Gold(I)-catalyzed protodecarboxylation of (Hetero)aromatic carboxylic acids

Article abstract of DOI:10.1002/chem.201303200

Readily available, inexpensive and easy to handle, carboxylic acids have been shown to be very effective, greener coupling partners compared to costly organometallic reagents for the formation of C-C bonds. The use of well-defined gold complexes furnished 3 in slightly better yield with butyric acid, and in quantitative yield with adamantane-1-carboxylic acid. All reactions reached completion within 16 h. As with silver systems, this reactivity trend highlights, as previously observed, the benefits of potential coordinating groups in the ortho position to the gold binding site, which possibly facilitate the decarboxylation step. Additional reaction time and increased temperatures were necessary to afford the gold aryl products in satisfactory yields. Yet, some substrates such as 2-nitrobenzoic acids reacted poorly and could only be transformed in 50% yield.

Full text of DOI:10.1002/chem.201303200

DOI: 10.1002/chem.201303200
Gold(I)-Catalyzed Protodecarboxylation of (Hetero)Aromatic Carboxylic
Acids
Stꢀphanie Dupuy and Steven P. Nolan*[a]
Readily available, inexpensive and easy to handle, carbox-
ylic acids have been shown to be very effective, greener cou-
volves the use of as much as 20 mol% of expensive Pd-
AHCTUNGTRENNUNG
(TFA)2 as the catalyst.[9] The use of high catalyst loadings
pling partners compared to costly organometallic reagents
also requires expensive and time-consuming product purifi-
cation steps, which are unattractive for applications in the
pharmaceutical industry, for example.
[1]
À
for the formation of C C bonds. In these transformations,
carbon nucleophiles are generated after decarboxylation of
the carboxylic acid and subsequently coupled with a range
of electrophiles.[2] However, these procedures have long suf-
fered from the requirement for high temperatures. Develop-
ing more effective catalyst systems for the decarboxylation
step, with the protodecarboxylation as a model reaction, has
appeared to be key in accessing lower temperature decar-
boxylative couplings for use with a broader range of sub-
strates. Moreover, protodecarboxylation, on its own, is of
particular interest as a method to remove carboxylic acid
moieties used as directing groups in the synthesis of com-
plex molecules.[3] Protodecarboxylation was extensively
studied in the 1970s with stoichiometric amounts of Hg,[4]
Cu[5] and Ag.[6] The first report of a catalytic protodecarbox-
ylation reaction appeared in 2007 and was pioneered by
Goossen and co-workers. The reaction was carried out with
10 mol% of Cu2O/phenanthroline/quinoline.[7] This group
showed that the use of chelating nitrogen ligands as well as
aromatic amines as solvent greatly enhanced the rate of the
decarboxylation. The procedure was applicable to a broad
range of benzoic acids and was particularly efficient with
a nitro group in the ortho position but could also be applied
to meta- and para-substituted benzoic acids. However, the
procedure required high temperatures (160–1908C) and the
use of the expensive and bulky bathophenanthroline (4,7-di-
phenyl-1,10-phenanthroline) ligand for non-activated benzo-
ic acids. The reaction also proceeds when the metal centre is
changed to silver[8] or palladium.[9] With the former, al-
though lower temperatures (ca. 1208C) can be accessed, the
system is limited to ortho-substituted benzoic acids and het-
erocyclic derivatives. Similarly, very low temperatures (ca.
708C) can be reached with palladium but the substrate
range is narrowed to very electron-rich benzoic acids such
as 1,3,5-trimethoxybenzoic acid derivatives. Moreover it in-
In 2010, we reported a straightforward procedure to
obtain stable aryl gold complexes by decarboxylation of
(hetero)aromatic carboxylic acids with the Brønsted base
[AuACHTNUTRGNEUNG(IPr)(OH)] (IPr=1,3-bis(2,6-diisopropylphenyl)-imida-
zol-2-ylidene) complex at low temperature.[10] This first in-
sight into the decarboxylation reaction with gold revealed
a similar reactivity trend to silver. Indeed, the methodology
was particularly effective with electron-rich ortho-substitut-
ed benzoic acids or heterocyles bearing a carboxylic moiety
in the a position. The gold(I)-mediated decarboxylation of
(hetero)aromatic carboxylic acids prompted us to explore
the catalytic potential of [AuACTHUNGTREN(NUNG NHC)(OR)] (NHC=N-
heterocyclic carbene) in the protodecarboxylation reaction.
AHCTUNGTRENNUNG
We report herein the protodecarboxylation of (hetero)aro-
matic carboxylic acids catalyzed by gold (Scheme 1).
Scheme 1. Proposed gold(I)-catalyzed protodecarboxylation of benzoic
acids.
The new system permits low-temperature decarboxylation
with activated substrates and most importantly combines the
advantages of both copper and silver methods (activated
and deactivated substrates). To the best of our knowledge,
this is the first report of a protodecarboxylation reaction
with a catalytic amount of gold(I).
[a] S. Dupuy, Prof. Dr. S. P. Nolan
EaStCHEM, School of Chemistry
University of St. Andrews
During our exploration of the reactivity of [Au-
North Haugh, St. Andrews, Fife, KY16 9ST (UK)
E-mail: snolan@st-andrews.ac.uk
A
ACHTUNGTRENNUNG(IPr)-
AHCTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303200.
at 1108C in wet toluene afforded 1,3-dimethoxybenzene (3)
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COMMUNICATION
instead of the expected gold aryl complex after 2 h. The in
situ formation of one equivalent of AcOH was believed to
be responsible for the cleavage of the Caryl–Au bond. We
surmised that it would be possible to develop a general one-
step protodecarboxlation reaction with only a catalytic
but led to variable results with pivalic acid (Table 1, en-
tries 13 and 14).
The use of well-defined gold complexes furnished 3 in
slightly better yield with butyric acid (Table 1, entry 15) and
in quantitative yield with adamantane-1-carboxylic acid
(Table 1, entry 16). To circumvent handling of hazardous,
amount of [AuACHTUNGTRENNUNG(NHC)(OR)] complexes under milder condi-
tions than previously reported. Initially, the catalyst system
(Table 1) was assessed.
corrosive and unpleasantly odorous acid, [Au
AHCTUNTGREGN(NUN O2CAd)](4) was finally selected as the optimum catalyst
ACHTUNGTRENNUNG(SIPr)-
system. Under these optimized conditions, 80% of the prod-
uct was formed after reaction times of only 8 h.
We then explored the substrate scope and substituent ef-
fects under the optimized conditions. Selected results are
summarized in Table 2. A range of electron-rich ortho-sub-
Table 1. Gold-catalyzed protodecarboxylation of 2,6-dimethoxy-benzoic
acid.[a]
Table 2. Gold-catalyzed protodecarboxylation of (hetero)aromatic car-
boxylic acids.[a]
Entry[a] Catalyst (mol%)
Acid
T [8C] Yield [%][b]
1[c]
2[c]
3[c]
4
5
6
7
8
9
10
11
12
13
14
15
16
17
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
N
G
–
–
110
110
110
110
110
110
120
120
120
120
120
120
100
20
23
50
100
55
85
100
70
63
35
90
90
100
95
100
65
AcOH
AcOH
AcOH
AcOH
AcOH
PivOH
TFA
PhCO2H
TfOH
(Piv)2O
C3H7CO2H 120
AdCO2H
–
–
–
[Au
[Au
N
120
120
120
120
G
E
G
ACHTUNGTRENNUNG
[a] Reaction conditions: [Au], additive (1 equiv), benzoic acid
(0.5 mmol), anhydrous toluene (1.5 mL), 16 h. [b] GC yield with n-tetra-
decane as internal standard. [c] Technical grade toluene.
[a] Reaction conditions: 4 (2 mol%), substrate (0.5 mmol), anhydrous
toluene (1.5 mL), 1208C, 16 h. Isolated yield given unless otherwise
stated. [b] NMR yield with either hexamethylbenzene or 1,3,5-trimethox-
ybenzene as internal standard. [c] 3 mol% catalyst.
Decreasing the catalyst loading to 5 mol% led to a signifi-
cant drop in the yield (Table 1, entry 2). Treatment of [Au-
ACHTUNGTRENNUNG(IPr)(OH)] in situ with AcOH did not dramatically affect
the yield of the reaction (Table 1, entry 3). Noteworthily, al-
though the reaction could be conducted in the absence of
additives, the results obtained were variable and reproduci-
bility issues were encountered. The use of anhydrous tolu-
ene was found to greatly enhance the rate and yield of the
reaction (Table 1, entry 4). Alternatively, changing the
ligand from IPr to SIPr (SIPr=1,3-bis(2,6-diisopropylphen-
yl)-4,5-dihydroimidazol-2-ylidene) resulted in quantitative
yield suggesting that, as previously reported,[7] more elec-
tron-rich (basic) complexes favour the decarboxylation step
(Table 1, entry 5).[11] An increase of the temperature to
1208C was necessary to maintain good yields when only
stituted benzoic acids and heteroaromatic carboxylic acids
(heteroatom in the a position) were found to undergo decar-
boxylation, affording the corresponding arenes in good to
excellent yields. All reactions reached completion within
16 h. As with silver systems, this reactivity trend highlights,
as previously observed, the benefits of potential coordinat-
ing groups in the ortho position to the gold binding site,
which possibly facilitate the decarboxylation step.[8] Gratify-
ingly, higher yields, compared to silver and copper, could be
obtained with cinnamic acid (76% with Ag and 0% with
Cu) and electron-deficient isoquinoline-1-carboxylic acid
(92% with Ag and 85% with Cu)[12] with our catalyst
system, although 3 mol% was necessary for the latter for
full conversion of substrate. As expected, pentafluorobenzo-
ic acid was also successfully employed as it is known that
the decarboxylation of polyfluorinated benzoic acids pro-
ceeds reasonably well.
3 mol% of [AuACHTUNGTRENNUNG(SIPr)(OH)] was used (Table 1, entry 7). The
role of mild to strong acid additives was then probed. Pivalic
acid, butyric acid and adamantane-1-carboxylic acid all af-
forded 3 in quantitative yield (Table 1, entries 8, 13 and 14).
A further decrease in catalyst loading still gave high turn-
overs with butyric acid and adamantane-1-carboxylic acid
Chem. Eur. J. 2013, 19, 14034 – 14038
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14035

S. P. Nolan and S. Dupuy
In our first report, we observed that decarboxylation of
ortho-substituted benzoic acids bearing electron-withdraw-
ing groups were a particular challenging class of substrates
with gold in comparison to copper and silver systems.[10] Ad-
ditional reaction time and increased temperatures were nec-
essary to afford the gold aryl products in satisfactory yields.
Yet, some substrates such as 2-nitrobenzoic acids reacted
poorly and could only be transformed in 50% yield. A
series of experiments was conducted with various electron-
deficient ortho-substituted benzoic acids under our opti-
mized conditions but led to modest results (ca. <50%
yield). Under adjusted reaction conditions, a wider range of
substrates could be successfully converted compared to our
initial report (Table 3).
the protodecarboxylation of meta- and para-toluic acid have
been reported under copper catalysis.[8a]
To expand the scope of the transformation and investigate
a possible similar reactivity trend between gold and copper
metals, we next embarked on an exploration of completely
de-activated aromatic carboxylic acids (Table 4).
Table 4. Gold-catalyzed protodecarboxylation of meta- and para-substi-
tuted benzoic acids.[a]
Table 3. Gold-catalyzed protodecarboxylation of substituted-benzoic
acids.[a]
[a] Reaction conditions: 4 (0.025 mmol), substrate (0.5 mmol), anhydrous
DMAc (1.5 mL), 1408C, 20 h. Isolated yield given unless otherwise
stated. GC yields are in parentheses. [b] 1658C. [c] 10 mol% catalyst for
48 h.
To our delight, under the same reaction conditions elec-
tron-rich para-substituted benzoic acids underwent smooth
decarboxylation to obtain the corresponding arenes in good
yields. In direct comparison, only 14% yield could be ob-
tained at 1608C with the silver system for the 4-methoxy-
benzoic acid.[12] At a higher temperature of 1658C, an even
wider range of deactivated carboxylic acids could be target-
ed, including m- and p-bromobenzoic acid and m- and p-ni-
trobenzoic acid (no conversion was observed under silver
catalysis).[12] However for the latter, the reactions could not
be pushed to the same degree of completion as 2-nitroben-
zoic acid. Even very electron-deficient 2-chloro-3,5-dinitro-
benzoic acids could be converted in 42% yield in 48 h with
10 mol% of catalyst. These results highlight the remarkable
reactivity enhancement induced by the (NHC)–gold(I)
system.
In 2009, Larrosa and co-workers reported the selective
monoprotodecarboxylation of aromatic dicarboxylic acids
exploiting the activating effect of the a heteroatom of pyri-
dines and ortho-substituents such as nitro or fluorine
groups.[8c]
Hence, to further explore the reactivity of our system, we
subjected (hetero)aromatic dicarboxylic acids, which were
previously used in the Larrosa study, to our methodology
(Table 5). Complete regioselectivity was observed with fluo-
rophthalic acids at 1208C to afford the resulting benzoic
acids in good yields.
[a] Reaction conditions: 4 (0.025 mmol), substrate (0.5 mmol), anhydrous
DMAc (1.5 mL), 1408C, 20 h. Isolated yield given unless otherwise
stated. [b] NMR yield with either hexamethylbenzene or 1,3,5-trimethox-
ybenzene as internal standard. [c] 3 mol% catalyst.
All reactions were analyzed after 20 h. Lower yields are
only due to incomplete conversion. In this regard, the proto-
decarboxylation proceeded smoothly with pyridyl rings and
nitro- and halogen-substituted benzoic acids that provide
opportunities for further functionalization of products. Only
traces of product were detected for very electron-deficient
2-chloropyridine and the reaction did not proceed at all with
2-cyanobenzoic acid. Interestingly, the latter substrate was
also reacted poorly with silver and copper systems (11%
with Ag and 0% with Cu).[13] As expected, lower reactivity
was observed with 2-nitrobenzoic acid, with the product iso-
lated in only 62% yield with our system which is in striking
contrast to copper and silver systems.[1c,1d,7–8] This suggests
that the mechanism may be different for Au/Cu/Ag. To our
satisfaction, even poorly activated ortho-toluic acid deriva-
tives were also amenable to the method, smoothly giving
the corresponding arenes in good yields. In comparison, the
latter were completely absent from the silver scope and only
The selected examples demonstrate that the scope of the
new Au system is broader than Pd systems and complemen-
tary to both Ag and Cu systems. This new procedure is not
only limited to ortho-substituted benzoic acids but also per-
forms well with meta- and para-substituted acids under simi-
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Gold(I)-Catalyzed Decarboxylation
COMMUNICATION
(OMe)2C6H3)]) have been previously reported.[10]
AHCTUNGTREGNU(NN IPr)ACHTNUGTREG(NNUN 2,6-CAHTNUGTRENNGUN
Table 5. Gold-catalyzed monoprotodecarboxylation of aromatic dicar-
boxylic acids.[a]
The reaction of 6 with adamantane-1-carboxylic acid in tolu-
ene at room temperature led to the quantitative formation
of 3 and 4.[14] Of note, isolated species 5 and 6 showed also
good (albeit lower) activity, in the protodecarboxylation of 2
and afforded 3 in 72 and 76% yield respectively.
On the basis of experimental observations, the following
mechanism for the gold-catalyzed protodecarboxylation
of (hetero)aromatic carboxylic acids can be proposed
(Scheme 3). The acid/base reaction between the carboxylic
[a] Reaction conditions: 4 (0.025 mmol), substrate (0.5 mmol), anhydrous
toluene (1.5 mL), 1208C, 20 h. Isolated yield given unless otherwise
stated. [b] 2 mol% of catalyst. [c] NMR conversion.
lar conditions to those used for Cu systems. A further ad-
vantage of our protocol is its simplicity. In toluene, the pure
product can be obtained after simple filtration to remove
the catalyst and acid. In N,N-dimethylacetamide (DMAc),
aqueous work-up allows for the removal of acids, and after
removing the solvent, the crude is triturated with pentane
and filtered off to eliminate the catalyst and afford the pure
product.
To shed light on the mechanism of the protodecarboxyla-
tion reaction, stoichiometric experiments were carried out
(Scheme 2). The reaction of 4 with 2 at 258C in toluene af-
Scheme 3. Proposed mechanism for the gold(I)-catalyzed protodecarbox-
ylation of (hetero)aromatic carboxylic acids.
acid 7 and 4 furnishes gold carboxylate species 8 that, after
liberation of CO2, leads to the formation of a gold aryl inter-
mediate 9. We believe that under catalytic conditions, both 7
and adamantane-1-carboxylic acid could be used as proton
À
sources to cleave the Au C bond of 9. This would most
likely be dependent on the pKa of the benzoic acid. Whereas
the reaction of 9 with adamantane-1-carboxylic would re-
generate catalyst 4, its reaction with starting material 7
would directly form the gold carboxylate species 8.
In summary, a general protocol for the Au-catalyzed pro-
todecarboxylation of (hetero)aromatic carboxylic acids has
been developed. It was successfully applied to activated and
de-activated benzoic acids and is compatible with a wide
range of functionalities. Notably, the reaction requires rela-
tively low catalyst loadings, and the isolation and purifica-
tion of the reaction products is straightforward; this protocol
is therefore both practical, and suitable for use in telescoped
reaction sequences for which column chromatographic pu-
Scheme 2. Isolation of intermediates in the protodecarboxylation reaction
mediated by gold(I).
forded a mixture of the gold carboxylate species 5 in equi-
librium with 4 in a 1/1 ratio (86% combined isolated yield).
Formation of AdCO2H during the reaction allows for the
À
À
cleavage of the newly formed Au O bond and makes the
process reversible. Intermediate 5 could nonetheless be ob-
rification is undesirable (e.g. in directed C H activation/pro-
todecaroboxylation/further functionalisation routes). More-
over, the isolation and full characterization of key reaction
intermediates has provided important insights into the
mechanistic details. Finally, the carboxylato complex 5 and
s-bonded gold aryl complex 6 have been shown themselves
to be active in the protodecarboxylation reaction and possi-
bly involved in the overall catalytic transformation. Further
mechanistic investigations on this and related transforma-
tions are currently underway in our laboratories.
tained cleanly by reaction of [AuACTHUNTGRNEUNG(SIPr)(OH)] with 2 as pre-
À
viously reported, due to the stability of the C Au bond to
presence of water.[10] A solution of 5 in anhydrous toluene
was heated at 1208C for 2 h to afford the gold aryl complex
6 in quantitative yield. Complexes 5 and 6 were fully charac-
terized by NMR spectroscopy and elemental analysis.
The crystal structures of the analogous complexes bearing
the IPr ligand ([AuCATHUNGTRENN(NUG IPr)AHCTNUTRGEGUN(NN O2C-2,6-ACHTUNGERTN(NUGN OMe)2C6H3)] and [Au-
Chem. Eur. J. 2013, 19, 14034 – 14038
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S. P. Nolan and S. Dupuy
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carboxylic acids: A mixture of carboxylic acid (0.5 mmol) and [Au
ACHTUNGTRNE(NUNG SIPr)-
ACHTUNGTRENNUNG(O2CAd)] (4) (0.025 mmol) in anhydrous DMAc (1.5 mL) was stirred at
1408C. After 20 h, the crude mixture was quenched with 2m HCl (2 mL)
and the aqueous phase extracted with Et2O (5 mL). The latter was
washed then with 1m NaOH (10 mL). The combined organic layers were
washed with brine (25 mL) and dried (MgSO4). The crude mixture was
concentrated and then triturated with pentane, filtered and washed with
pentane (3ꢁ2 mL). Concentration under reduced pressure gave the de-
sired pure product.
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Acknowledgements
We thank the EPSRC and the ERC (FUNCAT) for funding. SPN is
a Royal Society Wolfson Merit Award holder.
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Keywords: catalysis · decarboxylation · gold · N-heterocyclic
carbene · protodeauration
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Received: August 13, 2013
Published online: September 23, 2013
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Chem. Eur. J. 2013, 19, 14034 – 14038