Benzoylation of arenes using envirocat EPZG catalyst and microwave irradiation

Article abstract of DOI:10.1135/cccc20000644

Benzoylations of several benzene, naphthalene and thiophene derivatives with benzoyl chloride or benzoie anhydride, catalyzed with EPZG catalyst and in the absence of solvents were studied under microwave irradiation (MWI). Reasonable to good yields of products were achieved in short times.

Full text of DOI:10.1135/cccc20000644

644
Veverková, Gotov, Mitterpach, Toma:
BENZOYLATION OF ARENES USING ENVIROCAT EPZG® CATALYST
AND MICROWAVE IRRADIATION
1
2
1
3,
Eva VEVERKOVÁ , Battsengel GOTOV , Róbert MITTERPACH and Štefan TOMA *
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University,
1
2
842 15Bratislava, Slovak Republic; e-mail: organika@fns.uniba.sk, gotovyn@fns.uniba.sk,
toma@uniba.sk
3
Received February 7, 2000
Accepted April 14, 2000
Dedicated to Professor Otakar Červinka on the occasion of his 75th birthday.
Ben zoylation s of several ben zen e, n aph th alen e an d th ioph en e derivatives with ben zoyl ch lo-
ride or ben zoic an h ydride, catalyzed with EPZG® catalyst an d in th e absen ce of solven ts
were studied un der m icrowave irradiation (MWI). Reason able to good yields of products
were ach ieved in sh ort tim es.
Key w ord s: Acylation s; Ben zoylation s; Clays; En viron m en tal ch em istry; Heterogen eous ca-
talysis; Microwave ch em istry; Friedl–Crafts acylation .
Friedel–Crafts reaction s are very im portan t in dustrial processes with a con -
siderable en viron m en tal drawback, wh ich is caused prim arily by usin g alu-
m in um ch loride as catalyst. Th e fin al step of th e Friedel–Crafts acylation is
work-up of th e reaction m ixture with water resultin g in a great am oun t of
waste water. Th is is th e reason for th e exten sive search for a n ew, h eteroge-
n eous an d en viron m en t-frien dly catalyst. Acid clays with exch an geable cat-
ion s were foun d to be very good Friedel–Crafts acylation catalysts1–3
.
Clay-supported Lewis acids are especially good Friedel–Crafts catalysts4,5
an d are th erefore also com m ercialized, in spite of lon g reaction tim es6 (up
to 20 h ). Th e effect of structure of Clayzic catalyst on its catalytic activity
was th orough ly studied7. Rare earth salts supported on clays were described
as good Friedel–Crafts alkylation catalysts8, an d Lewis acids supported on
m esoporous silica were also used for th e sam e reaction s9,10. Rare earth m etal
ch loride supported on K-10 clay or silica gel are also very good catalysts for
Friedel–Crafts acylation of differen t arom atic com poun ds, especially th eir
m eth oxy derivatives11. Even graph ite was claim ed to be a solid state
Friedel–Crafts catalyst in m eth oxyben zen e ben zoylation 12, but it was
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Benzoylation of Arenes
645
proved th at th e real catalysts are th e m etal, especially iron , im purities in
graph ite13,14
.
Th e ben eficial effect of m icrowaves on m an y organ ic reaction s was de-
scribed15–20, am on g th em also on graph ite catalyzed Friedel–Crafts
ben zoylation 13,14 as well as differen t Lewis acids catalyzed Friedel–Crafts
ben zoylation of m eth oxyaren es21. An in terestin g m icrowave accelerated
in tram olecular Friedel–Crafts acylation catalyzed by K-10 clay was also de-
scribed22. We decided th erefore to exam in e if th e application of m icrowave
irradiation could sh orten th e reaction tim e of th e En virocat-catalyzed
ben zoylation s. Th e secon d aim of our work was to fin d out wh eth er isom er
ratio of th e products form ed could be in fluen ced by th is un con ven tion al
en ergy source an d wh eth er th e sam e catalyst can be reused several tim es.
EXPERIMENTAL
Gen eral
All m icrowave experim en ts were carried out in a Mileston e Lavis 1000 reactor, power settin g
400 W. Mileston e Lavis 1000 reactor is a m ultim ode m icrowave oven , with on e m agn etron ,
equipped with reflux con den ser an d an electrom agn etic stirrer. Reactor is operatin g in a
pulse m ode (5 s on 5 s off) an d power in put can be regulated from 10 to 1 000 W. 1H NMR
spectra (300 MHz) were m easured on a Varian Gem in i 2000 in strum en t, GC-MS an alyses
were perform ed on a HP 5890A Series II gas ch rom atograph in terfaced to HP 5971 A
m ass-selective detector. Meltin g poin ts were determ in ed on a Kofler h ot-stage in strum en t
an d are un corrected. En virocat EPZG® catalyst6 is a Zn Cl2-clay supported catalyst sim ilar to
Clayzic. En virocat EPZG® is a free flowin g, yellow to green powder, bulk den sity 0.76 g/cm 3,
surface area approxim ately 250 m 2/g.
Ben zoylation with Ben zoyl Ch loride (Meth od A)
A m icrowave pre-dried EPZG® catalyst (0.5 g) was added to a m ixture of aren e (40 m m ol)
an d ben zoyl ch loride (2.81 g, 20 m m ol) in a 50 m l flask. (Th e catalyst was pre-dried eith er
by azeotropic destilation with toluen e or by exposin g for 2 m in to MWI at 270 W.) Th e
m ixture was th en m icrowave-irradiated for a given tim e (Tables I an d II), th e fin al tem pera-
ture was m easured in dicated by th e IR th erm om eter wh ich is in th e reactor an d ch ecked by
in sertin g a con tact th erm om eter righ t after reaction was fin ish ed. Th e m ixture was th en
cooled to room tem perature. Dich lorom eth an e was added an d th e catalyst was filtered off.
Th e excess ben zoyl ch loride was destroyed by extraction with a water solution of trieth yl-
am in e an d th e dich lorom eth an e solution was dried over an h ydrous sodium sulfate. Th e sol-
ven t was evaporated an d th e residue was subjected to GC-MS an alysis or ch rom atograph ed
on a silica gel colum n usin g h exan e–eth yl acetate m ixture as eluen t.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

646
Veverková, Gotov, Mitterpach, Toma:
TABLE I
Ben zoylation of aren es with ben zoyl ch loride usin g EPZG® catalyst an d MWIa
Aren e
Tfin , °C
Yield, %
Isom ers ratiob, %
Ben zen e
103
152
105
105
130
136
185
185
185
165
178
102
19
12
18
23
36
41
83
74
72
68
68
61
Brom oben zen e
Fluoroben zen e
Fluoroben zen ec
Toluen e
13(o)
21(o)
9(o)
4(m)
0(m)
0(m)
6(m)
8(m)
0(m)
0(m)
0(m)
34(2)
75(6)
0(3)
83(p)
79(p)
91(p)
77(p)
72(p)
82(p)
82(p)
82(p)
17(o)
20(o)
18(o)
18(o)
18(o)
66(1)
0(1)
Toluen ed
Meth oxyben zen e
Meth oxyben zen ee
Meth oxyben zen ef
Naph th alen e
2-Meth oxyn aph th alen e
Th ioph en e
25(8)
100(2)
a
Meth od A: Aren e 40 m m ol, ben zoyl ch loride 20 m m ol, EPZG® 0.5 g, 20 m in , MWI = m icro-
b
c
d
wave irradiation ; position of ben zoyl in paren th eses; th erm al h eatin g for 72 h ; toluen e :
ben zoyl ch loride 1 : 2; with recycled catalyst; with twice recycled catalyst.
e
f
TABLE II
Ben zoylation of aren es with ben zoic an h ydride usin g EPZG® catalyst an d MWIa
Aren e
Tfin , °C
Yield, %
Isom ers ratiob, %
Ben zen e
105
130
182
166
183
106
0
0
Toluen e
Meth oxyben zen e
Naph th alen e
2-Meth oxyn aph th alen e
Th ioph en e
49
33
32
44
10(o)
90(p)
34(2)
18(8)
0(3)
66(1)
82(6)
100(2)
a
b
Meth od B: Aren e 20 m m ol, ben zoic an h ydride 20 m m ol, EPZG® 0.5 g, 20 m in ; position of
ben zoyl in paren th eses.
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Benzoylation of Arenes
647
Ben zoylation with Ben zoic An h ydride (Meth od B)
A m icrowave pre-dried EPZG® catalyst (0.5 g) was added to a m ixture of aren e (20 m m ol)
an d ben zoic an h ydride (4.53 g, 20 m m ol) in a 50 m l flask. Th e m ixture was m icrowave-
irradiated for 20 m in an d th en cooled to room tem perature. (Th e fin al tem perature was ap-
proxim ately th e sam e as in m eth od A.) Dich lorom eth an e was added an d th e catalyst was fil-
tered off. Th e solution was dried over an h ydrous sodium sulfate, th e solven t was evaporated
an d th e residue subjected to GC-MS an alysis or ch rom atograph ed on a silica gel colum n us-
in g h exan e–eth yl acetate m ixture as eluen t.
Th e 1H NMR spectra of isolated products are in accord with th e publish ed data: ben zo-
ph en on e23 , 4-fluoroben zoph en on e24, 4-brom oben zoph en on e25, 4-m eth ylben zoph en on e
an d 4-m eth oxyben zoph en on e26, 1- an d 2-ben zyln aph th alen es27, X-ben zoyl-2-m eth oxy-
n aph th alen es28 an d 2-ben zoylth ioph en e29
.
RESULTS AND DISCUSSION
Several ben zen e derivatives, n aph th alen e, 2-m eth oxyn aph th alen e an d
th ioph en e were ch osen as substrates for our Friedel–Crafts ben zoylation
study. Th e ben zoylation s were carried out eith er with ben zoyl ch loride or
ben zoic an h ydride usin g differen t ratios of th e substrate, ben zoylation re-
agen t an d EPZG® catalyst ratio (Tables I–III). Th e gen eral course of th e reac-
tion is outlin ed on th e Sch em e 1.
EPZG R
MWI
O
COCl
X
+
X
X = H, Me, MeO, F, Br
SCHEME 1
Structure of th e products was establish ed by 1H NMR spectra an d th eir
com parison with th e literature available data.
Th e first experim en ts were carried out with ben zoyl ch loride as an
acylation agen t (m eth od A). After several trials, we foun d th at 400 W in put
power of th e m icrowave oven was th e best for our experim en ts from th e
poin t of view of th e reactan ts con version an d th e reaction tim e. It is obvi-
ous (Table I) th at yields were h igh ly depen den t on electron ic effects of
substituen t as well as on th e boilin g poin t of th e substrate. Syn th esis of
ben zoph en on e in 67% yield usin g th e EPZG® catalyst was described previ-
ously6: th erm al h eatin g was applied an d th e reaction tim e was 30 h . We
ach ieved just a 19% yield after 20 m in of m icrowave irradiation . Th is can
be caused by perform in g th e reaction on a sm all scale an d m ost of ben zen e
was probably evaporated in to th e reflux con den ser. Th is is probably th e
reason wh y our ben zoylation of toluen e afforded just 36% yield of prod-
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

648
Veverková, Gotov, Mitterpach, Toma:
ucts. Th e obtain ed isom er ratio was (GC-MS) o : m : p = 17 : 6 : 77, wh ich is
sligh tly better th an th at described by Kodom ari11 (o : p = 25 : 75) usin g
graph ite as th e catalyst an d th erm al h eatin g. Our results are com parable
with th ose obtain ed by Laporte et al.13 wh o used graph ite as catalyst an d
m icrowave irradiation (MWI) (50% yield, o : m : p = 13 : 3 : 84). A rise in th e
ben zoyl ch loride substrate ratio to 1 : 2 did n ot ch an ge th e picture. It is n ot
surprisin g th at ben zoylation of substrates like fluoroben zen e an d brom o-
ben zen e resulted in lower yields of th e products (18 an d 12%, respectively).
We also perform ed ben zoylation of fluoroben zen e un der th erm al h eatin g.
Th e reaction tim e was very lon g (72 h ), th e yield of th e product was h igh er
an d th e reaction was m ore selective th an in th e m icrowave experim en t.
Th is can be explain ed by superh eatin g of th e solid catalyst in th e m icro-
wave experim en t30 wh ich m ean s th at reaction takes place locally at h igh er
tem perature th an is boilin g poin t of fluoroben zen e.
Th e ben zoylation of m eth oxyben zen e led to virtually total con version of
th e substrate (82% yield of isolated products) an d th e o : p isom er ratio was
18 : 82. Our results are com parable with ref.13 (86%), but better th an de-
scribed for a con ven tion al experim en t with EPZG® catalyst6 (60%, 15 h ).
Th e reaction proceeded less selectively un der our con dition s, as we
ach ieved th e o : p ratio 1 : 4.6 in con trast to th e described6,13 1 : 12.
Ben zoylation of n aph th alen e resulted in a 78% yield of th e m ixture of
ben zoyln aph th alen es; th e observed ratio of 1- to 2-ben zoyln aph th alen e
was 2.3 : 1 (GC-MS). Ben zoylation of 2-m eth oxyn aph th alen e resulted in a
total yield of 68% of th e 75 : 25 m ixture of 2-ben zoyl-6-m eth oxyn aph th a-
len e an d 1-ben zoyl-7-m eth oxyn aph th alen e. Th e absen ce of 1-ben zoylated
product can be explain ed by results of Giordan o et al.31, wh o foun d out
th at th e isom er ratio was tim e-depen den t an d th e acylation of 2-m eth oxy-
n aph th alen e is a reversible reaction .
An attem pt at ben zoylation of th ioph en e was also m ade. Th e reaction
wen t sm ooth ly an d 2-ben zoylth ioph en e was isolated in 68% yield.
Very sim ilar results were ach ieved usin g ben zoic an h ydride (m eth od B) as
th e acylation agen t (Table II).
Th e on ly differen ce is th at, due to th e less reactive acylation agen t, n o
ben zoylation of ben zen e an d toluen e was observed an d lower yields of
products were obtain ed at ben zoylation of oth er substrates. Th e very low
yields of ben zoylation of n aph th alen e an d 2-m eth oxyn aph th alen e can also
be explain ed by th e fact th at reaction is takin g place in th e m elt wh ere th e
local reaction tem perature could be very h igh , destroyin g th e catalyst. Th is
could be judged from its black colour after th e reaction .
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Benzoylation of Arenes
649
Our secon d question was wh eth er it is possible to observe a specific m i-
crowave effect an d wh eth er th e sam e catalyst could be reused several tim es.
Th e reaction with m eth oxyben zen e as th e substrate sh owed (Table I) th at
th e sam e catalyst can be reused twice with out con siderable loss of its activ-
ity. It was also proved th at th e 20 m in reaction tim e for ben zoylation of
m eth oxyben zen e was overestim ated. Th e sam e yield of ben zoyl(m eth oxy)-
ben zen e was isolated just after 1 m in of m icrowave irradiation . Th e reac-
tion m ixture reach ed tem perature 148 °C after 1 m in irradiation an d 185 °C
after 5 m in irradiation (see Table III).
A m uch lower yield of th e ben zoyl(m eth oxy)ben zen e was isolated wh en
th e reaction was carried out un der th e sam e con dition s (reaction tim e an d
tem perature) but a th erm al h eatin g was applied with th e reaction m ixture
im m ersed in an oil bath preh eated to a ch osen tem perature. As it was rath er
a surprisin g result, experim en ts were repeated at lower power in put (200 W),
but even at such con dition s th e reaction was over after 5 m in . An experi-
m en t with th erm al h eatin g was also perform ed, in wh ich th e reaction m ix-
ture was preh eated to 100 °C with out a catalyst, th en an EPZG® catalyst
was added an d th e m ixture was h eated to th e sam e tem perature for 10 m in ,
but th e yield of th e ben zoylated m eth oxyben zen e was just 8%. Th is could
be caused by a specific m icrowave effect or by th e local overh eatin g of th e
catalyst wh en exposed to m icrowave irradiation .
TABLE III
Ben zoylation of m eth oxyben zen e un der differen t con dition sa
Tim e, m in
Tfin , °C
Heatin g
Isolated yield, %
1
1
5
5
148
148
185
185
MWI
82
19
82
64
Oil bath
MWI
Oil bath
a
Meth oxyben zen e 40 m m ol, ben zoyl ch loride 20 m m ol, EPZG® 0.5 g.
This work was supported by the Slovak Grant Agency VEGA (grant No. 1/4166/97). We thank the
referees for their useful comments, Dr E. Solčániová for measuring the NMR spectra as well as Dr R.
Mračnová and Dr R. Kubinec for GC-MS analysis. Our thanks are also due to Contract Catalysts,
Prescot, for the generous gift of EPZG® catalyst.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

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Veverková, Gotov, Mitterpach, Toma:
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