Copper-catalyzed decarboxylation of aromatic carboxylic acids: En route to milder reaction conditions

Article abstract of DOI:10.1002/adsc.201201018

The copper-catalyzed decarboxylation of carboxylic aromatic acids has been advantageously achieved by using aliphatic amines like tetramethylethylenediamine (TMEDA) or hexamethylenetetraamine (HMTA) as ligands instead of the aromatic heterocyclic amines (quinoline, phenanthroline) used until now. The improvement is significant since the reaction can be performed at a lower temperature (ca. 50 °C less) and the reaction time is clearly shorter (15 min instead of 12 to 24 h). Copyright

Full text of DOI:10.1002/adsc.201201018

UPDATES
DOI: 10.1002/adsc.201201018
Copper-Catalyzed Decarboxylation of Aromatic Carboxylic
Acids: En Route to Milder Reaction Conditions
Gꢀrard Cahiez,a,* Alban Moyeux,b Olivier Gager,b and Maꢁl Poizatb
a
Chimie ParisTech, UMR CNRS 7223, 11 rue Pierre et Marie Curie, 75005 Paris France
E-mail: gerard-cahiez@chimie-paristech.fr
Universitꢀ Paris 13, UMR CNRS 7224, 74 rue Marcel Cachin, 93017 Bobigny, France
b
Received: November 16, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201018.
Abstract: The copper-catalyzed decarboxylation of
carboxylic aromatic acids has been advantageously
achieved by using aliphatic amines like tetramethyl-
carboxylation. A breakthrough was recently intro-
duced by Gooßen who showed that the decarboxyla-
tion of aromatic carboxylic acids can be performed by
using only a catalytic amount of copper to give the
protodecarboxylation product. Moreover, he de-
scribed the first copper/palladium-catalyzed decarbox-
ylative cross-coupling reaction between an aromatic
carboxylic acid and an aryl bromide.[6,7] Of course, the
use of cheap and readily available carboxylic acids as
the source of aryl nucleophiles instead of costly pre-
formed stoichiometric organometallic reagents make
this new procedure very attractive from economical
and environmental points of view.[3b–d]
ACHTUNGTRENNUNGethylenediamine (TMEDA) or hexamethylenetetra-
amine (HMTA) as ligands instead of the aromatic
heterocyclic amines (quinoline, phenanthroline)
used until now. The improvement is significant
since the reaction can be performed at a lower tem-
perature (ca. 508C less) and the reaction time is
clearly shorter (15 min instead of 12 to 24 h).
Keywords: aromatic carboxylic acids; aryl–aryl cou-
pling; copper; decarboxylative cross-coupling; pal-
ladium
Since the seminal reports on the copper-mediated
protodecarboxylation of aromatic carboxylic acids,[2–4]
almost no change was reported concerning the cata-
lytic system and the reaction was generally performed
in NMP or a mixture NMP/quinoline at 1708C in the
The decarboxylation of copper aromatic carboxylates presence of bipyridine or 1,10-phenanthroline as a li-
was extensively investigated from the 1960s[1] by gand.[5b–d] These last years, only one attempt to im-
Sheppard,[2] Cohen,[3] Nilsson[4] and others.[5] Sheppard prove this catalytic system was performed by Gooßen.
et al. reported that cuprous arylcarboxylates readily He has compared various aromatic diamines and he
decarboxylate on heating. Quinoline is generally used showed that 4,7-diphenyl-1,10-phenanthroline (batho-
as a solvent or cosolvent since the reaction then phenanthroline) is more efficient than 1,10-phenan-
occurs faster.[1] It should be noted that the tempera- throline with non-activated aromatic carboxylic
ture of decarboxylation is clearly dependent on the acids.[6b] Currently, one of the main challenge is to
nature of the starting cuprous aromatic carboxylate lower the reaction temperature and so far, this was
(100 to 2158C).[2,5] The results reported by Cohen, only possible by using silver[8] or palladium[9] instead
Sheppard and Nilsson support the formation of a s- of copper.
bonded arylcopper species as an intermediate in these
The mechanism of the reaction is not clear. Howev-
reactions. Thus, in a seminal report, Nilsson described er, as suggested by Cohen,[3b] it is reasonable to pro-
ꢀ
a one-pot decarboxylation-Ullmann reaction using pose the complexation of the copper atom to the C
cuprous 2-nitrobenzoate and iodobenzene or 2- and COO bond of the carboxylate (1!2), via a p-com-
4-iodoanisole.[4a] This was the first decarboxylative plexation to the aromatic ring or not.
coupling reaction. A few years later, Sheppard
Then, a copperACTHUNGTERNNU(G III) intermediate 3 would be
showed that the decarboxylation of cuprous penta- formed by oxidative addition. Finally, a rapid reduc-
fluorobenzoate leads to pentafluorophenyl copper in tive elimination would give an arylcopper 4 and
72% yield.[2] Thereafter, various studies pointed out carbon dioxide (Scheme 1). By considering this puta-
that chelating nitrogen ligands such as bipyridine[3b] or tive mechanism, the role of a s-donor ligand like
1,10-phenanthroline[3a] greatly enhance the rate of de- 1,10-phenanthroline would be to favour the oxidative
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Table 1. Influence of various amines on Cu-catalyzed proto-
decarboxylation of 2-nitrobenzoic acid.[a]
Entry Ligand
Time
Yield [%][c]
1
2
3
4
5
6
7
8
9
none
quinoline
N,N-dimethylaniline
2 h
2 h
72
89
45 min 82
N,N-diisopropylethylamine 45 min 82
DBU[d]
45 min 94
1 h
1 h
1 h
N,N-dimethylpiperazine
99
99
99
99
DABCO[e]
triethylamine
TMEDA[f]
5 min
Scheme 1. Putative mechanism for the Cu-catalyzed decar-
boxylation of aromatic carboxylic acids.
[a]
[b]
[c]
[d]
[e]
Reactions performed on a 2.5 mmol scale (c=0.5M).
Amine/ArCOOHꢁ30:1.
Yield determined by GC (decane as internal standard).
ꢀ
addition of the C C bond which is probably the limit-
DBU=1,8-diazabicyclo
ACHTUNGTRENNUNG
ing step of the mechanism described above
(Scheme 1). Indeed, the insertion of copper into the
DABCO=1,4-diazabicyclo
AHCTUNGTRENNUNG
1:1).
ꢀ
C C bond is due to a p back-donation from a filled
[f]
TMEDA=N,N,N’,N’-tetramethylethylenediamine.
ꢀ
metal orbital into the s* orbitals of the C C bond.
Such a process is favoured by increasing the electron-
ic density of the copper atom and it should be fav- TMEDA. As shown in Table 2, 10 mol% TMEDA
oured by the presence of s-donor ligands on copper. and 5 mol% Cu2O (ligand/Cu=1:1) are enough to
Thus, the good results obtained with quinoline, bypyr- perform quantitatively the reaction at 1408C in 5 min
idine or 1,10-phenanthroline are not surprising since (Table 2, entry 2). Even more interestingly, a satisfac-
amines are good s-donor ligands. On the other hand, tory yield was obtained in the presence of only 0.5%
it seemed to us very surprising to use aromatic Cu2O and 1% TMEDA (ligand/Cu=1:1), however
amines instead of aliphatic amines since the latter are the reaction lasts 1.5 h instead of 5 min (Table 2,
expected to be more efficient. In the light of these entry 3). To the best of our knowledge, this reaction
considerations, we decided to study the influence of had never been done with such a low copper catalyst
various amines on the copper-catalyzed protodecar-
boxylation of aromatic carboxylic acids.
In a first set of experiments, we performed the
copper-catalyzed decarboxylation of 2-nitrobenzoic
acid in NMP in the presence of stoichiometric
amounts of various simple amines at 1408C (ratio
ArCOOH/amineꢁ1:30).
Table 2. Cu-catalyzed protodecarboxylation of 2-nitrobenzo-
ic acid in the presence of TMEDA.[a]
As shown in Table 1, the improvement observed by
adding quinoline is not spectacular. The yield of pro-
todecarboxylation product was slightly enhanced
(89% instead of 72%) but the reaction time remained
unchanged (Table 1, entries 1 and 2). In fact, all the
other amines tested gave better results.
Entry Solvent Cu2O
TMEDA Temp. Time Yield
[mol%] [mol%]
[8C]
[%][b]
1
2
3
4
5
6
7
8
NMP
NMP
NMP
NMP
NMP
NMP
DMSO
5
5
0.5
5
5
340
10
1
10
10
10
10
10
140
14
5 min 99
5 min 99
140
120
110
100
140
110
1.5 h
93c)
As expected, aliphatic amines are more efficient
than aromatic amines (Table 1, entries 2–9). The dis-
appointing result obtained with (i-Pr)2EtN, a good s-
donor ligand (Table 1, entry 4) can probably be ex-
plained by steric hindrance. Similar effects were re-
ported with bulky ligands such as 2,9-dimethyl-1,10-
phenanthrolines.[6b] The best result (99% in 5 min)
was achieved by using TMEDA, a good bidentate s-
donor ligand (Table 1, entry 9). Of course, it was im-
portant to lower the amount of both copper and
15 min 99
8 h
12 h
99
99
5
5
5 min 99
24 h 61
toluene 5
[a]
The reactions were performed on a 5 mmol scale (c=
0.5M).
Yield determined by GC (decane as internal standard).
The reaction was performed on a 15 mmol scale.
[b]
[c]
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Copper-Catalyzed Decarboxylation of Aromatic Carboxylic Acids
a similar result was to replace copper salts by silver
salts.[8,9]
We applied our decarboxylation procedure to vari-
ous activated aromatic and heteroaromatic carboxylic
acids (Table 3).
In most cases, the protodecarboxylation product is
formed rapidly in high yields. All experiments were
performed at 1408C to avoid optimizing the tempera-
ture of the reaction for each case. However, it should
be noted that in many cases it is possible to work at
a lower temperature. For instance, 2-nitrobenzoic acid
Scheme 2. Cu-catalyzed protodecarboxylation of 2-nitroben-
zoic acid by using HMTA as ligand.
loading. The new catalytic system also allowed us to led to 99% of nitrobenzene in 12 h at 1008C and 2-
lower the reaction temperature down to 1008C acetoxybenzoic acid gave 86% of acetophenone in 3 h
(Table 2, entry 6). The yield of protodecarboxylation at 1208C.
product is then still quantitative but the decarboxyla-
Compared with the procedure developed by Goo-
tion takes place in 12 h instead of 5 min at 1408C. ßen[6b] which was generally used until now to decar-
NMP was used as a solvent but other high boiling boxylate activated aromatic acids, the catalytic system
polar solvents like DMSO can be used (Table 2, Cu2O/TMEDA can be used at lower temperatures
entry 7). On the other hand, non-polar solvents such (100–1408C instead of 1708C).
as toluene led to unsatisfactory results (Table 2,
entry 8).
Moreover, the reactions are faster (often less than
1 h instead of 12 h) and lead to higher yields. For ex-
It should be noted that HMTA,[10] a good s-donor ample, 2-methoxybenzoic and 2-thiophenecarboxylic
ligand since the lone pair of the nitrogen is very ac- acids were decarboxylated in respectively 75% and
cessible, is as efficient as TMEDA (Scheme 2). Final- 87% yields instead of 24% and 58% with the Goo-
ly, by comparison with the previous procedures de- ßenꢃs procedure.[5b] Moreover, the scope of the reac-
scribed by Gooßen,[6b] the use of TMEDA allows to tion is larger, thus, 2,6-dimethoxybenzoic acid led to
lower the reaction temperature of 50–708C.
92% yield whereas it does not react by using Cu2O/o-
The examples depicted in Figure 1 show that phenanthroline as a catalyst.[8c] The results listed in
TMEDA is much more efficient at 1208C. It must be Table 3 show that the reaction is chemoselective since
emphasized that, up to now, the only way to obtain various functional groups (nitro, chloro, acetoxy, ester
or methoxy) are tolerated. Finally, it is interesting to
note that the reaction was successfully applied to cin-
namic acid (Table 3, entry 13).
We then tried to use the efficient catalytic system
described above to perform some palladium-catalyzed
decarboxylative cross-coupling reactions. Of course,
to avoid the rapid protonation of the arylcopper inter-
mediates formed in the reaction mixture, it is not pos-
sible to start from the free carboxylic acid and we re-
placed them by their potassium salts. In the case of
potassium 2-nitrobenzoate the coupling occured at
1208C in the presence of 10% CuBr,[11] 10% TMEDA
and 3% PdACTHNURGTNEUNG(acac)2 but the reaction was very slow and
led to low yield of 2-nitrobiphenyl (22%, Scheme 3).
This unsatisfactory result is mainly due to the poor
solubility of potassium carboxylates in NMP. Thus, it
is possible to improve the yield by working in
DMPU[12] (40%) but to obtain good yields (90%) it is
necessary to heat to 1708C in NMP or DMPU.
Accordingly, to achieve the coupling efficiently at
a lower temperature, it is necessary to use a more
soluble metal carboxylate such as caesium 2-nitroben-
zoate (Scheme 4).
The coupling product was then formed at 1308C in
92% yield.
Various decarboxylative cross-coupling reactions
were efficiently achieved according to this procedure.
Figure 1. Protodecarboxylation of 2-nitrobenzoic and 2-thio-
phenecarboxylic acids. Comparison of two catalytic sytems:
o-phenanthroline/Cu2O and TMEDA/Cu2O.
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Table 3. Cu-catalyzed protodecarboxylation of various aro-
Table 3. (Continued)
matic and heteroaromatic carboxylic acids.[a]
Entry
ArCOOH
Time
Yield [%][b]
89[c]
14
40 h
1 h
Entry
1
ArCOOH
Time
5 min
Yield [%][b]
99
15
93[f]
[a]
The reactions were performed on a 5 mmol scale (c=
2
3
20 min
20 min
83
98
0.5M).
[b]
Yield determined by GC (decane or hexadecane as inter-
nal standards).
1708C.
20 mol% of Cu2O was used.
15 mol% of Cu2O was used.
The reaction was performed on a 15 mmol scale, isolated
yield.
[c]
[d]
[e]
[f]
4
45 min
81
5
6
45 min
1 h
87
95
Scheme 3. Cu/Pd-catalyzed decarboxylative cross-coupling
reaction between potassium 2-nitrobenzoate and bromoben-
zene.
7
8
9
4 h
90[c]
87
30 min
6 h
99[c]
10
11
40 min
1 h
75[d]
68[e]
Scheme 4. Cu/Pd-catalyzed decarboxylative cross-coupling
reaction between caesium 2-nitrobenzoate and bromoben-
zene.
The examples depicted in Table 4 show that the reac-
tion is chemoselective since fluoro, chloro, nitro,
cyano, ester or keto groups are tolerated. It should be
noted that various heterocyclic substrates were used
successfully.
This procedure can be used to synthezise polyaro-
matic molecules. Thus, 1,3-bisthienylbenzene was
easily prepared from 1,3-dibromobenzene (Scheme 5).
The last example (Scheme 6) shows that the catalyt-
ic system described above can be used efficiently on
a preparative scale (0.1 mol). Interestingly, it is then
12
13
2 h
92[f]
70[c]
24 h
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Copper-Catalyzed Decarboxylation of Aromatic Carboxylic Acids
Table 4. Cu/Pd-catalyzed decarboxylative cross-coupling reactions.[a]
Entry
ArCOOM
Coupling Product
Yield [%][b]
M=Cs[c]
M=K[d]
1
2
77
80
89
90
91
91
3
4
5
77
91
88
96
6
90[e]
91
7
8
82
74
71[f]
81[f]
9
–
70
10
11
12
69[f]
70
73[f]
–
61
66
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Table 4. (Continued)
Entry
ArCOOM
Coupling Product
Yield [%][b]
M=Cs[c]
M=K[d]
13
72[g]
73
[a]
The reactions were performed on a 2 mmol scale.
Yield of isolated product.
The reaction was performed at 1308C in DMPU.
The reaction was performed at 1708C in NMP.
20% CuBr/TMEDA, 40 h.
[b]
[c]
[d]
[e]
[f]
20% CuBr/TMEDA, 1408C.
2 equiv. of caesium carboxylate were used.
[g]
in the 1960s.[2-4] The reaction conditions described
herein also allow us to perform copper-palladium-cat-
alyzed decarboxylative cross-coupling reactions be-
tween 120 to 1408C instead of 1708C. The results re-
ported above could open the way to the development
of low temperature procedures. It is worthy of note
that, for large scale applications, it is very advanta-
geous to replace 1,10-phenanthroline by TMEDA
which is clearly less expensive.
Scheme 5. Cu/Pd-catalyzed decarboxylative cross-coupling
reaction from 1,3 dibromobenzene.
Experimental Section
Typical Procedure: Synthesis of 2-Nitrobiphenyl
(Scheme 6)
An oven-dried, nitrogen-flushed, 500-mL four-necked flask
equipped with a mechanical stirrer, a thermometer and
a condenser was charged with potassium 2-nitrobenzoic acid
(26.7 g, 130.00 mmol), bromobenzene (15.7 g, 100.00 mmol),
copper(I) bromide (144 mg, 1.00 mmol), TMEDA (116 mg,
Scheme 6. Example of preparative Cu/Pd-catalyzed decar-
boxylative cross-coupling reaction.
1.00 mmol), PdACHTNUTRGNE(NUG acac)2 (30.5 mg, 0.1 mmol) and NMP
(130 mL). The resulting mixture was stirred at 1708C for
20 h. When the reaction was complete, the mixture was
cooled to room temperature and filtered through celite. The
solid was rinsed with ethyl acetate (3ꢄ50 mL) and the re-
sulting filtrate was concentrated under vacuum,. The residue
was purified by column chromatography on silica gel
(eluent: petroleum ether/ethyl acetate, gradient 98:2 to
95:5) furnishing 2-nitrobiphenyl as a yellow oil; yield: 17.1 g
(86%).
possible to lower the charge of CuBr/TMEDA up to
1% and the one of PdACHTNUGRTENUNG(acac)2 to 0.1%.
In conclusion, we have shown that the use of
TMEDA as a ligand allows us to lower significantly
the temperature of the copper-catalyzed decarboxyla-
tion of activated aromatic carboxylic acids. Thus, in
the case of 2-nitrobenzoic acid the decarboxylation
takes place at 1008C instead of 1708C with the previ-
ous procedures. This new catalytic system was devel-
oped owing to an analysis of the reaction mechanism.
Until now, such a result was only possible by using
silver salts instead of copper salts.[8] Concerning the
reaction temperature, it should be noted that it is the
Acknowledgements
We thank the CNRS for its financial support and the Minis-
tꢀre de l’Education Nationale et de la Recherche for a grant
first significant improvement since the seminal reports to M. Poizat.
6
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Copper-Catalyzed Decarboxylation of Aromatic Carboxylic Acids
References
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[10] HMTA had already been used as ligand for the iron-
catalyzed coupling between aryl Grignard reagents and
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[11] With metal carboxylates, CuBr was used in place of
Cu2O since the reaction is generally faster and gives
better yields. For instance from potassium 2-nitroben-
zoate, at 1208C, the yield of coupling product was re-
spectively 22% and 10%.
[1] Some isolated examples of copper-catalyzed decarboxy-
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before but no systematic study was reported. For in-
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2217; b) S. G. Turnbull Jr, U. S. Patent 2,453,103, 1948.
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M. Nilsson, Acta Chem. Scand. 1971, 25, 3451.
[5] The copper-mediated decarboxylation of aromatic car-
boxylic acid is an old reaction. For instance, see:
a) A. F. Shepard, N. R. Winslow, J. R. Johnson, J. Am.
Chem. Soc. 1930, 52, 2083. For recent reviews, see:
b) R. Shang, L. Liu, Sci. China Chem. 2011, 54, 1670;
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Chem. 2008, 120, 3144; Angew. Chem. Int. Ed. 2008, 47,
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Blanchot, C. Linder, B. Zimmermann, Pure Appl.
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[6] a) L. J. Gooßen, G. Deng, L. M. Levy, Science 2006,
313, 662; b) L. J. Gooßen, W. R. Thiel, N. Rodriguez, C.
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c) L. J. Gooßen, B. Melzer, J. Org. Chem. 2007, 72,
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129, 4824; e) L. J. Gooßen, N. Rodriguez, C. Linder, J.
[12] DMPU=N,N’-dimethylpropyleneurea.
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8 Copper-Catalyzed Decarboxylation of Aromatic Carboxylic
Acids: En Route to Milder Reaction Conditions
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Gꢀrard Cahiez,* Alban Moyeux, Olivier Gager, Maꢁl Poizat
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