Transformation of cis- and trans-2,3-dimethyloxiranes on a Cu/SiO2 catalyst

Article abstract of DOI:10.1006/jcat.1997.1666

Transformation of cis- and trans-2,3-dimethyloxiranes on a copper catalyst was studied at 363 K at various hydrogen pressures as well as in the presence of deuterium, in a recirculating reactor. The main reactions are hydrogenolysis, isomerization, and deoxygenation (2-butanol, 2-butanone, and 2-butenes are formed). In the course of transformation, the surface of the catalyst is oxidized via deoxygenation, leading to the generation of Lewis acid-base ion pairs. 2-Butanol and 2-butenes are formed on Cu(O) atoms while the formation of 2-butanone occurs mainly on copper ions. Differences between the reactivities of the oxirane isomers and the characteristic features of the mechanism of the three reactions observed are discussed in detail. Finally, experimental observations are compared with the transformation of cis- and trans-1,2-dimethylcyclopropanes under similar conditions.

Full text of DOI:10.1006/jcat.1997.1666

JOURNAL OF CATALYSIS 169, 114–119 (1997)
ARTICLE NO. CA971666
Transformation of cis- and trans-2,3-Dimethyloxiranes
on a Cu/SiO Catalyst
2
1
Andr a´ s F a´ si, Ferenc Notheisz, and Mih a´ ly Bart o´ k
Department of Organic Chemistry and Organic Catalysis Research Group, Hungarian Academy of Sciences,
J o´ zsef Attila University, D o´ m t e´ r 8, H-6720 Szeged, Hungary
Received September 24, 1996; revised January 8, 1997; accepted January 27, 1997
Cu/SiO2 catalyst (6.36% ) was prepared by ion exchange.
Transformation of cis- and trans-2,3-dimethyloxiranes on Preparation and characterization of the catalyst have been
a copper catalyst was studied at 363 K at various hydrogen described (6). Fresh catalyst (20 mg) was added for each
pressures as well as in the presence of deuterium, in a recirculating
reactor. The main reactions are hydrogenolysis, isomerization, and
deoxygenation (2-butanol, 2-butanone, and 2-butenes are formed).
In the course of transformation, the surface of the catalyst is
measurement. Pretreatment was performed in a recirculat-
ing system; the catalyst was treated at 26.6 kPa hydrogen
or deuterium for 1 h at 573 K.
oxidized via deoxygenation, leading to the generation of Lewis Methods
acid–base ion pairs. 2-Butanol and 2-butenes are formed on Cu(0)
Measurements were carried out in a recirculation reactor
atoms while the formation of 2-butanone occurs mainly on copper
(
7). The volume of the reactor was 69 ml and the total vol-
ions. Differences between the reactivities of the oxirane isomers
and the characteristic features of the mechanism of the three
reactions observed are discussed in detail. Finally, experimental
observations are compared with the transformation of cis- and
ume of the system was 170 ml. The reactor was heated by an
air thermostat. The total sample volume was ca. 0.5 ml. In
the case of hydrogenolysis, analysis was carried out by a gas
chromatograph (HP 5890) equipped with a flame ionization
detector. For studies on deuterolysis, a gas chromatograph
trans-1,2-dimethylcyclopropanes under similar conditions.
�c 1997
Academic Press
(
HP 5890) with a quadrupole mass selective detector was
attached to the system. Separation was done in a Carbowax
INTRODUCTION
2
0M capillary column (HP-20M, 25 mm � 0.2 mm � 0.2 �m
film thickness). Calculations were made in a Data Apex
Chromatography Station for Windows 1.5 using a HP 5970
chromatogram analysis program.
Relatively few data are available in the literature regard-
ing the transformation of oxiranes on Cu catalysts (1–5). No
experimental observations have been published at all on
transformations of dimethyloxiranes on copper catalysts.
Therefore, it seemed an important task to study the trans-
formation of cis- and trans-2,3-dimethyloxirane on a copper
catalyst at various hydrogen pressures and in the presence
of deuterium. These studies allow conclusions to be drawn
regarding the stereochemistry of the reactions, the struc-
ture of the adsorbed form generated on the metal surface,
and, consequently, the mechanism of the transformation.
Hydrogen used for the measurements was generated in
a Matheson type 8326 electrolysis apparatus equipped with
a Pd diffusion cell. Deuterium was obtained by a General
Electric 15EHG generator (the purity of D2O was 99.8% ).
Deuterium was deoxygenated by a Model 1000 oxygen trap
(
CRS, U.S.A.).
Procedure
cis- or trans-Oxirane (1.33 kPa) was added for each mea-
surement. When the hydrogen pressure dependence was
studied, hydrogen pressure was varied within the range of
EXPERIMENTAL
0
–101.3 kPa. Reaction temperature was 363 K for both iso-
Materials
mers. The effect of temperature was studied at a hydrogen
pressure of 20 kPa, at 353–403 K.
cis- and trans-2,3-Dimethyloxiranes were purchased
from Aldrich. The purity of the isomers were 99 and 98% ,
respectively. The trans-isomer was shown to contain no de-
tectable amount of the cis-isomer. Before the reaction, oxi-
rane samples were subjected to several freeze–thaw cycles.
� 1
Initial rates (% /min) and turnover frequencies (min
)
were calculated. Based on temperature dependence, ap-
parent activation energies were also calculated.
Deuterolysis was performed at a deuterium pressure of
2
0 kPa and a reaction temperature of 363 K. The data were
1
To whom correspondence should be addressed.
used for the calculation of deuterium distribution.
114
0021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

Cu-CATALYZED RING OPENING OF OXIRANES
115
The mass spectra were measured in hydrogen and
deuterium, too. The measured deuterium spectra were
corrected by the natural deuterium distribution. The molec-
ular ion of 2-butanone at mass 72 was used for the calcula-
tion of deuterium distribution on the basis of the equation
Ix/6Ix � 100. Fragments at mass 43 and 58 are indicating
that CHD group is present in the molecule. In the case
of 2-butanol the peaks at mass 45 and 59 were selected
for the calculation. The two peaks together contains the
whole molecule and make it possible to calculate the deu-
terium distribution of the molecular ion. The mass spectra
at mass 45 were almost identical in hydrogen and in deu-
terium, indicating that there is no deuterium in the CH3 and
CH groups. Because of the experimentally demonstrated
OD–OH exchange during the chromatographic separation,
FIG. 1. Transformation of cis-2,3-dimethyloxirane over Cu/SiO cata-
2
the 2-butanol containing one C–D bond was considered as lyst (363 K; phydrogen = 20 kPa; poxirane = 1.33 kPa; catalyst = 20 mg).
d2, while the molecule containing two C–D bonds was con-
sidered as d3, and so on.
As a result of measurements of temperature dependence,
apparent activation energies were determined (Table 1).
RESULTS AND DISCUSSION
These were nearly identical for the two isomers and higher
than on Pd catalyst (10). The low activation energy on Pd in-
dicates that oxirane is adsorbed on the surface of the metal
at very low temperatures already and covers it completely.
Temperature plays a role in the hydrogenolysis of C–Pd
bond and this process is rate-limiting step at low temper-
atures. Conversely, the surface of copper is probably not
fully covered by oxirane and—in contrast to the process on
palladium—the rate limiting step is the cleavage of the C–O
Activity and Selectivity of Transformations
Hydrogenolysis and isomerization lead to the formation
of 2-butanol and 2-butanone, while deoxygenation pro-
duces cis- and trans-2-butene (Scheme 1).
The GC–MS recirculation system operating with a cap-
illary column did not allow the simultaneous separation
of volatile compounds and the components with higher
boiling point. Conditions were therefore selected so that
bond.
2
-butanol, 2-butanone, and oxiranes were separated from
HYDROGEN PRESSURE DEPENDENCE
OF RING-OPENING REACTIONS
each other, while cis- and trans-2-butene were eluted in one
peak.
Rates of formation of the products of the two isomeric
dimethyloxiranes are shown in Figs. 1 and 2.
The transformation rate of cis-2,3-dimethyloxirane on a
Cu/SiO2 catalyst at 363 K was only 20% higher than that of
the trans-isomer, unlike on a platinum or palladium catalyst
Dependence of the transformations upon hydrogen pres-
sure (Figs. 3 and 4) reveals that the rates of hydrogenol-
ysis and deoxygenation increase virtually linearly with
(
8, 9) where kcis � ktrans; i.e., in the case of dimethyloxiranes
experimental results obtained on copper resemble those
measured on nickel. The difference between reaction rates
on Cu/SiO2 may be accounted for by the difference between
the rates of the deoxygenation reactions of the two oxirane
isomers.
SCHEME 1. Reaction pathways in the transformations of cis- and trans-
,3-dimethyloxiranes.
FIG. 2. Transformation of trans-2,3-dimethyloxirane over Cu/SiO2
catalyst (363 K; phydrogen = 20 kPa; poxirane = 1.33 kPa; catalyst = 20 mg).
2

´ ´
FASI, NOTHEISZ, AND BARTOK
1
16
TABLE 1
Comparison of Activation Energies (Ea)
Ea (kJ/mol)
|
|
|
——
Q
Products
——
Q
ꢀ
ꢀ|
�
�
2
2
2
-Butene
-Butanone
-Butanol
64.3
60.7
36.9
60.0
77.1
77.7
47.1
76.3
Total
Note. 353–403 K; phydrogen = 20 kPa; poxirane = 1.33 kPa;
catalyst = 20 mg Cu/SiO2.
FIG. 4. Turnover frequency of product formation vs hydrogen pres-
sure at trans-2,3-dimethyloxirane (catalyst = 20 mg Cu/SiO2; 363 K).
increasing hydrogen pressure in the case of both isomers,
while the rate of isomerization displays a maximum (see
below). The increase is especially pronounced in the case
of deoxygenation; therefore, while at low and medium
hydrogen pressures the main products are 2-butanone and
high hydrogen pressures, therefore, reactions proceeding
on Cu(0) atoms may become dominant and those take place
on copper ions may be limited. Deoxygenation evidently
proceeds on Cu(0) atoms; therefore, it is to be expected
that the amount of 2-butene will increase with increasing
hydrogen pressure. Considering the similarity of hydrogen
dependences, the formation of 2-butanol is most proba-
bly also associated with the presence of Cu(0) atoms (in
other words, Cu(0) atoms are necessary for the formation of
2
-butene, at high hydrogen pressures it is unambiguously
deoxygenation that becomes predominant. In contrast to
platinum, palladium and nickel, hydrogenation of 2-butene
to yield butane is not observed. (Due to the significantly
higher adsorptivity of oxirane, the olefin is not hydro-
genated on Cu under these conditions, in the presence of
oxirane.)
2
-butanol).
It should be noted that the effect of hydrogen pressure
has an identical tendency in the case of both 2-butene and
Regarding isomerization, in the case of the cis-isomer the
process displays a slight maximum while the isomerization
of the trans-oxirane is more like a saturation curve in the
pressure range studied. Naturally, it cannot be excluded
that above 100 kPa a decrease in rate may occur also in the
case of the trans-isomer. The effect of hydrogen pressure
on reaction rate suggests that it is not electrophilic catalysis
alone that brings about the formation of 2-butanone.
It is a well-known fact that zero-valent copper has a low
affinity to alcohols (12). However, insertion of oxygen into
a copper surface generates positively charged active sites
that promote the adsorption of alcohols. For example, the
amount of methanol adsorbed on a partially oxidized cop-
per surface displays a maximum at 20% oxygen coverage.
Active centers are made up by Lewis acid–base ion pairs.
The alkoxide ion is adsorbed on the metal ion while the
hydrogen of the hydroxyl group is attached to the oxygen
2
-butanol and the formation of these products is not re-
pressed even at high hydrogen pressures, suggesting that
they are most probably formed on active sites of similar
structure.
Oxiranes oxidize the surface of the copper catalyst via de-
oxygenation (5). Due to the presence ofhydrogen, however,
the oxidized surface is reduced, leading to the formation of
a partially oxidized metal surface (11). As hydrogen pres-
sure is increased, the rate of the reduction and consequently
the ratio of Cu(0) atoms relative to Cu ions increases. At
(
13). By analogy, adsorption of oxiranes and cleavage of
their C–O bonds may proceed along similar lines. Cu(0)
atoms surrounding the active sites may play an important
role in hydrogen supply (5). Since isomerization necessi-
tates the presence of Cu–O ion pairs, maximal rate may be
expected at a hydrogen pressure which allows a maximal
surface concentration of these active sites.
The maximum observed around 40 kPa in the case of
the cis-isomer indicates a higher sensitivity to hydrogen
than that of the trans-isomer. This difference supports the
assumption that the decrease in the rate of isomerization
FIG. 3. Turnover frequency of product formation vs hydrogen pres-
sure at cis-2,3-dimethyloxirane (catalyst = 20 mg Cu/SiO2; 363 K).

Cu-CATALYZED RING OPENING OF OXIRANES
117
SCHEME 2. Mechanisms of 2-butanone formation.
observed is associated not only with a decrease in the sur-
There is a significant difference between the deu-
face concentration of copper ions but also with the disso- terium distributions of deuterated 2-butanol samples ob-
ciative character of the mechanism. The lower sensitivity of tained from cis- and trans-2,3-dimethyloxirane. Namely, the
the trans-isomer to hydrogen pressure suggests that in this cis-compound yielded more 2-butanol-d3, while the trans-
case the dissociation step plays a less significant role in the isomer gave rise to more 2-butanol-d2. The reason for this
mechanism. It seems highly probable that cleavage of the is that planar adsorption of cis-oxirane allows a more rapid
C–O bond necessary for ring opening proceeds with an formation of the transitionary �-allyl species which is ad-
identical rate whether adsorption is planar or edgewise. sorbed form of 2-butanol-d3. The mechanism represented
In the case of the cis-isomer, facial adsorption is possible, in Scheme 3 well demonstrates the formation of the two
while steric hindrance in the case of the trans-isomer fa- types of deuterated butanol.
vors edge adsorption. It is therefore probable that cleavage
In trans-oxirane, however, the formation of the above
of the C–O and C–H bonds of the cis-isomer is simultane- species is inhibited, since only one of the two hydrogen
ous, while cleavage of the C–O bonds of the trans-isomer atoms, C(2)–H or C(3)–H, is situated close to the surface
precedes that of its C–H bond, rendering this isomer less of the catalyst. Consequently, due to planar adsorption, the
sensitive to the effect of hydrogen (Scheme 2).
formation of the diadsorbed surface species is preferential,
resulting in a higher reaction rate for 2-butanol-d2.
Based on our experimental results, isomerization, hy-
drogenolysis, and deoxygenation may be interpreted as
Transformation Mechanisms: Nature of Active Sites
In the course of experiments in deuterium, no deuterium shown in Schemes 2 and 3.
was found in unreacted oxirane irrespective of the kind
According to the hypothetical mechanism outlined here,
of isomer studied, indicating that oxirane is irreversibly in the course of isomerization the ring is opened on
adsorbed on the Cu surface and is desorbed only as a
product. There was no traceable amount of deuterium in
2
-butenes either, evidence that deoxygenation proceeds
without cleavage of the C–H bond.
Isomerization resulted in the formation of mainly
2
-butanone-d0; however, a small amount of 2-butanone-d1
was also detected. The observed predominance of
-butanone-d0 indicates that it is essentially intramolecu-
2
lar hydrogen migration that happens.
Comparison of the data in Tables 2 and 3 with those
published regarding nickel (14) reveals that while on cop-
per the product is mainly 2-butanone-d0, on nickel mostly
2
-butanone-d1 is formed in the course of isomerization. This
allows to conclude that, in contrast to copper, on nickel
-butanone is formed mostly via the triadsorbed (or �-allyl)
form.
2
SCHEME 3. Mechanisms of 2-butanone-d1, 2-butanol-d2, and 2-
butanol-d3 formations.

´
´
1
18
FASI, NOTHEISZ, AND BARTOK
TABLE 2
Transformation of cis-2,3-Dimethyloxirane in Deuterium
Distribution of deuterium (% )
In 2-butanone
In 2-butanol
t (min)
Conv. (% )
S2-butanone
S2-butanol
S2-butene
d0
d1
d2
d3
d4
d2
d3
d4
5
5
0
4.6
12.7
23.1
0.43
0.36
0.30
0.13
0.13
0.14
0.44
0.51
0.56
70.9
55.2
37.2
26.0
37.8
45.2
3.1
6.1
13.9
0
0.9
3.3
0
0
0.4
91.6
82.5
66.8
8.4
17.5
24.7
0
0
8.5
1
5
Note. Catalyst = 20 mg Cu/SiO2; pdeuterium = 20 kPa; poxirane = 1.33 kPa; 363 K.
neighboring surface –Cu–O– atoms. The reaction itself is responding cyclopropane stereoisomers, published earlier
intramolecular migration of hydrogen, which proceeds in (6). Namely, what is the effect of the presence of the het-
an essentially similar way in both the cis- and the trans- eroatom in the three-member ring on transformation rate,
isomer. In the case of the cis-isomer, however, cleavage of on selectivities, and, last but not least, on stereochemical
C–O and C–H bonds may occur simultaneously due to pla- characteristics in the case of stereoisomers of similar ge-
nar adsorption, while steric hindrance makes adsorption of ometry?
the trans-isomer edgewise and therefore cleavage of C–O
precedes that of C–H.
Owing to significant differences between the electroneg-
ativities of C and O atoms, oxiranes are more readily ad-
2
-Butanol is formed on Cu(0) atoms. The copper atom sorbed on copper than are cyclopropanes. Consequently,
is inserted into the C–O bond, leading to the formation the rate of hydrogenative ring opening is considerably
of metalla-oxacyclobutane. The driving force of the reac- higher in the case of oxiranes.
tion is the strain energy of the three-member ring which
For both types of compounds, transformation of the
is markedly reduced upon formation of the four-member cis-stereoisomer proceeds at a higher rate. This observa-
ring. Insertion does not require planar adsorption; there- tion as well as other experimental data (dependence on hy-
fore, there is no significant difference between the transfor- drogen pressure, effect of temperature) yield information
mation rates of the two isomers.
regarding the geometry and structure of the adsorbed sur-
The active centers for the above reactions are supplied by face species, namely, that the cis-compounds are adsorbed
the partially oxidized copper surface generated by the op- in a planar and the trans-isomers in an edgewise manner.
posed processes of oxidation and reduction. Maximal cata-
Naturally, there is a large difference between the direc-
lytical activity is realized when an optimal copper/copper tions of the reactions as well. In the case of cyclopropanes
oxide surface structure is formed. The determinant factor only C–C cleavage occur, in the case of oxiranes both C–O
for isomerization is the presence of copper ions, while for and C–C cleavage may theoretically happen. However—
hydrogenolysis and deoxygenation it is that of Cu(0).
also as a consequence of what was stated above—the lat-
ter fails to come about. In the case of cyclopropanes there
is two-way hydrogenolysis (pentane and isopentane are
formed), while oxiranes undergo deoxygenation, isomer-
ization, and hydrogenolysis. The selectivity of the transfor-
Comparison of Transformation of Cyclopropanes
and Oxiranes
The results of experiments on oxirane stereroisomers de- mation is affected by differences due to the geometry of the
scribed above allow a comparison with data on the cor- stereoisomers.
TABLE 3
Transformation of trans-2,3-Dimethyloxirane in Deuterium
Distribution of deuterium (% )
In 2-butanone
In 2-butanol
t (min)
Conv. (% )
S2-butanone
S2-butanol
S2-butene
d0
d1
d2
d3
d4
d2
d3
d4
5
5
0
3.4
9.4
17.6
0.56
0.53
0.49
0.06
0.06
0.06
0.38
0.41
0.45
72.1
55.0
35.9
25.4
38.1
44.3
2.5
6.0
15.1
0
1.0
4.2
0
0
0.6
95.8
92.5
88.3
4.2
7.5
9.7
0
0
2.1
1
5
Note. Catalyst = 20 mg Cu/SiO2; pdeuterium = 20 kPa; poxirane = 1.33 kPa; 363 K.

Cu-CATALYZED RING OPENING OF OXIRANES
ACKNOWLEDGMENTS
119
Finally it must be noted that the catalytic reactions take
place on surface active sites of different types. In the case
of cyclopropanes it is mainly Cu(0) atoms that dominate;
in the case of oxiranes, in addition to Cu(0) atoms copper
ions also play a significant role (5).
Funding provided by the Hungarian National Science Foundation via
Grants OTKA T014315 and T016109 is gratefully acknowledged. The au-
thors express their gratitude to Arp a´ d Moln a´ r for having made available
´
the catalyst he himself had prepared and characterized.
CONCLUSIONS
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´
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´
8
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than on that of the trans-compound;
. The above results are interpreted by the different
geometry of the adsorption of the two compounds: the
cis-isomer is adsorbed on the surface in a planar manner,
while adsorption of the trans-isomer is edgewise.
4