The methoxycarbonylation of aryl chlorides catalysed by palladium complexes of bis(di-tert-butylphosphinomethyl)benzene

Article abstract of DOI:10.1039/b501868d

A catalyst system based on palladium-1,2-bis-(di-tert-butylphosphinomethyl) benzene (BDTBPMB) shows good activity for the methoxycarbonylation of strongly activated aryl chlorides, like 4-chloromethylbenzoate or 4-chlorocyanobenzene. Surprisingly, the use of less activated aryl chlorides, like 4-chloroacetophenone, leads to the formation of dimethyl terephthalate amongst other products arising from organic reactions of methoxide ion and/or CO. Less nucleophilic alcohols such as 2,2,2-trifluoroethanol promote the formation of carbonylation products even from 4-chloroacetophenone and chlorobenzene. Labelling studies involving CD₃OH, CD₃OD or ¹³CO give information on the origin of many of the products. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501868d

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F U L L P A P E R
The methoxycarbonylation of aryl chlorides catalysed by palladium
complexes of bis(di-tert-butylphosphinomethyl)benzene
Cristina Jimenez-Rodriguez,a Graham R. Easthamb and David. J. Cole-Hamilton*a
a School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST.
E-mail: djc@st-and.ac.uk; Fax: 44-(0)1334-463808; Tel: 44-(0)1334-463805
b Lucite International, Technology Centre, PO Box 90, Wilton, Middlesborough, Cleveland, UK
TS6 8JE. E-mail: graham.eastham@lucite.com; Fax: 44-(0)1642-447119;
Tel: 44-(0)1642-447109
Received 7th February 2005, Accepted 6th April 2005
First published as an Advance Article on the web 25th April 2005
A catalyst system based on palladium-1,2-bis-(di-tert-butylphosphinomethyl)benzene (BDTBPMB) shows good
activity for the methoxycarbonylation of strongly activated aryl chlorides, like 4-chloromethylbenzoate or
4-chlorocyanobenzene. Surprisingly, the use of less activated aryl chlorides, like 4-chloroacetophenone, leads to the
formation of dimethyl terephthalate amongst other products arising from organic reactions of methoxide ion and/or
CO. Less nucleophilic alcohols such as 2,2,2-trifluoroethanol promote the formation of carbonylation products even
from 4-chloroacetophenone and chlorobenzene. Labelling studies involving CD3OH, CD3OD or 13CO give
information on the origin of many of the products.
NEt3 as additives to Pd-PCy3 complexes has been reported to
Introduction
provide a turnover number of 1000 in amidation reactions of
The palladium-catalysed carbonylation of aryl halides is a
electron poor aryl chlorides.7 Amidation (aminocarbonylation)
reaction of industrial interest, since a great variety of products,
reactions of aryl halides have also been reported.8
such as amides, esters (Scheme 1), aldehydes and acids, can
Recently, Beller and co-workers developed systems based
be obtained.1 These reactions proceed reasonably well for
on palladium complexes of 1,4-bis(diphenylphosphino)butane
bromides and iodides; however, aryl chlorides are much less
(dppb) or bis(diphenylphosphino)ferrocene (dppf) which repre-
active, probably because of the severe conditions needed for the
sent highly efficient catalysts for the methoxycarbonylation of
oxidative addition of the C–Cl bond to a zerovalent palladium
activated aryl chlorides9,10 and heteroaryl chlorides.10 Further
species. There is particular interest in replacing aryl bromides
work by Beller and co-workers showed that non-activated and
and iodides by the chloride analogues due to their low cost,
deactivated aryl chlorides can be methoxycarbonylated as well
easy preparation, high stability and wide availability. However,
as activated aryl chlorides by employing ferrocenyldiphosphines
the lower activity exhibited by aryl chlorides than by other
as the ligand.11 This catalytic system was improved using
aryl halides, means that efficient catalysts are required to
NaOAc as a base and bis(dicyclohexylphosphino)ferrocene as
activate them. Carbonylation of aryl chlorides requires harsher
the ligand,11 again showing the advantages that can be obtained
conditions than other C–C coupling reactions because of the
presence of a good p-acceptor ligand like CO, which not only
reduces the tendency towards oxidative addition at the metal
centre but may also promote the formation of palladium clusters.
by using highly electron donating bidentate phosphines with
large bite angles.
We have been interested in the use of bis(di-tert-
butylphosphinomethyl)benzene (BDTBPMB, Scheme 1)) pal-
ladium complexes in catalysis. They show very high activity for
the methoxycarbonylation of ethene12 via a hydride mechanism13
and we have recently reported its use for the highly regioselective
methoxycarbonylation of long chain alkenes14 and of vinyl
acetate under very mild conditions.15 Since BDTBPMB is
another example of a strongly electron donating ligand with
a wide bite angle and the potential to become unidentate, we
have studied its use in the carbonylation of aryl chlorides. We
now report the results of these studies.
Scheme 1 Methoxycarbonylation of an aryl choride catalysed by a
Pd/BDTBPMB complex.
[(chloroarene)Cr(CO)3] complexes can be carbonylated using
Experimental
palladium catalysts under mild conditions.2 Milstein and co-
workers reported the carbonylation of aryl chlorides using Pd
complexes of 1,3-bis(di-isopropylphosphino)propane (dippp).3
dippp proved to be the only ligand capable of catalysing the
reaction and provided the only Pd0 complexes which could
undergo rapid oxidative addition of aryl chlorides to give cis-
chloro aryl complexes.4,5 The high electron donating power and
the flexibility of the ligand backbone, which allows it to become
unidentate are both important in allowing oxidative addition to
occur.4,5 The introduction of basic ligands also overcomes the
problem of palladium agglomeration and palladium clustering.
The most widely used catalytic system for this purpose is based
on PCy3 modified palladium complexes.6 The use of K2CO3 and
Palladium chloride (Lancaster), BDTBPMB (Lucite) and
potassium tert-butoxide (Lancaster) were kept under an ar-
gon atmosphere in the glovebox. Methanol was dried by
distillation and degassed. 4-Chloromethylbenzoate (Aldrich),
4-chlorobenzonitrile (Aldrich), 4-chloroacetophenone (Lan-
caster), 4-nitrobenzene (Aldrich), KOH (Aldrich) and NEt3
(Avocado) were used as received.
All the experiments were carried out using standard Schlenk
line and catheter tubing techniques in dried and degassed
glassware and Hastelloy autoclaves.
All catalytic solutions were prepared as follows: PdCl2 (32 mg,
0.1 mmol), BDTBPMB (100 mg, 0.5 mmol) and KOBut (800 mg,
1 8 2 6
D a l t o n T r a n s . , 2 0 0 5 , 1 8 2 6 – 1 8 3 0
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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8 mmol) were weighed into a Schlenk tube in the glovebox.
Methanol, 1-propanol or trifluorethanol (10 cm3) was added
with a syringe. The solution was warmed to dissolve all the
solids. When the base used was KOH (8 mmol) or NEt3 (8 mmol)
they were added in place of KOBut. The substrate (8 mmol)
was added once all the solids were dissolved. The solution was
transferred to a dry and degassed Hastelloy autoclave via a
canula. It was pressurised with 20 bar of CO and heated to
100 ◦C for 48 h, unless otherwise stated. The resulting solutions,
which were black, indicating catalyst instability, were analysed
by GC and GCMS.
Gas chromatographic analyses were carried out on a Hewlett-
Packard 5890 series gas chromatograph equipped with a flame
ionisation detector for quantitative analyses and a Hewlett-
Packard 5890 series mass selective detector for qualitative analy-
ses. A Supelco MDN-35 35% phenyl/ 65% methyl-polysiloxane
column, 30 m long and 250 lm diameter◦was employed. The
flow rate of helium was 2.0 cm3 min−1 at 50 C for 4 min then the
temperature was raised at 20 ◦C min−1 to 130 ◦C, held for 2 min,
then raised at 20 ◦C min−1 to 220 ◦C and held for 23.5 min.
Scheme
2
Methoxycarbonylation of 4-chloronitrobenzene and
4-chlorocyanobenzene.
The reduction of the nitro group resulted in the formation
of 4-chloroaniline, which underwent carbonylation to afford
anilides and carbamates. Formation of carbamates from ni-
troaromatic compounds is well known,16–20 however, to the
best of our knowledge, this synthesis of anilides has not
been previously reported. Nucleophilic aromatic substitution
of the chlorine by the methoxy group occurred very effec-
tively. Surprisingly, the expected 4-nitromethylbenzoate was not
formed. 4-Cyanomethylbenzoate was produced in 30.8% yield
from 4-chlorocyanobenzene but competitive hydrolysis of the
cyano group also occurred and further esterification led to 4-
chloromethylbenzoate, which underwent methoxycarbonylation
and nucleophilic aromatic substitution. The major product was
obtained from displacement of the cyanide by methoxide.
Based on these results, we next examined the methoxycar-
bonylation of 4-chloroacetophenone to yield 4-acetylmethyl-
benzoate, which has a weaker electron withdrawing group in the
position para to the chlorine. Table 1 lists the products observed
and their yields. In the first attempts (Runs 1–3, Table 1) the
reaction was carried out under identical conditions to those
described for 4-chloromethylbenzoate, using KOBut as the base.
Surprisingly, a mixture of seven products (one, Mr = 200, could
not be identified) was obtained. The most abundant were 4-
chloromethylbenzoate, 4-methoxy acetophenone and dimethyl
terephthalate; however, the expected methoxycarbonylation
product, 4-acetylmethylbenzoate was not obtained. Dimethyl
terephthalate was not obtained at the lower temperature of
Results and discussion
The methoxycarbonylation of the activated chloroaromatic,
4-chloromethylbenzoate using PdCl2 and BDTBPMB in the
presence of a strong base like KOBut (required to neutralise
the HCl which may be produced in the reaction) was carried
out in batch reactors at 100 ◦C under 20 bar of CO for 24 h.
Under these conditions methoxycarbonylation to afford the
expected dimethyl terephthalate (16.4%) occurred. However,
there were two other major products: 4-methoxymethylbenzoate
(22.6%) and methylbenzoate (24.2%). The formation of 4-
methoxymethylbenzoate suggests that nucleophilic aromatic
substitution, which requires harsh conditions, can occur. In
order to confirm the influence of the base, the reaction was
performed in the presence of NEt3. The use of NEt3 instead of
KOBut led to an increase in the catalyst efficiency. Methoxy-
carbonylation was significantly more selective and the side-
reactions which occurred in the previous case were surpressed,
since only the desired product was produced in 35% yield after
18 hours. Slight reaction (4%) was observed in the absence of
base.
We then investigated the methoxycarbonylation of 4-
chloronitrobenzene and 4-chlorocyanobenzene, which also have
a strong electron withdrawing group in the position para to the
chlorine. Scheme 2 shows the products and the yields obtained.
◦
80 C, although 4-chloromethylbenzoate was formed in 27.7%
yield (Run 4, Table 1). This suggests that 4-chloroacetophenone
Table 1 Methoxycarbonylation of 4-chloro-acetophenone catalysed by palladium-BDTBPMB
Amount (%)
Run
Conditions
?
1
2
3
4
5
6
7
8
9
t/h
24
48
75.5
36.6
17
57.5
58
0.9
87
0
2.1
1.1
8.8
0
0
60
0
14.5
11
2.2
6.8
0
7.8
21.1
18.7
27.7
0
0
0
0
0
22.5
0
0.8
1.1
2.9
0
0
2.1
0
17.3
0
3.2
3.3
8.8
20.9
27.5
8.7
0
4
0
3.5
16.6
18.4
0
38.3
29
11
0
1.5
1.1
3.2
6.1
3.7
4
2
0
10.5
0
0
1.5
3.6
0
0
0
0
0
0
0
56
48a
Base
NEt3
KOH
None
Yesb,c
Yesd
Noe
Nob
Catalyst
19.4
0
78.5
55.6
1.5
0
0
0
10
11
16.5f
88
0
a T
=
80 ◦C. PCO
=
0
bar, KOBut present. c 1-Hydroxyethylbenzene (33.4%), phenylpropanone and phenylbutanone (8.5%),2,4-
b
dimethoxyacetophenone (6.9%). d PCO = 0 bar, KOBut absent. e PCO = 20 bar, KOBut present. f 4-Methoxyacetophenone and 4-methoxymethylbenzoate
could not be separated.
D a l t o n T r a n s . , 2 0 0 5 , 1 8 2 6 – 1 8 3 0
1 8 2 7

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Table 2 Alkoxycarbonylation of 4-chloroacetophenone with primary alcohols (ROH) using KOBut as the base
Amount (%)
Alcohol
MeOH
n-PrOH
CF3CH2OH
48.5
20
0
3.5
0
0
1.2
0
0
12.8
20
4
1.5
1.8
0
13.8
1.1
0
10.8
19
6
0
38
90
is first converted into 4-chloromethylbenzoate and that this
undergoes carbonylation to afford dimethyl terephthalate. It
is also noteworthy that nucleophilic aromatic substitution of
chloride by the methoxide and the conversion of the methyl
ketone into a methyl ester are fast compared with the remaining
transformations occurring during the reaction.
The choice of the base is crucial since its role is not only to
neutralise the HCl formed in situ but also to produce the alkoxide
anion which attacks the palladium acyl complex. Once again,
in the presence of NEt3 (Run 5 Table 1), dimethyl terephthalate
was produced selectively and in higher yield than observed when
starting from 4-chloromethylbenzoate. If an inorganic base like
KOH was used (Run 6, Table 1), acetophenone was obtained in
60% yield, whereas dimethyl terephthalate was produced in 29%
yield, suggesting that 4-chloromethylbenzoate is formed and
consumed rapidly and that dehalogenation becomes competitive
with the formation of the esters.
To examine the generality of the alkoxycarbonylation of 4-
chloroacetophenone and especially to attempt to reduce the
formation of side products, the reaction was carried out using
primary alcohols (2,2,2-trifluoroethanol (TFE) and n-propanol)
other than methanol (see Table 2). Alkoxycarbonylation of
4-chloroacetophenone was successfully initiated in both of
these alcohols. The use of the less nucleophilic, TFE, was
especially successful because it hindered the formation of 4-
chloroalkylbenzoate, bis-trifluorethyl terephthalate and prod-
ucts arising from non-carbonylative ring dechlorination, but
promoted the methoxycarbonylation, so that 90% selectiv-
ity to the desired alkoxycarbonylation product was observed
when KOBut was the base and 97% alkoxycarbonylation (3%
bis-trifluoroethyl terephthalate) when triethylamine was the
base (see Table 3). In methanol, direct carbonylation of 4-
chloroacetophenone does not occur, whereas in either n-PrOH
or TFE carbonylation is clearly favoured relative to nucle-
ophilic aromatic substitution, presumably because the anions
are less nucleophilic (TFE) or bulkier (TFE and propanol) (see
Scheme 3).
Scheme 3 Nucleophilic aromatic substitution vs. carbonylation.
TFE also promoted the carbonylation of the unactivated aryl
halide, chlorobenzene, although the conversion to trifluorethyl
benzoate was only 20% after 24 h (100% selectivity, Table 3).
Mechanistic studies
In order to gain an understanding on how 4-chloromethyl-
benzoate and dimethyl terephthalate are produced during the
attempted methoxycarbonylation of 4-chloroacetophenone, we
investigated the influence of the CO, base and catalyst (see
Table 1). In the absence of CO (Run 8 Table 1) no starting
material was recovered at the end of the reaction. However,
a major product (17.3%) was 4-chloro-1-hydroxyethylbenzene
formed via transfer hydrogenation. Nucleophilic aromatic sub-
stitution of chlorine by the methoxy group also occurred.
Neither 4-chloromethylbenzoate nor dimethyl terephthalate
were produced under these conditions. The absence of esters
suggests that CO is involved in the replacement of the ketone by
an ester function. It is interesting that even in the absence of both
CO and base (Run 9, Table 1) small amounts of acetophenone
were formed, but neither nucleophilic aromatic substitution nor
carbonylation reactions occurred.
In TFE (NEt3 as the base), the alkoxycarbonylation of
4-chloromethylbenzoate occurred with 97% conversion (93%
selectivity to bis-trifluorethyl terephthalate), the only other
product being 4-chloro(trifluorethyl)benzoate (7%, Table 3),
which arises from transesterification of the starting ester.
Knowing that CO is required for the formation of the
two esters, we investigated which processes are catalysed by
palladium-BDTBPMB complexes and which are uncatalysed
organic reactions, by performing the methoxycarbonylation
with and without the palladium/BDTBPMB.
In the absence of KOtBu, CO and the Pd catalyst, i.e.,
heating 4-chloroacetophenone in MeOH at 100 ◦C, no reaction
occurred. In contrast, nucleophilic aromatic substitution turned
out to be very effective when only KOBut was present, affording
4-methoxyacetophenone in 88% yield (Run 11, Table 1). We
found that in the presence of KOtBu and CO but with no catalyst
present (Run 10, Table 1), 4-chloromethylbenzoate (22.5%) was
produced, proving that only base and CO are involved in
the transformation of the ketone into an ester. However, no
dimethyl terephthalate was detected, confirming that dimethyl
terephthalate is not produced from direct methoxycarbonylation
of 4-chloroacetophenone but arises from tandem ester formation
followed by methoxycarbonylation of 4-chloromethylbenzoate
and that palladium-BDTBPMB complexes are required in
the last step. The origin of the various products from the
Table 3 Alkoxycarbonylation of 4-ClC6H4Rꢀ in TFEa
Amount (%)
Rꢀ
Base
COMe
COMe
CO2Me
H
KOBut
NEt3
NEt3
NEt3
0
0
3
4
0
7
—
6
3
90
—
90
97
0
80
20
a Reaction time 24 h.
1 8 2 8
D a l t o n T r a n s . , 2 0 0 5 , 1 8 2 6 – 1 8 3 0

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methoxycarbonylation of 4-chloroacetophenone in the presence
of KOBut is shown in Scheme 4, together with the agents required
for their synthesis.
We have carried out labelling studies to try to understand
the mechanism of this unusual transformation, for which both
CO and base, but not catalyst, are required (see above). Con-
firmation that the methoxy group in 4-chloromethylbenzoate
is provided by the methanol and not by the methyl group
in 4-chloroacetophenone comes from reactions carried out
in CD3OH. Analysis of the products obtained revealed the
formation of d3-4-chloromethylbenzoate, d3-methylbenzoate, d3-
4-methoxyacetophenone, d6-4-methoxymethylbenzoate and d6-
dimethyl terephtalate. No H/D exchange takes place into the
ring, since unreacted 4-chloroacetophenone is recovered with
no deuterium incorporation.
Reactions performed with 13CO in MeOH and n-PrOH
confirm that only one of the two acetyl groups of either dimethyl
terephthalate or dipropyl 1,4-benzenedioate contains 13C, i.e.
is formed by palladium catalysed alkoxycarbonylation. This
shows that the carbonyl group in the ester derived from the
ketone is that which was originally in the ketone. This is further
confirmed by the lack of 13C in 4-chloromethylbenzoate. Careful
GC analysis of all the products obtained, both in the liquid and
gas phases shows that 13C only appears in the terephthalates and
in methyl or propyl formate. The formates are also formed in
the absence of substrate, so they are probably not relevant to
the transformation of the ketone into an ester. We have not been
able to identify the fate of the methyl group which “leaves” the
ketone.
Evidence concerning the origin of 4-methoxyacetophenone
and 4-methoxymethylbenzoate is provided by reactions car-
ried out in CD3OD (see Table 4). The unreacted 4-
chloroacetophenone contains 1–6 D atoms (predominantly 2–
5). Up to 3 D atoms can be explained by keto–enol tautomerism
and exchange of the enol H with D from the solvent. The other
two D atoms must reside on the ring. A possible mechanism for
their incorporation involving activation of an ortho C–H bond
of an O bound substrate molecule is shown in Scheme 5. The
small amount of incorporation of a 6th D atom suggets that
more general H/D exchange into the ring is also possible. The
predominance of d4-acetophenone suggests that it contains 3
D atoms on the methyl group (keto–enol exchange with the
Scheme 4 Products from the methoxycarbonylation of 4-chloroace-
tophenone and their origins. (i) KOtBu, MeOH; (ii) CO, KOtBu, MeOH;
(iii) Pd/BDTBPMB, CO, KOtBu, MeOH.
The formation of 4-chloromethylbenzoate from 4-
chloroacetophenone is unexpected since it appears to involve the
nucleophilic displacement of methyl (a very poor leaving group)
by methoxide. Zhang and Vozzolo21 have recently reported a
similar kind of C–C bond cleavage to produce aryl carboxylic
acid esters by treatment of aryl methyl ketones with dimethyl
formamide–dimethyl acetal (DMF·DMA) in methanol. The
mechanism proposed for the formation of methyl esters in this
manner involves initial attack of DMF·DMA so is clearly not
applicable in our system. A previous report by Zabjek and
Petric22 showed that, after treating aryl methyl ketones with
an excess of KOH in DMF at 65–68 ◦C, the carboxylic acid
was produced in more than 80% yield. No mechanism was
proposed.
Table 4 Deuterium labelling with CD3OD
Deuterium content(%)
Substrate
d0
0
d1
d2
d3
d4
d5
d6
d7
d8
0
3.1
20.1
47
12.5
15.3
2
0
0
0
0
0
7.5
0
31.8
0
55.5
4.4
5.2
28
0
0
0
0
60.4
7.2
0
0
3.4
62.3
10.1
20.9
3.3
0
0
0
0
0
0
0
0
16
0
69
0
11
4
4
0
0
0
86.7
9.3
0
0
0
0
0
0.7
84.1
13.8
1.4
D a l t o n T r a n s . , 2 0 0 5 , 1 8 2 6 – 1 8 3 0
1 8 2 9

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that in 4-chloroacetophenone, selective carbonylation can only
be achieved by using alcohols of low nucleophilicity, such as
2,2,2-trifluoroethanol. More nucleophilic alcohols give many
side products arising from nucleophilic aromatic substitution,
reductive dehalogenation and an unusual transformation of the
methylketone into a methyl ester. Labelling studies show that this
reaction formally occurs by displacement of the methyl group
by methoxide. Nitro and cyano groups on the ring also undergo
severe side reactions.
Scheme 5 Proposed mechanism for ring deuteriation by CD3OD in
4-chloroacetophenone. A second D can be incorporated into the 5
position by the same mechamism.
solvent) and a D atom in the 4 position. This must come
from the OD group of the solvent since it is H when CD3OH
is used. The comparitive lack of acetophenone containing >
4 D atoms suggests that it forms early in the reaction and
does not itself undergo ortho C–H/D exchange. Reductive
dechlorination of chloroaromatics, the reaction which produces
acetophenone from 4-chloroacetophenone, has been reported to
be catalysed by Pd0/dippp complexes using KOH/methanol or
sodium formate in alcohols or DMF as the reducing agent.23
In our system, it appears to require base (Run 11, Table 1) or
catalyst (Run 9, Table 1) but not CO.
4-Methoxyacetophenone contains 4–7 D atoms (predomi-
nantly 6) showing that little ring deuteriation has occurred and
especially that it is formed by nucleophilic substitution of the
chloride ((i) in Scheme 6) rather than by a benzyne mechanism
((ii) in Scheme 6) which would be expected to give much more d7
product. Again, ortho C–H/D exchange does not seem to occur
readily in this case and is not very prevalent in the compounds
derived from 4-chloromethylbenzoate.
Acknowledgements
We thank Lucite International for a studentship, P. Pogorzelec
for assistance with the product analysis and Drs N. P. Botting
and R. A. Aitken for helpful discussions.
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1992, 73, 173.
Selective carbonylation of aromatic chlorides to carboxylic acid
esters is catalysed by Pd/BDTBPMB complexes in alcohols
in the presence of base, provided that the aromatic ring is
very electron poor. For less activated aromatic rings such as
1 8 3 0
D a l t o n T r a n s . , 2 0 0 5 , 1 8 2 6 – 1 8 3 0