A novel mode of reactivity for gold(I): The decarboxylative activation of (hetero)aromatic carboxylic acids

Article abstract of DOI:10.1002/adsc.201100109

Gold(I) salts are found to mediate the decarboxylation of a variety of aromatic and heteroaromatic carboxylic acids at significatively lower temperatures (as low as 60°C) than the currently used copper(I) (180-190°C) and silver(I) (80-140°C) systems. In contrast to silver(I)- and copper(I)-mediated decarboxylations, the resulting aryl-gold(I) complexes are stable towards protodemetallation and can be readily isolated.

Full text of DOI:10.1002/adsc.201100109

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DOI: 10.1002/adsc.201100109
A Novel Mode of Reactivity for Gold(I): The Decarboxylative
Activation of (Hetero)Aromatic Carboxylic Acids
Josep Cornella,a Martin Rosillo-Lopez,a and Igor Larrosaa,*
a
Queen Mary University of London, School of Biological and Chemical Sciences, Joseph Priestley Building, Mile End
Road, London E1 4NS, U.K.
Fax : (+44)-(0)20-7882-7427; phone: (+44)-(0)20-7882-8404; e-mail: i.larrosa@qmul.ac.uk
Received: February 13, 2011; Published online: May 17, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100109.
Abstract: Gold(I) salts are found to mediate the de- decarboxylations, the resulting aryl-gold(I) com-
carboxylation of a variety of aromatic and heteroaro- plexes are stable towards protodemetallation and
matic carboxylic acids at significatively lower tem- can be readily isolated.
peratures (as low as 608C) than the currently used
copper(I) (180–1908C) and silver(I) (80–1408C) sys- Keywords: aryl-gold(I) species; decarboxylation;
tems. In contrast to silver(I)- and copper(I)-mediated gold; halogenation; homogeneous catalysis; silver
Introduction
In this vein, we have recently reported the first exam-
ple of gold(I) mediated C H activation of electron-
À
In the last decade, homogeneous gold catalysis has poor arenes which leads to the corresponding aryl-
given rise to a wide array of novel transformations for gold(I) complexes,[2] and opens the door to gold(I)-
[1]
[3]
À
À
C C bond formation. The majority of these new mediated catalytic C H functionalization of arenes.
À
transformations are based upon two main modes of However, as is common in C H activation, regiose-
reactivity: (i) the use of gold(I) or gold
(III) salts as lectivity can be a problem, and so far this methodolo-
powerful p-acids for the activation of alkynes, al- gy only provides access to aryl-gold(I) complexes
kenes, and allenes toward reaction with nucleophiles bearing two electron-withdrawing ortho substituents.
À
and (ii) in the case of gold
(III), the C H activation of
Decarboxylative transformations have emerged in
À
electron-rich arenes. Although these modes of activa- the past few years as an alternative to C H function-
tion were known for other metals, the unique proper- alization in which the regioselectivity is easily con-
ties of gold have led to a variety of new transforma- trolled by the position of the carboxylic acid function-
tions. Therefore, the development of novel modes of al group (Scheme 1).[4] Furthermore, the by-product
reactivity for gold would further expand homogene- generated during the activation is CO2, placing it
ous catalysis by opening access to new methodologies. amongst the greenest activation methods for cross-
Scheme 1. Decarboxylative transformations vs. undesired side reactions.
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Scheme 2. Proposed gold(I)-mediated decarboxylation (L=PR3, NHC).
couplings. The use of copper, silver, palladium and
rhodium salts has been reported for the decarboxyla-
tive activation of benzoic acids.[5–9] Nevertheless, these
methods suffer from several drawbacks including the
requirement of very high temperatures (180–1908C
for copper), low substrate compatibility (for palladi-
um and rhodium), disproportionation and dimerisa-
tion (for palladium) or instability and tendency to-
wards protodemetallation (for silver). These unde-
sired processes significantly lower the yields of subse-
quent transformations (Scheme 1).
Table 1. Optimization of the gold(I)-mediated decarboxyla-
tive auration of potassium 2,6-difluorobenzoate.[a]
Entry LAuX
Additive Solvent Yield [%][b]
1
2
3
G
–
–
–
–
–
DMF
DMF
DMF
DMF
DMF
0
0
0
0
Having the same electronic configuration as cop-
per(I), silver(I) and palladium(II), we envisaged that 4[c]
5[c]
98 (92)[d]
89
gold(I) may also be able to mediate decarboxylative
activations (Scheme 2). Furthermore, based on our
previous experience with aryl-gold(I) complexes, we
expected that the resulting aryl-Au(I) species would
6
AgTFA DMF
7[e]
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
DMF
DMSO 95
NMP
PhCH3
99 (94)[d]
8[e]
9[e]
96
0
be significantly more stable than their silver, copper
10[e]
or palladium counterparts, thus reducing or eliminat-
ing the side reactions observed when further transfor-
mations are attempted. This knowledge contrasted
with a previous report indicating that Ph3PAuOCOPh
invariably led to deposition of Au(0) and radical-
mediated processes when heated at reflux in tolu-
ene.[10] A further point of concern was the recent
report by Nolan on the carboxylation of aryl-gold(I)
compounds, which suggested that the decarboxylation
process may be thermodynamically disfavoured.[3] De-
spite these precedents we set out to explore the possi-
11[e]
dioxane <5
[a]
Reactions were carried out using 1.0 equiv. of 1a,
1.0 equiv. of LAuX and 1.0 equiv. of the additive.
All yields are calculated by NMR analysis using an inter-
nal standard.
[b]
[c]
R=o-Ph-C6H
(t-Bu)2P; 2e should be formulated as
(MeCN)} SbF6.
{Au[(t-Bu)2biphenP]
G
ACHTUNGTRENNUNG
[d]
[e]
Yield of isolated pure material.
Carboxylic acid 4a, was used instead of the salt 1a
bility of using gold(I) salts to catalyse the activation ic gold(I) species {Au
of C CO2H bonds.
G
ACHTUNGTRENN(UNG MeCN)} SbF6,
À
We now report that cationic gold(I) complexes are 1008C in 3 h affording the corresponding aryl-gold(I)
able to mediate the decarboxylation of aromatic and complex 3ad in excellent yield (entry 5). Gratifyingly,
heteroaromatic carboxylic acids. The reactivity of the use of 1 equivalent of AgTFA as an additive to
these gold(I) species can be modulated via the ligand gold chloride 2d also afforded an excellent yield of
to achieve decarboxylations at temperatures as low as 3ad, indicating that a cationic source of gold(I) is nec-
608C.
essary for the decarboxylation to proceed (entry 6).
The use of basic Ag2O allowed for the direct use of
the carboxylic acid 4a, instead of its salt 1a (entry 7).
A survey of reaction solvents showed that polar or co-
ordinating solvents were necessary for the reaction to
proceed, with DMF being the optimal (entries 7–11).
Results and Discussion
Optimization
Initially, we explored the reaction of the potassium
salt of 2,6-difluorobenzoic acid, 1a, with several Lowering the Temperature of Decarboxylation
gold(I) complexes under a variety of conditions
(Table 1). Gold chloride salts 2a–d were found to be Having demonstrated that gold(I) is able to mediate a
inactive when heated at 1008C in DMF (entries 1–4). decarboxylation process (entry 5), we set out to ex-
On the other hand, the commercially available cation- plore the possibility of significantly lowering the tem-
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A Novel Mode of Reactivity for Gold(I): The Decarboxylative Activation
perature required for decarboxylation by carefully complexes at 1008C (entries 4, 6, 8 and 10), but mod-
choosing the ligand in the gold(I) complex. Due to erate to low yields at 608C. The (t-Bu)3P-bearing
the greater number of commercially available gold gold(I) chloride 2a proved to be more reactive, af-
chlorides, the protocol using a silver(I) salt to form fording excellent yields of 3aa at temperatures as low
the cationic gold in situ was used (entry 7).
as 608C (entries 11–13). Remarkably, even at 508C
We therefore examined the reaction of Ag2O this complex was able to mediate the decarboxylative
(1.0 equiv.) and carboxylic acid 4a (1.0 equiv.) with aureation, albeit longer reaction times were required
the gold(I) chloride complexes of a variety of ligands (entry 14). It is noteworthy that complexes 3aa–af
at different temperatures (Table 2). The reaction tem- were easily purified through SiO2 treated with 2.5%
perature using gold(I) chloride 2d could be lowered Et3N, highlighting the remarkable stability of these
to 708C (entry 2), albeit increased reaction times complexes towards protodemetallation.[12]
were required. On the other hand, only 36% decar-
boxylation product 3ad was observed at 608C even
after 40 h of reaction (entry 3). Gold(I) complexes 2a, Scope of the Gold(I)-Mediated Decarboxylation
2f, 2c and 2b bearing (t-Bu)3P, PEt3, PPh3, and the N-
heterocyclic carbene IPr, respectively, also afforded Subsequently, we explored the scope of this decarbox-
excellent yields of the corresponding aryl-gold(I) ylative auration reaction with regards to the carboxyl-
ic acid partner (Table 3). The reactions were per-
formed both at 1108C, which required short reaction
Table 2. Gold(I)-mediated decarboxylative auration of 2,6-
difluorobenzoic acid.
times, and at lower temperatures to test the lower
limits for decarboxylation. The reaction protocol was
compatible with benzoic acids bearing ortho electron-
withdrawing substituents, such as Cl, Br, NO2 and
MeO (entries 1–9), affording the corresponding com-
plexes in excellent yields. Benzoic acid, on the other
hand, with no ortho substituents, failed to react
(entry 10). Heteroaromatic carboxylic acids were also
found to undergo decarboxylative auration under the
reaction conditions (entries 11–17), provided the car-
boxylic acid is a to the heteroatom (4h–j) or ortho to
an electron-withdrawing group (4k). In most cases,
decarboxylation was possible at temperatures as low
as 60–808C.
Entry LAuCl
1[c]
2
3
Aryl-gold(I)
complex
t
T
Yield
[h] [8C] [%][a]
3
100 94[b]
RAuCl (2d)
20 70
40 60
93
36
These results highlight a high degree of functional
group tolerance for the gold(I)-mediated decarboxyla-
tive activation. The requirement of an ortho electron-
withdrawing group or an a-heteroatom for the decar-
boxylation to proceed, suggests a similar pattern of
chemoselectivity to that observed previously for
silver-mediated decarboxylations,[6] but without the
disadvantage of protodemetallation. Interestingly,
these results contrast with the recently reported
gold(I)-mediated carboxylation of arenes and suggest
that this process may be reversible.[3]
Et3PAuCl
(2f)
4
5
3
100 97[b]
20 60 29
Ph3PAuCl
(2c)
6
7
2
100 95[b]
20 60 37
8
9
IPrAuCl (2b)
2
100 98[b]
20 60 37
Mechanistic Investigations
ACHTUNGTRENNUNG
10
2
100 94[b]
Our initial experiments using the cationic gold(I) salt
2e (Table 1, entry 5) showed that gold on its own is
able to mediate the decarboxylative activation of po-
tassium salt 1a. To confirm the generality of this pro-
cess the potassium salts of other acids, 4b and 4j, rep-
resentative of the reaction scope, were also tested
under these silver-free conditions, affording excellent
isolated yields of the corresponding gold(I)-complexes
3bd and 3jd (Figure 1). However, in light of recent ex-
11
12
13
14
16 70
20 60
40 60
60 50
95
53
99
53
[a]
Unless otherwise noted, yields are calculated by
1H NMR analysis using an internal standard.
Yield of isolated pure material.
[b]
[c]
R=o-Ph-C6H4ACHTUNGTRENNUNG(t-Bu)2P.
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Table 3. Scope of gold(I)-mediated decarboxylative auration.
Entry
Ar-CO2H
Aryl-gold(I) complex
T [8C]
t [h]
Yield [%][a]
1
2
110
80
3
60
96
(85)
3
4
110
80
2
60
72
(87)
5
6
110
80
3
60
78
(95)
7
8
100
70
4
40
92
(90)
9
110
110
2
75
0
10
16
11
12
110
70
3
60
87
(89)
13
14
100
70
3
60
80
(85)
15
16
100
60
3
40
85
(75)
17
110
3
70
[a]
1
Yields are of isolated pure material. Yields in brackets are calculated by H NMR analysis using an internal standard.
amples where reactivity initially attributed to gold or observed when compared with silver(I)-mediated de-
iron has been shown to originate from trace impuri- carboxylations, we decided to investigate the possibili-
ties of palladium or copper,[13,14] respectively, and ty of traces of silver(I) impurities being responsible
given the similarity in the chemoselectivity pattern for this decarboxylation process. Therefore, the gold
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A Novel Mode of Reactivity for Gold(I): The Decarboxylative Activation
1
was followed by H and 31P NMR analysis. Stirring 4a
and Ag2O in DMF at 508C for 5 min afforded quanti-
tative formation of the corresponding silver(I)-car-
boxylate 5 (Figure 2, a). After addition of Ph3PAuCl,
1
a small change was observed by H NMR (Figure 2,
b) while 31P NMR indicated complete conversion of
Ph3PAuCl (32.7 ppm) to species assigned as gold(I)-
carboxylate 6 (28.5 ppm). Upon heating of 6 at
1008C, clean conversion to aryl-gold(I) 3ac was ob-
served by 1H (Figure 2, c and d) and 31P NMR
(43.0 ppm) after 3 h. These observations are consis-
Figure 1. Aryl-gold(I) complexes formed from reaction of
the potassium salts of 4b and 4j with {Au[(t-Bu)2biphenP]-
(MeCN)} SbF6 (2e) using the conditions described in
Table 1, entry 5.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
complex 2e and the carboxylate salt 1b, were analysed tent with gold(I) mediating the decarboxylation of 4a
by ICP, which showed a content of silver of <0.01%. even in the presence of silver(I) salts (path b in
Control experiments showed no decarboxylation of Scheme 3).
1b in the absence of the gold complex, or in the pres-
Finally, in order to investigate whether this decar-
ence of 1% of AgSbF6 thus confirming that gold(I) boxylation process is reversible we examined the re-
and not trace impurities of silver(I) is responsible for action of aryl-gold(I) complex 3ab with CO2 under
the observed reactivity.
the conditions reported for the gold(I)-mediated car-
Although these experiments prove that cationic boxylation process (CO2 saturated THF, À1008C to
gold(I) is able to mediate the decarboxylation step, it room temperature).[3] However, unreacted 3ab was re-
is not clear that this is the case when using the proto- covered quantitatively after reaction times of 2–10 h.
col that employs Ag2O as an additive (Table 2 and Further studies to clarify the reversibility of the decar-
Table 3). In these cases, two possible reaction path- boxylation step are under way.
ways can be envisaged (Scheme 3). Initially, the sil-
ver(I) salt I, will be formed by reaction of Ag2O with
the carboxylic acid. Subsequently, carboxylate I could Synthetic Transformations
undergo decarboxylation and the aryl-silver(I) species
II thus generated then transmetallate to LAuCl, with Aryl-gold(I) complexes and alkenes have recently
concomitant formation of AgCl (path a). On the been shown to undergo a variety of transformations,
other hand, silver(I) carboxylate could undergo ligand including halogenations and Pd-catalysed cross-cou-
exchange with LAuCl, thus generating gold(I) carbox- plings,[16,17] which could be combined with our new de-
ylate III. Decarboxylation of III would then afford carboxylative aureation protocol. In order to demon-
aryl-gold(I) complex IV directly (path b). Gold(I) car- strate the potential of our new entry point to aryl-
boxylates are generally prepared from the corre- gold(I) complexes, we explored the possibility of a
sponding silver(I) salts with AgCl formation, support- one-pot transformation from benzoic acids to haloar-
ing the latter hypothesis.[15]
enes in a process that is reminiscent of the Huns-
In order to determine the identity of the different diecker reaction. However, to the best of our knowl-
species involved when using these conditions, the re- edge, the standard version of this transformation does
action of benzoic acid 4a with Ph3PAuCl and Ag2O not proceed satisfactorily with aromatic carboxylic
Scheme 3. Two proposed reaction pathways for the decarboxylative gold(I) auration (R=EWG).
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1
Figure 2. 1H NMR study of the reaction corresponding to entry 6, Table 2. (a) H NMR of 2,6-difluorobenzoic acid (4a)+
1
1
Ag2O (1.0 equiv.) in DMF/CDCl3. (b) H NMR 5 min after addition of Ph3PAuCl. (c) H NMR after 30 min at 1008C. (d)
1H NMR after 2 h at 1008C.
acids, having a very limited substrate scope and low cating that the gold(I)-mediated decarboxylation
reproducibility.[18] Compounds 4a, 4d and 4k were re- could be used to develop a general methodology for
acted with (t-Bu)3PAuCl and Ag2O for 3 h, followed the halodecarboxylation of arenes. We are currently
by addition of 1.1 equiv. of NBS or NIS (Scheme 4). investigating the development of a catalytic version of
Gratifyingly, analysis after work-up showed good to this procedure.
excellent yields of the corresponding haloarenes indi-
Conclusions
In conclusion, we report a novel mode of reactivity
for gold(I) that provides access to a variety of
gold(I)-arene substrates from cheap and readily avail-
able carboxylic acids, thus competing with current
methodologies that require the use of organometallic
reagents as the aryl donors.[19] The decarboxylation
process occurs at temperatures as low as 608C, a con-
siderable decrease when compared with other metals
such as copper or silver. In addition, the higher stabil-
ity of aryl-gold(I) species towards protodemetallation
as compared to aryl-silver(I) and copper(I) intermedi-
ates is showcased by the possibility of isolating and
characterising these organometallic compounds. Their
potential as reactive synthetic intermediates is show-
Scheme 4. One-pot procedure for the decarboxylative halo-
genation of (hetero)aromatic carboxylic acids. Yields calcu-
lated by 1H NMR using an internal standard.
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A Novel Mode of Reactivity for Gold(I): The Decarboxylative Activation
d) L. J. Gooßen, C. Linder, N. Rodriguez, P. P. Lange,
cased by their clean one-pot transformation into iodo-
and bromoarenes. Our current work aims at applying
these stoichiometric studies to the development of
new gold(I) catalysed synthetic methodologies.
Chem. Eur. J. 2009, 15, 9336; e) L. J. Gooßen, B. Zim-
mermann, C. Linder, N. Rodriguez, P. P. Lange, J. Har-
tung, Adv. Synth. Catal. 2009, 351, 2667.
[6] For recent work on silver and palladium decarboxyla-
tive transformations, see: a) C. Wang, I. Piel, F. Glorius,
J. Am. Chem. Soc. 2009, 131, 4194; b) J. Cornella, P.
Lu, I. Larrosa, Org. Lett. 2009, 11, 5506; c) L. J.
Gooßen, P. P. Lange, N. Rodriguez, C. Linder, Chem.
Eur. J. 2010, 16, 3906; d) F. Zhang, M. F. Greaney,
Angew. Chem. 2010, 122, 2828; Angew. Chem. Int. Ed.
2010, 49, 2768; e) J. Zhou, P. Hu, M. Zhang, S. Huang,
M. Wang, W. Su, Chem. Eur. J. 2010, 16, 5876; f) K.
Xie, Z. Yang, X. Zhou, X. Li, S. Wang, Z. Tan, X. An,
C.-C. Guo, Org. Lett. 2010, 12, 1564; g) J. Cornella, H.
Lahlali, I. Larrosa, Chem. Commun. 2010, 46, 8276.
[7] For silver(I)-mediated protodecarboxylations, see: a) J.
Cornella, C. Sanchez, D. Banawa, I. Larrosa, Chem.
Commun. 2009, 7176; b) P. Lu, C. Sanchez, J. Cornella,
I. Larrosa, Org. Lett. 2009, 11, 5710; c) L. J. Gooßen, C.
Linder, N. Rodriguez, P. P. Lange, A. Fromm, Chem.
Commun. 2009, 7173; d) L. J. Goossen, N. Rodriguez,
C. Linder, P. P. Lange, A. Fromm, ChemCatChem 2010,
2, 430.
Experimental Section
Representative Procedure for the Decarboxylative
Auration of 4b (Table 3, entry 1)
A mixture of (t-Bu)3PAuCl (26.1 mg, 0.06 mmol), 2-chloro-
5-nitrobenzoic acid (12.1 mg, 0.06 mmol) and Ag2O
(13.9 mg, 0.06 mmol) in 0.9 mL of DMF was stirred at
1108C for 3 h. After this time the reaction mixture was fil-
tered through cotton wool, washed with AcOEt and evapo-
rated to dryness. Column chromatography on silica gel treat-
ed with 2% triethylamine (hexanes:EtOAc 9:1) afforded
compound 3ba as a white solid; yield: 29.5 mg (96%);
Acknowledgements
We gratefully acknowledge the Engineering and Physical Sci-
ences Research Council and the Royal Society for generous
funding, the EPSRC National Mass Spectrometry Service
(Swansea), QMUL for a studentship (J.C.), and the Nuffield
Foundation for a summer bursary (M.R.-L.).
[8] For recent copper(I)-mediated protodecarboxylation
reactions, see: a) L. J. Gooßen, N. Rodriguez, B.
Melzer, C. Linder, G. Deng, L. M. Levy, J. Am. Chem.
Soc. 2007, 129, 4824; b) L. J. Gooßen, W. R. Thiel, N.
Rodriguez, C. Linder, B. Melzer, Adv. Synth. Catal.
2007, 349, 2241; c) L. J. Gooßen, F. Manjolinho, B. A.
Khan, N. Rodriguez, J. Org. Chem. 2009, 74, 2620.
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