ea=RTÞz0:1
(3)
In addition, the OD-II intermediate is thermodynamically more
stable than OD-I isomer: the difference between enthalpies of
formation for these isomers is 1.4 kcal/mol. Hence, main products of
the reaction observed in experiment should be formed via further
transformations of exactly the OD-II intermediate.
The main decomposition path for OD-I intermediate (Fig. 2)
involves a hydrogen shift and gives isobutanal 1 and nitrogen (path
1, Ea ¼ 17.5 kcal/mol). Decomposition of OD-I intermediate with the
cleavage of the CeC bond leading to formaldehyde 5 and 2-
diazopropane 7 (path 2) is also possible. However, the contribu-
tion of such reaction should be very small due to a higher activation
barrier (Ea ¼ 22.9 kcal/mol). The theoretical fraction of formalde-
hyde 5 among all carbonyl products does not exceed 0.001%, which
agrees with the experimentally observed absence of this product.
Owing to the structural features of OD-II intermediate, it cannot
3. Conclusions
Isobutene is a valuable component of the C4 fraction, which is
formed as a by-product of oil refining via cracking or pyrolysis. A
detailed experimental and quantum-chemical study of isobutene
reaction with nitrous oxide, which was performed for the first time
in this work, extends knowledge on the mechanism of 1,3-dipolar
cycloaddition of N2O to olefins and possible routes of their oxida-
tive transformation by this promising alternative oxidant.
The experimental study on the liquid phase oxidation of iso-
butene with N2O at 453e513 K revealed that the main route of the
reaction involves the addition of N2O oxygen to the second carbon
atom in the double bond with the formation of 4,5-dihydro-[1,2,3]-
oxadiazole intermediate, which decomposes with the cleavage of
the CeC bond in the oxadiazole cycle. The obtained results along
with earlier published experimental data on the oxidation of 1-
butene and 2-butene were used to elucidate the effect of substi-
tution degree of the double bond in butene isomers on their reac-
tivity toward N2O, the contribution of different reaction routes and
the composition of products. In distinction to 2-butene and 1-
butene, where contribution of the cleavage route is 8 and 39%,
respectively, the main part of isobutene molecules (~90%) un-
dergoes the cleavage. Therewith, the reactivity decreases in the
following order: 1-butene > 2-butene > isobutene. It was found
that these pronounced distinctions are related to the presence of
the 1,1-disubstituted double bond in isobutene. It has been shown
that electron-donor alkyl substituents at the double bond in ter-
minal olefins exert an orienting effect on the regioselectivity of N2O
cycloaddition. As a result, the N2O oxygen is preferentially added to
the second carbon atom.
The quantum-chemical simulation (the CBS-QB3 method)
revealed that the reaction actually proceeds via the 1,3-dipolar
cycloaddition mechanism and confirmed a very high contribution
of the cleavage route for isobutene. In particular, detailed
Fig. 2. Possible routes of isobutene oxidation with N2O. The structures of activated
complexes are shown in square brackets.
Please cite this article in press as: Semikolenov S, et al., Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide,
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6
S. Semikolenov et al. / Tetrahedron xxx (2018) 1e7
information on the structure of 1,2,3-oxadiazole intermediates, the
way the C]C bond is cleaved, and the products distribution was
obtained. A good quantitative agreement between theory and
experiment was achieved.
The oxidation of isobutene with the cleavage yields acetone and
the highly reactive singlet methylene, which can readily react with
the CeH bonds of alkanes or unsaturated bonds of aromatic com-
pounds and olefins to form cyclopropane derivatives in the latter
case. The reaction of isobutene with N2O is an unusual example of
in situ generation of methylene with a high yield via the oxidation
of a simple and accessible compound.
126.96 (2C, CH), 131.28 (2C, CH).
The isobutene conversion (XR) was calculated from the amount
of the reaction products in the liquid phase that formed from iso-
butene. The liquid phase obtained after the oxidation of isobutene
with N2O in n-hexadecane solution was additionally analyzed by
GC using a capillary column (SE-52, 50 m, 0.2 mm) and FID. Other
details of experiments were given earlier.48
4.3. Quantum-chemical methods
The exploration of possible reaction pathways of isobutene
oxidation with nitrous oxide were carried out assuming gas-phase
conditions. Reactions rates were estimated using Transition State
theory. Potential Energy Surfaces (PESs) of relevant reaction steps
were constructed at the Density Functional Theory (DFT) level us-
ing the B3LYP functional.54 Kinetic control of the reaction means
that activation energy of the preferred path is lowest. These gas-
phase activation energies were calculated using CBS-QB3 quan-
tum chemical model55 that was applied in our previous work.49
Detailed discussion of the accuracy of this model with respect to
the reactions similar to ones considered here is given else-
where.44,49 All calculations were performed with the Gaussian 09
software.56 Calculations were performed at the Siberian Super-
computer Centre SB RAS. Visualization of the results was performed
using the MOLDEN program.57
4. Experimental section
4.1. Chemicals
Isobutene (99%) and benzene (99.9%) were purchased from
Sigma-Aldrich. Medical grade nitrous oxide (99.8%) was obtained
from Cherepovets Azot Plant.
4.2. Typical procedure for isobutene oxidation
The liquid phase oxidation of isobutene with nitrous oxide was
carried out in a benzene solution using a 100 ml Parr reactor
equipped with a stirrer. In a typical experiment, benzene (50 ml)
was loaded to the reactor. To remove air, the reactor was purged
with helium and then filled with 0.12 mol isobutene and
0.08 mol N2O by the method described elsewhere.48 The reactor
was heated at a 6 K/min ramp to the required reaction temperature
(453e513 K) and kept for 2e12 h under stirring (300 r.p.m.). An
additional experiment on the oxidation of isobutene (0.05 mol)
with N2O (0.3 mol) in a solution of n-hexadecane (20 ml) was
performed using similar procedure at 523 K for 24 h.
Acknowledgements
This work was supported by the Russian Academy of Sciences
and Federal Agency of Scientific Organizations (project V.44.1.6, N
0303-2016-0006).
ꢀ
Appendix A. Supplementary data
After termination of the reaction, the reactor was cooled to room
temperature. The composition of the gas phase (N2O, N2, О2, СО,
СО2) was analyzed by gas chromatography (GC) using a TCD and a
column packed with Porapak Q. N2 was the only gaseous product,
whose amount corresponded to that of the carbonyl products
formed in the liquid phase. The N2O conversion was calculated by
the equation:
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.tet.2018.05.022.
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Please cite this article in press as: Semikolenov S, et al., Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide,
Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.05.022