Glass wool catalysed regioselective isomerization of styrene oxides

Article abstract of DOI:10.1002/jccs.201200269

Glass wool is widely used as an insulating material. Here we report for the first time, the function of glass wool as a mild heterogeneous catalyst under vapor phase conditions - particularly for the rearrangement of styrene oxides including halogen-substituted styrene oxides to the corresponding phenyl acetaldehydes. Using this methodology, 4-isobutyl-α-methyl styrene oxide is smoothly converted to 4-isobutyl α-methyl phenyl acetaldehyde which is the precursor of the API "Ibuprofen" - an important pharmaceutical agent.

Full text of DOI:10.1002/jccs.201200269

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Glass Wool Catalysed Regioselective Isomerization of Styrene Oxides
Govindarajan K. Ramaswamy,a,b,* Angalan Somasundaram,a Balasubramanian K. Kuppuswamya
and Murugesan Velayudhamb
aShasun Research Centre, Vandaloor-Kelambakkam Rd, 27 Keelakottaiyur, Chennai, India
bCentre for Research, Anna University, Guindy, Chennai, India
(Received: May 10, 2012; Accepted: Aug. 6, 2012; Published Online: ??; DOI: 10.1002/jccs.201200269)
Glass wool is widely used as an insulating material. Here we report for the first time, the function of glass
wool as a mild heterogeneous catalyst under vapor phase conditions - particularly for the rearrangement
of styrene oxides including halogen-substituted styrene oxides to the corresponding phenyl acetalde-
hydes. Using this methodology, 4-isobutyl-a-methyl styrene oxide is smoothly converted to 4-isobutyl
a-methyl phenyl acetaldehyde which is the precursor of the API “Ibuprofen” - an important pharmaceuti-
cal agent.
Keywords: Glass wool; Heterogeneous catalysis; Styrene oxide; 4-Isobutyl-a-methyl phenyl
acetaldehyde.
INTRODUCTION
Epoxides are one of the most useful and versatile sub-
acetals), a sweet leaf odour (diethyl acetals), or a tangy
aroma (diphenyl acetals).9
stances in organic chemistry due to their high reactivity and
easy availability through a wide variety of methods that
permit relative and absolute stereo control.1,2,3 One of the
most frequently used atom-economical reactions of epox-
ides is their rearrangement to carbonyl compounds. Anum-
ber of reagents - including a variety of Lewis acids, palla-
dium catalysts and Indium(III) chloride - have been dem-
onstrated as being effective for this rearrangement reac-
tion.4,5,6 One of the well-known procedures for preparation
of phenyl acetaldehydes in an industrial scale is by de-
hydrogenation of phenyl ethanols.7 However, in this ap-
proach, only partial conversion is possible, and separation
of starting material from the end product involves huge
losses (mainly owing to phenyl acetaldehydes being ther-
mally unstable). Additionally, formation of auto-condensa-
tion products during fractionation is another drawback of
this approach. Finally, halogen-substituted phenyl acetal-
dehydes cannot be prepared by this approach since elimina-
tion of halogen can occur.7 An alternate approach involves
rearrangement of epoxides using homogeneous or hetero-
geneous catalysts.8 For instance, the isomerization of sty-
rene oxide to phenyl acetaldehyde is an acid catalyzed reac-
tion, which is used for the production of fragrant chemicals
in an industrial scale as well as production of pharma-
ceuticals, insecticides, fungicides and herbicides.8 Further-
more, phenyl acetaldehyde is a valuable intermediate for
producing more stable acetals with a honey aroma (glycol
Homogeneous catalysts such as phosphoric acid,
BF3, FeCl3, ZnBr2, as well as heterogeneous catalysts such
as SiO2, Al2O3, ZnO, WO3, supported metals and various
precipitated phosphates have been employed for isomeri-
zation reactions.8 These acid catalysts are generally used in
stoichiometric quantities. However, these catalysts are cor-
rosive on the equipments used, in addition to necessitating
procedures involving aqueous work up and challenging
isolation techniques. In addition, large amounts of salts are
produced during neutralization which can lead to heavy
metal pollution. Chemo- and regio-selective conversion of
epoxides to carbonyl compounds in 5 M lithium perchlor-
ate (in diethyl ether medium) has been reported.10 Lithium
perchlorate in refluxing benzene may be a useful reagent
for the rearrangement of several epoxides since this reagent
shows higher selectivity compared to strong Lewis acids.11
However, the perchlorate salts are not eco-friendly and are
explosive in nature, and this method is not suitable for large
scale preparation. Several solid catalysts have been used to
study the rearrangement of various styrene oxides under
gas and liquid conditions.12 Homogeneous catalysts are
used in industry although they are not regenerable and pro-
duce voluminous corrosive waste streams. On the other
hand, heterogeneous catalysts like TiO2, P2O5/SiO2, Y-alu-
mina, B2O3/SiO2 and bentonite have been used for this re-
action.8 One of their advantages is that they can be used in
gas-phase reactions and therefore continuous processes
* Corresponding author. Tel: +91-44-23642210; Fax: +91-44-47406190; E-mail: krg.geetha@gmail.com
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Article
Ramaswamy et al.
can be created with relatively little technical effort. How-
ever, they do have some drawbacks including incomplete
conversion, formation of mixtures of ketones and alde-
hydes and the formation of aldol condensation products.
The formation of these condensation products is the first
step in the formation of coke and therefore limits the life-
time of these catalysts.13 Different styrene oxides can be re-
arranged in a fixed bed reactor under gas phase condi-
tions.14 When conventional oxides are used, one of the ma-
jor side products is 1,3,5-triphenyl benzene formed via
aldol condensation mechanism. The side reactions can be
suppressed by the use of zeolites such as ZSM-5 (Si/Al =
18.8) that hinder aldol condensation and subsequent prod-
ucts because of steric constraints on the framework of the
catalyst.8 Some of the problems associated with these cata-
lysts are challenging preparation methods, poor selectivity,
difficulty in separating the by-products formed (due to
auto-cyclization) and poor lifetime of the catalyst (caused
by coating of the surface). Moreover, these catalyst sys-
tems are not useful for halogenated starting materials.
While studying the isomerization of halogen substituted
styrene oxides, we found that glass wool15 can effectively
and selectively isomerize styrene oxides to phenyl acetal-
dehydes under vapor phase conditions.
Scheme I Styrene oxides isomerization
and by comparison with an authentic sample. It is impor-
tant to note that this isomerization did not take place to an
appreciable extent in the absence glass wool, (Table 1 entry
5). We have also observed that silica gel (desiccant form as
well as silica gel for column chromatography) and alumi-
num oxide (used as pellets) did not catalyze the reaction to
the same extent as that of glass wool under identical condi-
tions. It is interesting to note that synthesis of aldehydes
from oxiranes using silica gel as reagent is known under
liquid phase conditions.16
We have attempted styrene oxide isomerization under
reflux conditions as such without any catalyst (194-196 °C)
and also in presence of ceramic beads and glass wool sepa-
rately. We did not observe any rearrangement and obtained
the starting material as such in all the cases, indicating that
styrene oxide was stable under reflux conditions and in
presence of these materials. We have successfully isome-
rized various styrene epoxides using glass wool as the het-
erogeneous catalyst under vapor phase condition as shown
in Table 2. Halogen substituted styrene oxides like 4-chlo-
rostyrene oxide 3 (Table 2 entry 1); 3-chlorostyrene oxide 5
(Table 2 entry 2) underwent rearrangement smoothly to the
corresponding phenyl acetaldehydes under vapor phase
RESULTS AND DISCUSSIONS
Styrene oxides isomerization is schematically repre-
sented in Scheme I. Styrene oxide 1 served as the model
compound for our studies and the reaction conditions for
this vapor phase isomerization reactions were evaluated
(Table 1). Styrene oxide (1) vapors (6 mL/h) were fed in to
the pre-activated glass wool (450 mg) placed inside the tu-
bular reactor kept at 300 °C under N2 flow, the product col-
lected at the receiver was found to be phenylacetalydyde
(Table 1 entry 3), identified on the basis of its spectral data
Table 1. Styrene oxide 1 (density 1.052 g/mL) isomerization
Remarks
% of styrene
oxide in the
product by GC
Flow rate of Glass wool-
GC purity of
Yield
w/w
Entry
T °C
WHSV*
1 mL/h
mg
2-% area
1
2
3
4
5
300
300
300
300
300
1.5
3
450
450
450
450
Nil
3.50
7.01
14.02
210.3
-
80
89.4
91.5
21
34.6
54.5
9117
87
Nil
Nil
6
0.28
76.6
77
90
3
22
60
*WHSV-Weight Hourly Space Velocity. (Weight of feed flowing per unit weight of catalyst per
hour).
2
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CHEMICAL SOCIETY
Glass Wool Catalysed Isomerization of Styrene Oxides
Table 2. Various epoxides isomerization using glass wool as catalyst under gas phase
Glass wool
Rate of flow
(mL/h)
Yield
w/w
Entry
Epoxide
Temp
Product
Loading (mg)
20
O
21
CHO
1
300 °C
450
450
450
3
3
3
62%
Cl
Cl
Cl
Cl
(4)
(3)
22
O
23
CHO
2
3
300 °C
300 °C
70%
(6)
(5)
24
O
25
Cl
Cl
CHO
80%
(7)
(8)
26
O
27
CHO
4
300 °C
450
3
60%
(10)
(9)
16
O
28
CHO
5
6
7
300 °C
300 °C
300 °C
450
450
450
3
3
3
52%
H3CO
H3CO
(12)
(11)
No isomerization has occurred.
Starting epoxide was obtained
with GC purity of 93.4%
O
-
-
Cl
(13)
29
O
O
No isomerization has occurred.
Starting epoxide was obtained
with GC purity of 96.02%
(14)
30
O
O
No isomerization has occurred.
Starting epoxide was obtained
with GC purity of 89.2%
8
300 °C
450
3
-
CHO
(15)
conditions using glass wool as the heterogeneous catalyst
while our attempts to rearrange them using conventional
systems did not yield desired results.10,11 Likewise, 3-chlo-
ro-a-methyl styrene oxide 7 underwent smooth rearrange-
ment to the corresponding phenyl acetaldehyde under va-
por phase conditions using glass wool as catalyst (Table 2
entry 3). 4-Isobutyl-a-methyl phenyl acetaldehyde 10
which is precursor of the API “Ibuprofen”, a popular anti-
inflammatory and analgesic drug, has been prepared by the
isomerisation of 4-isobutyl-a-methylstyrene oxide 9 (Ta-
ble 2 entry 4) in moderate yield of 60% (Table 2 entry 4).
4-Methoxystyrene oxide 11 also underwent this transfor-
mation without any problem. (Table 2 entry 5). The cata-
lytic activity of glass wool is specific to rearrangement of
styrene oxides. Epoxides that are not attached to aromatic
systems did not undergo isomerization under these condi-
tions. For example, epichlorohydrin 13 (Table 2 entry 6),
phenyl glycidyl ether 14. (Table 2 entry 7) and 4-oxiranyl-
methoxy benzaldehyde 15 (Table 2 entry 8) did not un-
dergo the rearrangement to the corresponding aldehydes
under these conditions.18,19 Time Controlled Desorption
measurement and Temperature Programmed Desorption
profile which are simple and inexpensive methods nor-
mally used to measure the number and strength of acid sites
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Article
Ramaswamy et al.
Scheme II Possible mechanism of styrene epoxides isomerization over glass wool
on solid catalysts were extended to glass wool.31 Time Con-
trolled Desorption (TCD) measurement and Temperature
Programmed Desorption – NH3 (TPD) measurement on
glass wool revealed strong and weak acidic sites in the
glass wool molecule. These acidic sites are believed to aid
in the isomerization reaction of aryl ethylene oxides to cor-
responding aryl aldehydes. This transformation can be
rationalized as shown in Scheme II. Non-benzylic epoxides
did not undergo rearrangement in presence of glass wool
due to the difficulty in the formation of the carbocations
that are not resonance stabilized.
vated glass wool placed inside the tubular reactor at a flow
rate of 6 mL/h. The temperature for the rearrangement reac-
tion was maintained at 300 °C. The product vapors are
cooled and collected downwards at the receiver, with 91%
recovery of phenyl acetaldehyde (2) with a GC purity of
91.3% a/a.
1H NMR (300 MHz, CDCl3): d 3.7 (d, 2H, J = 3 Hz),
7.31-7.23 (m, 2H), 7.41-7.33 (m, 3H), 9.75 (t, 1H, J = 3
Hz).
Example 2: 4-Chloro phenyl acetaldehyde (4) (Table
2, entry 1)
4-Chloro styrene oxide (3) vapors are fed in to the
pre-activated glass wool, placed inside the tubular reactor
at a flow rate of 3 mL/h. The rearrangement reaction was
conducted under N2 flow at 300 °C. The product vapors are
cooled and collected at the receiver, with 62% recovery of
4-chloro phenyl acetaldehyde (4).
1H NMR (300 MHz, CDCl3): d 3.69 (d, 2H, 2 Hz),
7.16 (d, 2H, J = 8 Hz), 7.36 (d, 2H, J = 8 Hz), 9.75 (t, 1H, J
= 2 Hz).
CONCLUSION
In conclusion we have shown that glass wool behaves
as a heterogeneous catalyst system under vapor phase con-
ditions, particularly for the regioselective rearrangement of
styrene oxides to the corresponding phenyl acetaldehydes.
This method also provides very good yields with halo-
gen-containing starting materials which are very difficult
to prepare by other methods. Further, the use of glass wool
as a catalyst appears to be associated with several advan-
tages over current approaches, including green transforma-
tion, enhanced selectivity, increased catalyst lifetime,
heightened availability and lower cost. Taken together,
these results establish that glass wool could be used as a
novel heterogeneous mild acid catalyst in organic chemis-
try reactions under vapor phase.
Example 3: 3-Chloro phenyl acetaldehyde (6) (Table
2, entry 2)
3-Chloro styrene oxide (5) vapors are fed in to the
pre-activated glass wool, placed inside the tubular reactor
at a flow rate of 3 mL/h. The temperature for the rearrange-
ment reaction was maintained at 300 °C. The product va-
pors are cooled and collected at the receiver, with 70% re-
covery of 3-chloro phenyl acetaldehyde (6) with a GC
purity of 94.3% a/a.
General Procedure for the isomerization of styrene
epoxides to phenyl acetaldehydes
A tubular catalyst chamber of desired width and
length is loaded with the catalyst glass wool of desired vol-
ume. The catalyst is activated by heating to 500 °C for 5 h
and cooled to the desired temperature under nitrogen. The
aryl ethylene oxide is vaporized and the vapors are passed
through the externally heated tubular reactor loaded with
the catalyst, at a desired flow rate using a syringe infusion
pump. The vapors flow through the hot catalyst and the re-
sultant product vapors are cooled and collected.
Example 1: Phenyl acetaldehyde (2) (Table 1, entry
3)
1H NMR (300 MHz, CDCl3): d 3.68 (d, 2H, J = 2 Hz),
7.09 (d, 1H), 7.15 (s, 1H), 7.21-7.34 (m, 2H), 9.74 (t, 3H, J
= 2 Hz); m/z value is 154.
Example 4: 3-Chloro-a-methyl phenyl acetaldehyde
(8) (Table 2, entry 3)
3-Chloro-a-methyl styrene oxide (7) was prepared
according to the literature procedure. This epoxide was fed
into the reactor over the pre-activated glass wool at 300 °C,
at a flow rate of 3 mL/h, with 80% yield of 3-chloro-a-
methyl phenyl acetaldehyde and a GC purity of 94.3% a/a.
1H NMR (300 MHz, CDCl3): d 1.45 (d, 3H, J = 7 Hz),
3.63 (q, 1H, J = 7 Hz), 7.08 (d, 1H), 7.15 (s, 1H), 7.21-7.34
Styrene oxide (1) vapors are fed in to the pre-acti-
4
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Glass Wool Catalysed Isomerization of Styrene Oxides
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control facilities. We acknowledge and thank Prof. Ranga
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