Pd-benzothiazole carbene catalysed carbonylation of aryl halides in ionic liquids

Article abstract of DOI:10.1016/S0022-328X(01)01401-2

The carbonylations of aryl halides with the Pd-carbene catalyst 1 were studied both in molecular solvents and in ionic liquids (ILs). Among the ILs, tetrabutylammonium bromide (TBAB) was found to give better results. Under these conditions the catalyst, when recovered and reused, did not show a significant decrease of activity. The role of TBAB on the catalyst stability is discussed.

Full text of DOI:10.1016/S0022-328X(01)01401-2

Journal of Organometallic Chemistry 645 (2002) 152–157
www.elsevier.com/locate/jorganchem
Pd–benzothiazole carbene catalysed carbonylation of aryl halides
in ionic liquids
Vincenzo Cal o` , Potenzo Giannoccaro, Angelo Nacci *, Antonio Monopoli
Centro CNR ‘‘MISO’’, Dipartimento di Chimica, Uni6ersit a` di Bari, Via Amendola 173, 70126 Bari, Italy
Received 2 August 2001; accepted 22 October 2001
Abstract
The carbonylations of aryl halides with the Pd–carbene catalyst 1 were studied both in molecular solvents and in ionic liquids
(
ILs). Among the ILs, tetrabutylammonium bromide (TBAB) was found to give better results. Under these conditions the catalyst,
when recovered and reused, did not show a significant decrease of activity. The role of TBAB on the catalyst stability is discussed.
2002 Elsevier Science B.V. All rights reserved.
©
Keywords: Carbonylation; Aryl halides; Ionic liquids; Palladium–carbene complexes
1
. Introduction
both in conventional solvents as dimethylformamide
DMF) or dimethylacetamide (DMA) and in ionic liq-
uids (IL) as tetrabutylammonium bromide (TBAB) [8–
0].
(
The transition-metal catalysed carbonylation of or-
ganic halides, in the presence of a nucleophile, is the
most common method for the synthesis of aromatic
acids and derivatives (Scheme 1) [1]. Among various
catalysts, the Pd–phosphane complexes have been
widely employed mostly under homogeneous conditions
1
[2,3].
However, as a result of the notorious phosphane
In the latter case, besides very high conversion rates,
degradation by PꢀC bond cleavage [4,5], an excess of
phosphane is often required in order to avoid the
catalyst deactivation. Furthermore, the carbonylation of
less reactive aryl halides often requires high reaction
temperatures, which decompose the catalyst and impede
further applications. Even more robust catalysts such as
Hermann’s palladacycle [6] proved to be inactive.
In the context of our effort to develop efficient and
stable catalysts for the Heck reaction [7], we synthesised
the Pd–catalyst 1 with benzothiazole carbenes as lig-
ands; they are stable to high temperatures, oxygen and
moisture, which catalyses the CꢀC coupling reactions
the catalyst was efficiently recycled several times without
metallic palladium deposition. Pd–phosphane catalysts
have also been employed for carbonylation processes in
IL [11,12], but no example has been reported on the use
of Pd–carbene complexes.
This paper reports some results of our studies on the
carbonylation of aryl halides catalysed by 1 and the role
exerted by IL on the catalyst activity and stability.
2
. Results and discussion
Initially, we tested the efficacy of 1 as a catalyst in
DMA (Scheme 2). Iodobenzene and activated aryl bro-
Scheme 1. X=Cl, Br, I; Nu=OH, OR, NR2.
*
Corresponding author. Fax: +39-080-544-2924.
E-mail address: nacci@chimica.uniba.it (A. Nacci).
Scheme 2.
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01401-2

V. Cal o` et al. / Journal of Organometallic Chemistry 645 (2002) 152–157
153
Table 1
Carbonylation of aryl halides in DMA catalysed by complex 1
a
Run
Substrate
Base
Cat (%)
Nucleophile
T (°C)
Time (h)
Conv. (%)
Product (yield%) b
1
2
3
4
5
6
7
Iodobenzene
Iodobenzene
Iodobenzene
p-Bromoacetophenone
p-Bromoacetophenone
p-Bromoacetophenone
Bromobenzene
Bromobenzene
Bromobenzene
Et N
AcONa
NaOH
AcONa
1
1
1
2
2
2
2
2
2
MeOH
80
90
2
7
5
10
5
6
14
14
14
100
100
95
100
95
95
\1
40
C H CO Me (\99)
3
6
5
2
c
c
H O
C H CO H (\99)
2
6 5 2
d
−
OH
100
130
130
130
150
130
130
C H CO H (90)
6 5 2
H O
4-CH COC H CO H (95)
2
3 6 4 2
Et N
MeOH
4-CH COC H CO Me (91)
3 6 4 2
3
Et N
Et NH
4-CH COC H CONEt (95)
3
2
3 6 4 2
Et N
MeOH
MeOH
MeOH
C H CO Me (traces)
6 5 2
3
e
8
Et N
C H CO Me (35)
3
6 5 2
f
9
Et N
89
C H CO Me (85)
6 5 2
3
a
b
c
d
e
f
In all experiments, DMA (6 ml), haloarene (4 mmol), MeOH (2 ml), Et N (2 ml) and catalyst 1 (1–2 mol%).
GLC yields evaluated by using diethylene glycol di n-butyl ether as internal standard.
3
AcONa·3H O (12 mmol) was used.
2
NaOH (12 mmol) was used.
PPh3 (4 mol%) was added.
PPh3 (4 mol%) and TBAB (1 g) were added.
Table 2
Carbonylation of less reactive aryl halides in DMA catalysed by 1 in the presence of TBAB/PPh3 at atmospheric pressure of CO a
Run
X
R
NuH
T (°C)
Time (h)
Products (yield) b
C H CO H (95)
1 c
2
Br
Br
Br
Br
Br
Br
Br
Br
H
H
H
CH3
CH3
CH3
OCH3
OCH3
OCH3
H O
130
130
130
130
130
130
130
130
130
130
130
130
140
140
140
140
140
140
7
4
5
12
5
7
7
7
7
7
2
6
5
2
MeOH
C H CO Me (95)
6 5 2
d
3
4
Et NH
C H CONEt (85)
6 5 2
2
c
H O
p-CH C H CO H (87)
2
3 6 4 2
5
6
7
MeOH
p-CH C H CO Me (\99)
3 6 4 2
Et NH
p-CH C H CONEt (91)
2
3 6 4 2
c
H O
p-CH OC H CO H (85)
3 6 4 2
2
8
9
0
MeOH
p-CH OC H CO Me (95)
3 6 4 2
Br
Et NH
p-CH OC H CONEt (93)
3 6 4 2
2
c
1
1-Br-naphthalene
1-Br-naphthalene
1-Br-naphthalene
Cl
Cl
Cl
Cl
Cl
Cl
H O
1-NaphthylCO H (55)
2
2
1
1
MeOH
7
7
1-NaphthylCO Me (66)
2
1
3
2
Et NH
1-NaphthylCO NEt (90) d
2
2
2
c
1
CH CO
CH CO
CH CO
NO2
NO2
NO2
H O
10
10
10
9
10
10
p-CH COC H CO H (45)
3
2
3 6 4 2
1
1
4
5
MeOH
p-CH COC H CO Me (53)
3 6 4 2
3
Et NH
p-CH COC H CONEt (17)
3
2
3 6 4 2
c
1
6
H O
p-CH COC H CO H (30)
3 6 4 2
2
1
1
7
8
MeOH
p-CH COC H CO Me (15)
3 6 4 2
Et NH
p-CH COC H CONEt (39)
3 6 4 2
2
a
In all experiments, DMA (6 ml), haloarene (4 mmol), MeOH or Et NH (2 ml), Et N (2 ml), TBAB (1g), PPh (8 mol%) and catalyst 1 (4
2
3
3
mol%) were used.
b
GLC yields.
c
AcONa·3H O (12 mmol) was used.
2
d
Isolated product.
mides were carbonylated with good yields at atmo-
spheric pressure of CO and under relatively mild tem-
perature conditions (Table 1, runs 1–6), but less
reactive aryl halides such as bromobenzene, required
the addition of triphenylphosphane and TBAB to give
in good yields the methoxycarbonylation (runs 7–9).
These reaction conditions were also extended to the
carbonylation of other less reactive substrates in the
presence of various nucleophiles. In all cases, to obtain
high conversions, it was necessary to add both phos-
phane and TBAB (Table 2).
The beneficial effect exerted by TBAB prompted us
to test the catalytic activity of 1 towards the butoxycar-
bonylation directly in TBAB melt and other ILs. This is
important since most of the traditional solvents used in
organic synthesis have been found ‘‘damaging to the
environment’’, so the utilisation of ILs can contribute
to decrease the environmental contamination by toxic
and volatile solvents.
Thus, iodobenzene or 4-bromoacetophenone were al-
lowed to react with a carbon monoxide pressure
(PCO=1–8 atm) into TBAB melt, in the presence of

154
V. Cal o` et al. / Journal of Organometallic Chemistry 645 (2002) 152–157
Table 3
Butoxycarbonylation of aryl halides in TBAB melt
a
Run
Aryl halide
Catalyst
T (°C)
PCO (atm)
Conv. (%)
Butyl ester yield (%) b
1
2
3
4
5
6
7
Iodobenzene
1
1
1
1
100
130
130
130
130
130
130
1
1
4
8
8
8
8
89
10
25
78
95
30
96
85
3
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
Bromobenzene
22
76
78
27
85
c
Pd(OAc) /PPh
1
1/PPh3
2
3
c
Bromobenzene
a
In all experiments a 5:1 ca. molar ratio TBAB–aryl halide has been used, which produces, at reaction temperature, a quite fluid melt to be
easily stirred: TBAB (6 g, 18.6 mmol), ArX (4 mmol), BuOH (4.8 mmol), Et N (5.2 mmol), catalyst (1 mol% of Pd); reaction time 14 h.
3
b
GLC yields evaluated by using diethylene glycol di n-butyl ether as internal standard.
c
PPh3 (3 mol%) was used.
catalytic amount of 1, small stoichiometric excess of
14-electron complex, which, by reaction with TBAB,
affords a stable and catalytically active 16-electron
BuOH, as nucleophile, and NEt as base.
3
(
−) (+)
The results in Table 3 show that only iodobenzene
was carbonylated at atmospheric pressure, while for
other substrates higher pressures are needed (Table 3,
runs 1–4) and the butoxyester yields rise when the CO
pressure increases. This behaviour could be accounted
for by an increasing of CO solubility into the IL as well
as for a faster insertion of CO into the palladium–car-
bon bond of the generally accepted aryl intermediate
complex. It is well known [2b] that aryl halides car-
bonylation is initiated by a Pd(0) species which forms
an arylꢀPdꢀX intermediate followed by coordination
and CO insertion into the Pdꢀaryl bond. The nucle-
ophilic attack on the resulting Pdꢀacyl complex fur-
nishes the carboxylic derivative and regenerates the
initial active species (Fig. 1). Also under these reaction
conditions, the less reactive aryl halides, such as bro-
complex [L PdBr]
NR . This would not be sur-
2
4
prising since in TBAB the bromide ion, being poorly
solvated, would be a good nucleophile for palladium.
In this view, ILs having poorly coordinating anions as
tosylate or fluoroborate should not stabilise the 14-elec-
tron complex L Pd(0). Consistent with these observa-
2
tions are our data showing the inefficacy of 1 to
catalyse the carbonylation in butylpyridinium tosylate
or imidazolium salts (Table 4, entries 1 and 8). This
anion effect was also observed [16] in imidazolium
tetrafluoroborate. Concerning the activating effect of
halides, our data show the following efficacy order:
−
−
−
Cl :I BBr (Table 4, entries 4–6). These results
could be accounted with a different coordinating power
−
of the anions. Indeed, while the I ion is less coordinat-
−
−
ing than the Br , Cl is too coordinating towards
palladium, thus it would make slower the successive CO
insertion process.
mobenzene, required addition of PPh to be carbony-
3
lated (Table 3, runs 6–7).
As far as the influence of the cation is concerned, the
2.1. Catalyst acti6ity and role of TBAB and phosphane
(
−) (+)
formation of a large [L PdBr]
NR complex, by
2
4
In the literature some scattered data on the influence
imposing a Coulombic barrier for collision, should
impede the formation of palladium clusters. Further-
more, the ammonium cation could electrostatically as-
sist the polarisation or deligation of the bromide ion
of ILs on the catalysts performance are reported but no
explanation was given. It appears that both the cation
and anions of IL play an important role on the catalyst
activity and stability. Reetz et al. [13] found that reduc-
tion of a Pd salt in THF, in the presence of tetrabuty-
lammonium acetate or formate, gave Pd-nanoparticles
stabilised by the large ammonium cation. Furthermore,
Neghishi et al. [14] and Amatore and Jutand [15],
demonstrated that (PPh ) Pd(0), the proposed catalyst
3
2
in the Heck reaction, is unstable in the absence of
halide or acetate ions which transform this complex in
a more stable and catalytically active 16-electron an-
(
−)
ionic complex as [Pd(PPh ) X] . The stabilisation of
3
2
catalytic systems by halide salts was also demonstrated
by extending the lifetime of the Herrmann palladacycle
[
6]. We believe that in our system the reduction of 1
Fig. 1. Generally accepted catalytic cycle for the carbonylation of aryl
halides.
leads to an underligated L Pd(0), a rather unstable
2

V. Cal o` et al. / Journal of Organometallic Chemistry 645 (2002) 152–157
155
Table 4
Another intriguing point is the role of PPh on the
3
a
IL effect on butoxycarbonylation of 4-bromoacetophenone
carbonylation of the less reactive halides. A question is
whether in these cases the benzothiazole ligand is still
bonded to palladium. In order to get some insight we
carried out the carbonylation of 4-bromoacetophenone
Yield b
Run
IL
1
2
3
4
5
6
7
8
[bmim+] [BF −4 ] c
[bmim ] [Cl
[bmim ] [Br
Bu NCl
Bu NBr
Bu NI
Aliquat
N-Butylpyridinium tosylate
3
B1
16
30
76
32
40
B5
+
−
]
]
in TBAB using Pd(II)–acetate/PPh as catalyst (Table
+
−
3
3
, run 5). In this case we observed the formation of
4
small amounts of metallic palladium, which caused the
deactivation of catalyst when it was reused (Fig. 2). On
the contrary, only a slight deactivation was observed in
the same reaction catalysed by 1 (see below).
4
4
d
a
In all experiments, 4-bromoacetophenone (4 mmol), catalyst 1 (1
mol%), T=130 °C, reaction time 14 h, PCO=8 atm, IL/substrate
ratio 5:1.
This behaviour could be justified by assuming that
the addition of PPh does not cause the substitution of
3
all benzothiazole ligands and the formation of a
b
GLC yields.
c
‘‘(PPh ) Pd(0)’’ intermediate, as would be in the case of
1
-Butyl-3-methyl imidazolium tetrafluoroborate.
3 2
d
Tricaprylmethyl ammonium chloride.
the reaction catalysed by Pd–acetate. Thus, these re-
sults accounted for an active species containing both
the benzothiazolylidene and the phosphane ligands.
Recently, a number of two-coordinated palladium(0)
bis(carbene) [18] and mixed Pd(0) or Pd(II) carbene–
phosphane complexes [18–20], which efficiently catalyse
cross coupling reactions of the aryl halides, have been
isolated and characterised.
Finally, a last question concerns the recycling of the
catalyst. To check it, after reaction the butyl ester was
extracted with diethylether and the solid residue, con-
taining the catalyst and TBAB, was reused under the
same reaction conditions to carbonylate other substrate
(see Section 3 for details). This procedure was repeated
several times. After each reaction, which we consider as
a cycle, the organic products were analysed for the
yields. The trend of yields for the carbonylation of
iodobenzene and p-bromoacetophenone versus cycles
from the anionic Pd(II) pentacoordinated complex
(
−)
(+)
[
L PdArBr ] NR4 , deriving from the oxidative ad-
2 2
dition with aryl bromides, and this would render the
Pd(II)-complex more electrophilic for a fast CO inser-
tion. This is conceivable since it was calculated, for
analogous Pd-complexes with imidazolyl carbenes lig-
ands, that the removal of bromide from the
(
−)
[
L PdArBr ]
complex is a strongly endothermic pro-
2
2
cess [17].
Other evidence on the influence of the cation arises
from the comparison of the carbonylation efficiency in
+
−
NBu Br and [bmim ] [Br ] (Table 4, entries 3 and 5).
4
We believe that the better performance of TBAB as
solvent could be ascribed to the structural differences
between the [ NBu ] and [bmim ] cations, which
could influence the anion behaviour. Indeed, the bulki-
ness of the tetrahedral ammonium ion forces the bro-
mide ion away from the cation and this decreased
electrostatic interaction would render the bromide ion
more available for palladium complex stabilisation or,
in other words, more ‘‘nucleophilic’’. On the contrary,
+
+
4
(
Fig. 2), shows that the catalytic activity decreases very
+
the planar [bmim ] cation, by binding the anion
tightly, would render this latter less available for
palladium.
In an attempt to gain evidence on the formation of
II
the active (benzothiazolylidene) Pd(0) species, the Pd -
2
catalyst 1 was reduced in THF at 50 °C with a slight
excess of sodium formate. The resulting red–brown
solution was submitted to a stream of CO and its IR
spectrum was recorded. No absorption bands ascribed
to the stretching of CO were observed. Addition of
iodobenzene did not cause the appearance of a band
assignable to the CO of the expected acyl complex, but
when the reaction temperature was raised to 90 °C, the
IR spectrum displayed two weak bands centred at 1770
Fig. 2. Recycling of catalytic system TBAB/1. Reaction conditions:
−
1
and 1630 cm . These bands could be assigned, respec-
tively, to the wCO of Pd–COR group and to the wCO of
iodobenzoyl formed by reductive elimination.
halide (4 mmol), Et N (1.2 equivalents) BuOH (1.3 equivalents),
3
catalyst 1 (1 mol%), PCO 1–8 atm. T=100–130 °C (see Table 3).
Isolation of products by soxhlet extraction.

156
V. Cal o` et al. / Journal of Organometallic Chemistry 645 (2002) 152–157
slowly. In fact, after each cycle the yields are still very
close to the initial value and the observed slight de-
crease is most likely due to a loss of catalyst during
handling and transferring of materials, rather than to
its decomposition. Though the by-product triethylam-
monium halide did not influence the catalyst activity,
its increase after each cycle, by rendering more viscous
the reaction medium, prevented further reaction after
six times.
In conclusion, while some aspects of the catalytic
cycle involving Pd–carbene complexes in ILs are not
well understood, our results show that ILs cannot be
considered only as simple high polarity solvents, but
their efficacy is due to several factors which are actually
studied by us.
placed in a stainless steel autoclave (50 ml) and pres-
surised with CO (8 atm of CO). The autoclave was
heated to the desired temperature (130 °C) and allowed
to react with stirring for 14 h. At the end of the
reaction, the orange was cooled, the resultant solid was
powdered, placed in a Soxhlet apparatus and the or-
ganic products were extracted with Et O (70 ml). The
2
solid residue containing TBAB, catalyst and Et N·HBr
3
was used in another carbonylation process to test the
catalyst stability. The reactions with other substrates
were carried out according to the above procedure.
Reaction conditions are given in Table 3.
Acknowledgements
This work was in part financially supported by Min-
istero dell’Universit a` e della Ricerca Scientifica e Tec-
nologica, Rome, and the University of Bari (National
Project: ‘‘Stereoselezione in Sintesi Organica:
Metodologie ed Applicazioni’’).
3
. Experimental
All chemicals and solvents were purchased from
Fluka and used without further purification. The CO
gas (purity 99.9%) was purchased from Air Liquide.
The catalyst 1 [bis (3-methyl-2,3-dihydro-benzothiazole-
References
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by GLC on a HP 5890A gas-chromatograph by using a
capillary column (ZB-1, 30 m, 0.25 mm i.d.) and di-
ethylene glycol di-n-dibutyl ether as internal standard.
IR spectra were recorded on a Perkin–Elmer 883
spectrophotometer.
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[
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