WCl6/DMF as a new reagent system for the phosphine-free Pd(0)-catalyzed aminocarbonylation of aryl halides

Article abstract of DOI:10.1039/c4ra04673k

WCl₆ in dimethyl formamide (DMF) is introduced as a new reagent system for aminocarbonylation of aryl halides in the presence of PdCl₂ as pre-catalyst without any phosphorous ligand. Aryl iodides, bromides as well as chlorides were efficiently converted to their corresponding N,N-dimethyl amides in good to high yields. In this protocol, WCl₆/DMF is responsible for the generation of both Pd(0) catalyst as well as the formation of a Vilsmeier imminium type intermediate.

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WCl6/DMF as a new reagent system for the phosphine-free Pd(0)-
catalyzed aminocarbonylation of aryl halides
Nasser Iranpoor,*a Habib Firouzabadi,*a Zeinab Tavangar Rizi,a and Soodabeh Erfana
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
WCl6 in dimethyl formamide (DMF) is introduced as a new reagent system for aminocarbonylation of
aryl halides in the presence of PdCl2 as preꢀcatalyst without any phosphorous ligand. Aryl iodides,
bromides as well as chlorides were efficiently converted to their corresponding N,Nꢀdimethyl amides in
good to high yields. In this protocol, WCl6/DMF is responsible for the generation of both Pd(0) catalyst as
10 well as the formation of a Vilsmeyer imminium type intermediate.
POCl3 has been reported for preparation of different
formamides.12
50 Recently, we have reported the aminocarbonylation of aryl
halides using the in situ generated Mo(CO)4NBD13 and also the
use of POCl3/DMF in the presence of nanoꢀpalladium catalyst
Introduction
Carbonylation reaction is among the effective transformations for
synthesis of esters, amides, and heterocyclic compounds. Among
the carbonyl compounds, amides are one of the important
15 functional groups, due to their wide biological activities as
antibacterial and antifungal agents.1 Synthesis of amides from
aryl halides in the presence of transition metal catlysts is an
important reaction in organic synthesis. Regarding this synthesis,
Heck has reported the first preparation of aryl and alkenyl amides
20 from their corresponding halides in the presence of palladium
catalyst, CO gas, and primary amines.2 After this report, some
other metal catalyzed methods were developed for
aminocarbonylation of aryl halides in the presence of CO gas.3,4
In order to remove the handling problem of carbon monoxide, the
25 use of Mo(CO)6,5 Cr(CO)6,5 W(CO)6,5 carbamoylstannanes6 and
carbamoylsilanes,7 as sources for the in situ generation of carbon
monoxide was considered as a practical method. However, the
use of metal carbonyls, still have some handling problem due to
the generation of CO gas, and in the other hand
30 carbamoylstannane and carbamoylsilanes have thermal instability
and are not commercially available.
The use of DMF as source of carbonyl group in transition metal
catalyzed aminocarbonylation reaction of aryl halides with
amines can be considered as an alternative reagent system for
35 amide formation.8ꢀ10 This reaction in strongly basic condition has
been reported both under thermal8 and microwave irradiation.9
Hallberg et al. reported the Pdꢀcatalyzed aminocarbonylation
using DMF as source of CO and an amine.9 However, in this
reaction, imidazole was added as an additive and the reaction
40 temperature was very high (180ꢀ190 °C). The use of excess
POCl3 in DMF was also reported as an example of COꢀfree
aminocarbonylation of aryl and alkenyl halides.10 Later on, this
reagent system was used for aminocarbonylation reaction of aryl
halides in the presence of Pd/C catalyst.11 The main disadvantage
45 of the mentioned methods is their limited application to only aryl
iodides. Most recently, the use of Pd(OAc)2/xantphos as catalyst
for COꢀfree aminocarbonylation of aryl halides in the presence of
Pd(0)/SDPP (SDPP
= silicadiphenylphosphinite) for amide
formation.14 In continuation of our recent studies on the
55 amidation of aryl halides, herein, we introduce a new palladiumꢀ
catalyzed aminocarbonylation of aryl halides using WCl6/DMF as
a new combinatorial carbonylation reagent system.
Results and Discussion
In order to obtain approperiate conditions for the
60 aminocarbonylation of aryl halides using DMF as CO source, we
decided to use different metal halides such as WCl6, MoCl5,
ZrOCl2, ZrCl4, TiCl4, and FeCl3 in the presence of Pd(II) as preꢀ
catalyst.The effect of different factors on aminocarbonylation of
iodobenzene are demonstrated in Table 1. As shown in this table,
65 among the studied metal halides, WCl6 is the most efficient one
and gave quantitative conversion within 5h. The use of Pd(OAc)2
instead of PdCl2 as a preꢀcatalyst was also examined and the
obtained result showed that the reaction time is nearly the same
as using PdCl2 (Table 1, entry 4). Since, PdCl2 is cheaper than
70 Pd(OAc)2, it was selected as the Pd precursor. The influence of
parameters such as temperature and catalyst loading was also
examined on the model reaction.
Running the reaction with optimized 2.5 mol% of PdCl2 in the
presence of WCl6 as the reagent at 120 °C showed that the yield
75 of the desired product was too low after 18 h (45% GC
conversion) (Table 1, entry 10)
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Table 1. Study of Different Parameters on the Aminocarbonylation of
15 and biphenyl wao obtained as byꢀproduct in 30% yield (Table 1,
entry 2). From these experiments, it was concluded that the
presence of both Pd catalyst and WCl6 are essential to produce
the corresponding product. When the amounts of WCl6 was
reduced from two equivalents to one, the reaction time was
20 increased from 5 to 12 h (Table 1, entry 11). In order to see the
effect of phosporous ligand on the process of the reaction, the
model reaction was conducted in the presence of PPh3 (Table 1,
entry 12). In comparison with the ligandꢀfree reaction (Table 1,
entry 3), the presence of PPh3 not only didn’t improve the
25 progress of the reaction, but also increaed the reaction time. The
elongation of reaction time could be possibly due to the
complexation of PPh3 with WCl6.
Since it was the first time that WCl6/DMF is used for this
reaction, we decided to find out its scope and applicability for
30 aminocarbonylation of aryl halides. We therefore, studied the
possibility of performing this new aminocarbonylation reaction in
the presence of WCl6 as the most efficient metal halide and apply
this ligandꢀfree reaction to other aryl halides. Under our
optimized reaction conditions (0.5 mmol of aryl halide, 1.0 mmol
35 of WCl6 5.0 mL of DMF, 4.4 mg (2.5 mol%) of PdCl2 under
nitrogen at 140 °C), the desired products were obtained in
moderate to excellent yields for a wide array of aryl halides
(Scheme 1).
Iodobenzene with Metal Halides at 140 °C a
O
I
H
O
CH3
CH3
Pd(II)
CH3
Metal Halide
N
N
+
CH3
140 °C, under N2
1a
2a
Entry
Metal Halide
Pd(II)
(mol%)
Time (h)
Yield
(%)b
1
2
WCl6
None
WCl6
WCl6
MoCl5
ZrOCl2
ZrCl4
FeCl3
TiCl4
WCl6
WCl6
WCl6
none
2.5
2.5
2.5
2.5
3.5
5
24
24
5
o
30c
3
4
5
100
100d
100
100
70
None
None
45e
90f
100g
4.5
8.5
12
24
24
24
18
12
12
6
7
8
5
9
5
10
11
12
2.5
2.5
2.5
5
a Reaction conditions: iodobenzene (0.5 mmol), metal halide (1.0 mmol),
DMF (5.0 mL). b Conversion yield was based on GC analysis. c Biphenyl
was obtained as the product. d Pd(OAc)2 was used instead of PdCl2. e The
reaction was performed at 120 oC. f One equimolar of WCl6 was used. g
2.0 mg of PPh3 was used.
40
10 As a controlled reaction, we also checked the aminocarbonylation
reaction in the absence of Pd catalyst and once in the absence of
metal halide. In the absence of Pd catalyst, the reaction did not
proceed even after 24 h (Table 1, entry 1). In the absence of
WCl6, most of the starting material was remained intact after 24 h
45
O
X
R
PdCl2 (2.5 mol%)
R
N
R
R
N
R
O
+
R
WCl6 140 oC, N2
R = Me, Et
CH3
N
O
O
O
O
O
CH3
CH3
N
CH3
CH3
CH3
N
N
N
CH3
CH3
CH3
CH3
O2N
O
H3C
X = I
2a: 5h, 93%
X = I
2b: 8h, 90%
X = I
2d: 6h, 83%
X = I
2c: 10h, 92%
X = I
2e: 12h, 76%
O
N
O
O
N
O
O
N
CH3
CH3
CH3
N
CH3
CH3
CH3
N
CH3
CH3
CH3
CH3
O2N
NC
O2N
CH3
CH3
X = Br
2j: 48h, 53%
X = Br
X = I
2f: 12h, 78%
X = Br
2h: 48h, 81%
X = I
2g: 7h, 69%
2i: 48h, 90%
O
N
O
O
N
O
O
CH3
CH3
CH3
N
CH3
CH3
N
N
CH3
CH3
CH3
CH3
O2N
H3C
O
O2N
NO2
X = Cl
CH3
X = I
2j: 8h, 91%
X = I
2k: 20h, 85%
X = Cl
2i: 48h, 88%
X = Br
2k: 48h, 57%
2l: 4h, 88%
Scheme 1. Ligandꢀfree aminocarbonylation of aryl halides in the presence of WCl6. Reaction conditions: aryl halide (0.5 mmol), PdCl2 (4.4 mg, 2.5 mol
%), WCl6 (1.0 mmol), DMF (5.0 mL) at 140 °C under N2. All yields are isolated compounds.
2
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As shown in Scheme 1, different aryl halids can be converted to
their corresponding amid derivatives under our optimized
this reaction. Interestingly, the presence of PPh3 as ligand is
required for occurance of the reaction. Using DEF, iodobenzene
conditions in the presence of WCl6/DMF reagent system. Aryl 20 and 4ꢀmethoxy iodobenzene reacted efficently to give their
iodids reacted faster than bromides and chlorides conterparts. The
reaction conditions are more effective for aryl halides containing
electronꢀwithdrawing groups so that electronꢀpoor substrates
reacts faster than electronꢀrich ones.
corresponding N,Nꢀdiethylamides in high yields under optimized
conditions, but in the presence of PPh3 ligand (Scheme 1,
compounds 2j, k).
5
In order to study the pathway of Pd(II) reduction under
Steric hindrance in some substrates such as 1ꢀiodonaphthalene, 1ꢀ 25 phosphineꢀfree condition, we studied the following experiments.
iodoꢀ2ꢀmethylbenzene
and
1ꢀiodoꢀ2ꢀmethylꢀ4ꢀnitrobenzene
First, we added PdCl2 to the solution of WCl6 in DMF and heated
it at 140 °C for 1h or 80 C for 3h. Then we studied the UVꢀ
o
10 caused a decrease in the yield of the desired product (Scheme 1,
compounds 2e-g). This catalytic system was also efficient for the
electronꢀdeficient aryl bromides and aryl chlorides (Scheme 1,
compounds 2h-i).
In order to see if the reaction can be applied to other formamides,
15 we used N,Nꢀdiethylformamide (DEF) instead of DMF for
aminocarbonylation of some aryl iodides under optimized
conditions. Unfortunatly, we couldn’t obtain any product from
spectrum of both solutions. Disappearance of the band around
400 nm was indicative of the absence of Pd(II) under these
30 conditions. In the other experiment in order to see the effect of
DMF, a combination of PdCl2 and DMF was stirred at 140 °C for
3 h in the absence of WCl6. The UVꢀspectrum of this mixture
showed that no reduction of Pd(II) to Pd(0) has been occurred
(Fig. 1a).
35
Fig. 1 a) UVꢀSpectrum of PdCl2 in the presence of DMF and WCl6. b) UVꢀSpectrum of PdCl2 in the presence of DMF and MoCl5. c) UVꢀSpectrum of
PdCl2 in the presence of DMF and ZrOCl2
In order to have a comparison with other metal halides, we also
performed a similar experiment sing MoCl5 in DMF at 140 C.
decomposition product of DMF, the mixture of WCl6 in DMF
was also heated at 80 C in a sealed tube and its gc analysis was
o
o
40 The UV spectrum of the mixture showed that Pd(II) was 60 compared with a blank solution of DMF under similar condition.
completely converted to Pd(0) after 2 h which is slightly slower
than using WCl6 (Fig 1b). When we studied ZrOCl2 for the same
purpose, it was observed that after 2 h, Pd(II) was still present in
the media, which shows that the reduction of Pd(II) to Pd(0) is
45 much slower in the case of ZrOCl2 (Fig 1c). This study clearly
shows that the reduction of Pd(II) to Pd(0) occurs more
efficiently in WCl6/DMF.
The formation of dimethyl amine in the mixture of WCl6/DMF
and its absence in the blank solution confirms the proposed
pathway for the generation of dimethyl amine.
65
According to the reported results in the literature, DMF has been
shown to act as reducing agent for the conversion of Ag(I) to
50 Ag(0).15 On this basis, it can be suggested that DMF can act as a
reducing agent for the conversion of W(VI) to W(IV) as shown in
Fig 2. In the following, the produced W(IV) acts as a reducing
agent for the reduction of Pd(II) to Pd(0). Detection of dimethyl
amine by gc analysis in the reaction mixturte is an evidence for
55 the proposed mechanism.
Fig. 2 Conversion of W(VI) to W(IV) in the presence of DMF
Since formation of Pd(0) from Pd(II) was not observed in DEF, it
can be concluded that DEF can not act as a reducing agent for the
70 conversion of Pd(II) to Pd(0), thus we needed to add PPh3 as a
phosphine ligand to the reaction mixture for the reduction of
In order to show that the formation of dimethyl amine occurs
through the reduction processes and is not
a thermic
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Pd(II) to generate the Pd(0) catalyst.
this transformation, N,Nꢀdimethylformamide (DMF) has been
Although the mechanism for this reaction is not clear at this time, 35 used as the source of carbonyl group. Amonge the studied metal
we propose a plausible reaction pathway according to our
findings and also the litrature as shown in Scheme 2.10, 15 In the
first step, reduction of WCl6 in DMF is occurred and W(IV) is
possibly formed. Then this compound can reduce Pd(II) to Pd(0).
halides, WCl6 showed to be the most efficient one. A possible
reason for the more efficiency of WCl6 was shown to be due to
the easier reduction of Pd(II) to Pd(0) in WCl6/DMF compared
with those reagent systems using MoCl5, ZrOCl2 or ZrCl4 in
5
The Vilsmeier imminium salt reagent [Me2N+=CHCl] is also 40 DMF. The reaction of aryl iodides in DEF can also occure to give
produced from the reaction of DMF and WCl6. The Intermediate
the corresponding N,Nꢀdiethyl amides but in the presence of PPh3
I which is produced from the oxidative addition of aryl halide to
as a reducing ligand.
10 Pd(0) reacts with the Vilsmeier imminium salt. Then the reaction
proceeds via the Heckꢀtype addition of aryl halides to the
imminium species as shown in Scheme 2.
Experimental section
The progress of reactions was followed by TLC on silica gel
45 SILG/UV 254 plates or GLC analysis on a Shimadzu model GCꢀ
10A instrument. IR spectra were run on a Shimadzu FTIRꢀ8300
spectrophotometer.The 1H NMR and 13C NMR spectra were
recorded on a BrukerꢀAvance DPX 250 FTꢀNMR spectrometer
O
CH3
N
O
CH3
N
CH3
R
CH3
WCl6
H2O
Cl
50 General Procedure for Aminocarbonylation of Aryl Halides
with WCl6 in the Presence of PdCl2 as Catalyst in DMF
WCl6 (1.0 mmol, 0.415 g) and dry DMF (5.0 mL) were added to
a flask containing a magnetic stirring bar. The resulting mixture
was stirred for 15 min at room temperature under nitrogen. Then
55 aryl halide (0.5 mmol), and PdCl2 (2.5 mol%, 4.4 mg) were added
to the reaction mixture. After 15 min, the mixture was heated at
140 °C for the appropriate time. After the consumption of the
starting material (monitored by TLC or GC analysis), the reaction
mixture was cooled down to room temperature. The resulting
60 mixture was added to a saturated aqueous solution of NaHCO3
(25 mL). The organic layer was extracted with ethyl acetate (25
mL×4) and then the solvent was evaporated to obtain the desired
product. Further purification was performed by silica gel column
chromatography [60 Merck (230ꢀ240 mesh)] using n-
65 hexane/ethyl acetate (4:1) as the eluent to give the pure product in
moderate to excellent yields.
Cl
CH3
N
CH3
CH3
Cl
N
H
R
CH3
CH3
H
CH3
N
R
Pd
X
Pd
X
H-Pd-X
R
Pd (0)
X
R
HX
W (VI)
W (IV) + Pd (II)
Scheme 2 The proposed mechanism
15 Since formation of the Vilsmeier imminium salt reagent
[Me2N+=CHCl] is the required step in this catalytic cycle, the use
of at least stoichiometic amount of WCl6 is required to generate
this intermediate through the reaction with DMF.
In order to show the formation of Vilsmeier imminium reagent
20 proposed in the mechanism, in an experiment, indole (1.0 mmol)
was reacted with WCl6 (1.0 mmol) in DMF (2 mL) at 80 oC.
After 2 h, indol 3ꢀcarbaldehyde was obtained in 75% isolated
yield. This experiment strongly confirms the generation of
Vilsmeier imminium reagent under our optimized conditions.
General Procedure for Aminocarbonylation of Aryl Halides
with WCl6 in the Presence of PdCl2/PPh3 as Catalyst in N,N-
Diethylformamide
70 WCl6 (1.0 mmol, 0.396 g) was dissolved in N,N-
diethylformamide (5.0 mL). Then, aryl halide (0.5 mmol), PdCl2
(2.5 mol%, 4.4 mg) and PPh3 (2.0 mg) were added to the vessel
containing the mixture of DEF and WCl6. After 15 min, the
mixture was stirred for the specified time at 140 °C under
75 nitrogen (Table 5). After consumption of the starting material
(TLC or GC), the reaction mixture was cooled down to room
temperature. Saturated solution of NaHCO3 (25 mL) was added
to the reaction mixture and was then extracted with ethyl acetate
(25×4 mL). Evaporation of the solvent gave the desired pure
80 amide. Further purification, if was necessary, was performed by
silica gel column chromatography using nꢀhexane/EtOAc as the
eluent to give the pure product in high to excellent yield.
H
H
N
WCl6, 80 oC, 2h
N
CH3
N
O
+
75%
CH3
O
H
25
Scheme 3 Synthesis of indoleꢀ3ꢀcarbaldehyde representing the formation
of Vilsmeier imminium reagent from DMF/WCl6 system
Acknowledgements
Conclusions
The authors would like to acknowledge the support of this work
85 by Shiraz University Research Council and the grant from Iran
National Elite Foundation. Technical assistance of Dr. S.
Motevalli is also acknowledged.
30 In this study, we have introduced
a new method for
aminocarbonylation of aryl halides in the presence of WCl6 as an
efficient metal halide in the presence of catalytic amounts of
PdCl2 as precatalyst in the absence of any phosphine ligand. For
4
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Notes and references
a Chemistry Department, College of Sciences, Shiraz University, Shiraz,
Iran. Fax: +987116460788; Tel: +987116460724;
Email: iranpoor@susc.ac.ir; firouzabadi@susc.ac.ir
5
10
15
20
25
30
35
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† Electronic Supplementary Information (ESI) available: Copy of 1H
NMR and 13C NMR spectra of compounds. See DOI: 10.1039/b000000x/
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