Ru-catalyzed decarboxylative annulations of α-keto acids with internal alkynes: Dual roles of cooh as directing group and leaving group

Article abstract of DOI:10.1002/chem.201405715

Carboxylic acid serving as both directing and leaving group was discovered in Ru-catalyzed decarboxylative annulations of α-keto acids with alkynes. The wellestablished protocol showed high functional group tolerance, which provided an efficient and alter

Full text of DOI:10.1002/chem.201405715

DOI: 10.1002/chem.201405715
Communication
&
Catalysis
Ru-Catalyzed Decarboxylative Annulations of a-Keto Acids with
Internal Alkynes: Dual Roles of COOH as Directing Group and
Leaving Group
Hui Tan,[a] Hongji Li,*[a] Jiawang Wang,[a] and Lei Wang*[a, b]
a
directing group in cross-coupling reactions [Scheme 1,
Eq. (3)].[8] For selected example, Yu and co-workers recently
found Pd-catalyzed arylation of sp2 CÀH bonds in simple car-
boxylic acids.[8a,b] Daugulis et al. had developed two Pd-cata-
lyzed methods for the ortho-arylation of benzoic acids.[8c] In
fact, other types of carboxylic acid have also been studied due
to the ease of their decarboxylation.[9] For a-keto acid as a typi-
cal example, Ge,[10a–c] Kim,[10d,e] Wang,[10f] and others[10g–k] report-
ed the successful application of a-keto acids in group-directing
acylation reactions using a transition metal as catalyst. Al-
though carboxylates as leaving groups or directing groups in
cross-coupling reactions have made major breakthrough, there
are still some shortcomings that cannot be ignored, such as
the use of expensive metal, complicated synthesis of function-
alized substrates, and so on. As part of our ongoing effort in
decarboxylative cross-coupling reactions,[11] we focus on the re-
action of readily available and cheap a-keto acids with simple
alkynes. Herein, we wish to report the first highly regioselective
Ru-catalyzed decarboxylative annulations of a-keto acids with
internal alkynes to the construction of coumarone backbone.[12]
It is important to note that the dual roles of COOH serving as
both directing group and leaving group were observed in the
reactions [Scheme 1, Eq. (4)].
Abstract: Carboxylic acid serving as both directing and
leaving group was discovered in Ru-catalyzed decarboxy-
lative annulations of a-keto acids with alkynes. The well-
established protocol showed high functional group toler-
ance, which provided an efficient and alternative route to
the coumarone backbone.
Recently, decarboxylative coupling reactions have received in-
creasing attention because carboxylic acids represent one of
the most stable and available organics at low cost from natural
and synthetic sources.[1] Particularly, a number of transforma-
tions of carboxylic acids in the past decade have been realized
and applied in the synthesis of many useful organic com-
pounds.[2] In this field, the pioneering work by Myers and co-
workers has contributed to the advance of decarboxylation,
which showed us a novel Pd-catalyzed decarboxylative Heck-
type coupling of olefin with aromatic carboxylic acids.[3] Then
Cu,[4a–f] Rh,[4g,h] Ni[4i] and other metals[4j–n] were also found to be
effective in the decarboxylative coupling reactions.[4] Very re-
cently, Lei et al. developed a visible-light mediated decarboxy-
lation/oxidative amidation of a-keto acids with the assistance
of photocatalyst [Ru(phen)3]Cl2.[5] It should be noted that Ru
and its complexes have never been directly used as catalyst in
the decarboxylative coupling reactions.
Generally, carboxylic acids can serve as synthetic equivalents
of acyl, aryl, or alkyl halides, or of organometallic reagents in
the defined catalytic system.[6] From this point, we think that
the carboxylic acids can be extended to other coupling reac-
tions. Basically, the carboxylic acid mainly functions as a leaving
group in the above decarboxylative reactions [Scheme 1,
Eqs. (1) and (2)].[7] For instance, Goossen,[7a,b] Liu,[7c] and Bone-
si[7d] reported the direct coupling of a variety of benzoic acids
with aryl halides or triflates in the suitable catalyst systems, re-
spectively. Moreover, the carboxylic acid also could be used as
[a] H. Tan, Dr. H. Li, J. Wang, Prof. Dr. L. Wang
Department of Chemistry, Huaibei Normal University
Huaibei, Anhui 235000 (P.R. China)
E-mail: leiwang@chnu.edu.cn
[b] Prof. Dr. L. Wang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Shanghai 200032 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405715.
Scheme 1. The roles of COOH: directing group and/or leaving group.
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The investigation began with the decarboxylative annulation
of 2-oxo-2-phenylacetic acid (1a) and 1,2-diphenylethyne (2a)
with a transition-metal catalyst (5 mol%) in the presence of
Cu(OAc)2 as an oxidant at 1208C in air (Table 1). We initially
found that Pd(OAc)2, Pd(TFA)2 or RuCl3·nH2O did not catalyze
the decarboxylative annulations (entries 1–3). To our delight, the
model reaction proceeded smoothly using [Cp*RuCl2]n as cata-
lyst and generated the desired product 3a in 13% isolated yield
(entry 4). It was found that [Ru(p-cymene)Cl2]2 showed better
catalytic activity under the same reaction conditions (entry 5).[13]
Then the use of potassium 2,4,6-trimethylbenzoate (MesCOOK)
as additive had no effect on the decarboxylative annulation of
1a with 2a (entry 6). Interestingly, it was found that 2,4,6-trime-
thylbenzoic acid (MesCOOH) as additive was required for the im-
proved yield of the model reaction, and actually 52% yield of
3a was obtained (entry 7). Based on these findings, 2,2-dime-
thylpropanoic acid (PivOH) was next tried as additive in this
model reaction under similar reaction conditions, and afforded
3a in 69% isolated yield (entry 8).[13] However, employing strong
organic acid as additive, such as trifluoroacetic acid (TFA) or p-
toluenesulfonic acid (TsOH), resulted in inferior yields of 3a (en-
tries 9 and 10). Further optimization showed that increasing the
amount of Ru catalyst had no apparent effect on the reaction
(entry 11). Subsequently, screening of oxidants in the reaction
of 1a with 2a was examined. It was found that di-tert-butyl
peroxide (DTBP), di-tert-amyl peroxide (DCP), Ag2CO3 and
PhI(OAc)2 did not help in promoting this catalytic reaction (en-
tries 12–15). Additionally, we found that this reaction was not
sensitive to water, and a comparable yield was observed with
Cu(OAc)2·nH2O as oxidant instead of anhydrous Cu(OAc)2
(entry 16). The further optimization of reaction time and tem-
perature did not improve the yield of 3a (entries 17–19). Then
the molar ratio of 1a/2a (1.5:1) was found to be requisite for
the model decarboxylative annulation (entries 8, 20 and 21).
After an extensive screening, it turned out that DMF was the
optimal solvent, and the desired product 3a was obtained in
satisfactory yield (Table S1 in the Supporting Information).
Based on the optimized reaction conditions, the substrate
scope was investigated with respect to the a-keto acids and
internal alkynes (Scheme 2). The reaction could tolerate a wide
range of substitutes within a-keto acids, and moderate to
good yields were obtained under the optimal conditions. The
electron-donating groups (Me, MeO, tBu and nBu) at para-posi-
tion of the phenyl rings seemed to give lower yields of corre-
sponding products (3a–3e). In addition, halogens, such as F
and Cl, allowed the formation of the desired products 3 f and
Table 1. Optimization of the reaction conditions.[a]
Entry
Catalyst
Oxidant
Additive
Yield [%][b]
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(TFA)2
RuCl3·nH2O
[Cp*RuCl2]n
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
[RuCl2L]2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DTBP
–
–
–
–
0
0
0
13
35
37
52
69
–
MesCOOK
MesCOOH
PivOH
TFA
9
trace
10
10
11
12
13
14
15
16
17
18
19
20
21
TsOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
70[c]
n.r.
n.r.
n.r.
n.r.
64
DCP
Ag2CO3
PhI(OAc)2
Cu(OAc)2·nH2O
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
61[d]
65[e]
66[f]
70[g]
47[h]
[a] Reaction conditions: 2-oxo-2-phenylacetic acid (1a, 0.30 mmol), 1,2-di-
phenylethyne (2a, 0.20 mmol), [Ru(p-cymene)Cl2]2 (0.01 mmol, containing
5.0 mol% Ru), oxidant (0.20 mmol), additive (0.20 mmol), DMF (0.50 mL)
at 1208C in air for 12 h; [b] isolated yield; [c] 10 mol% Ru catalyst;
[d] 1108C; [e] 1308C; [f] 20 h; [g] 1a/2a=2:1; [h] 1a/2a=1:2; Cp*=pen-
tamethylcyclopentadienyl, L=p-cymene; n.r.=no reaction.
Scheme 2. Substrate scope of internal alkynes. [a] Reaction conditions: 2-
oxo-2-arylacetic acid (1, 0.30 mmol), alkyne (2, 0.20 mmol), [Ru]=[Ru(p-cym-
ene)Cl2]2 (0.01 mmol, containing 5.0 mol% Ru), Cu(OAc)2 (0.20 mmol), PivOH
(0.2 mmol), DMF (0.50 mL) at 1208C in air for 12 h. [b] Isolated yields.
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3g, in comparable yields. Strangely, there is no obvious steric
hindrance observed in this investigation (3b, 3h, and 3i vs.
3k). It is worth noting that hydroxyl group (HO) within a-keto
acid had little effect on the decarboxylative annulation, and
a moderate yield of 3j was achieved. Moreover, this reaction
was not limited to a-keto acids with phenyl groups. For exam-
ple, the reaction of 2-(naphthalen-1-yl)-2-oxoacetic acid and 2-
oxo-2-(thiophen-2-yl)acetic acid with 1,2-diphenylethyne still
produced the corresponding product 3l in 68% yield, and 3m
in 51% yield, respectively. Next, a number of symmetric inter-
nal alkynes were synthesized and tested in the above decar-
boxylative annulation. 1,2-Diarylethynes with Me and F at the
4,4’- or 3,3’-position of phenyl ring gave good yields of desired
products (3n, 3o, 3q and 3r), with the exception of Cl with
which a lower yield of 3p was obtained. Further investigation
regarding the reaction of oct-4-yne with 2-oxo-2-phenylacetic
acid was performed under similar conditions, and 3s was ob-
tained in 56% isolated yield. Gratifyingly, hex-3-yne could react
with 2-oxo-2-phenylacetic acid to produce 3t in higher yield.
As expected, several mono- or disubstituted phenylglyoxylic
acids were effective in this decarboxylative annulation and
generated the desired products (3u–3z) in moderate to good
yields (62–77%).
Scheme 5. Proposal reaction mechanism.
sults indicate that only a carboxyl or carbonyl group within the
substrate cannot start the annulation. In contrast, co-existence
of carboxyl group and carbonyl group within the a-keto acid is
beneficial to the occurrence of decarboxylative annulation. Ad-
ditionally, CO2 release from the catalytic system was clearly ob-
served by FT-IR (Figure S1 in the Supporting Information).
Hence, the COOH group within the a-keto acid might serve as
an effective directing group and leaving group in the Ru-cata-
lyzed decarboxylative annulation.
In order to determine the role of the carboxyl group within
a-keto acid, the following control experiments were carried
out under standard conditions, as shown in Scheme 3. It was
found that benzaldehyde or 2-phenylacetic acid did not react
with internal alkyne 2a. When benzoic acid was used as sub-
strate, only 7% yield of 3a was obtained. Compared with the
former substrates, 2-oxo-2-phenylacetic acid with 2a had high
activity and product 3a was isolated in 69% yield. These re-
In order to probe the possible mechanism, an intermolecular
competition experiment between proton and deuteron 1a
was performed under optimized conditions, and the KIE value
of 4.0 showed that CÀH bond cleavage might be involved in
the rate-limiting step [Scheme 4, Eq. (1)]. In addition, the Ru-
catalyzed decarboxylative annulation of 1a with a-keto acid
2a was carried out under an isotope-labeled oxygen (18O2) at-
mosphere. A significant molecular ion peak for product 3a
containing 18O was observed by HRMS (in the Supporting In-
formation) which indicated that the oxygen atom in product
3a came from O2 [Scheme 4, Eq. (2)].
According to our experimental results and previous re-
ports,[14] a possible mechanism is outlined in Scheme 5. The ini-
tial anion exchange of [Ru(p-cymene)Cl2]2 with Cu(OAc)2 afford-
ed the complex I, which then reacted with a-keto acid to form
intermediate intermediate II with
Scheme 3. Control experiments. [a] Reaction conditions: A (0.30 mmol), 1,2-
diphenylethyne (2a, 0.20 mmol), [Ru]=[Ru(p-cymene)Cl2]2 (0.01 mmol, con-
taining 5.0 mol% Ru), Cu(OAc)2 (0.20 mmol), PivOH (0.20 mmol), DMF
(0.50 mL) at 1208C in air for 12 h. [b] Isolated yields.
releasing HOAc. The coordina-
tion of internal alkyne 2 with II,
and subsequent migratory inser-
tion produced intermediate III,
which then released CO2 to form
the key intermediate IV with the
assistance of O2 in air.[4e] The
final reductive elimination of IV
led to the formation of prod-
uct 3 and realized the catalytic
Scheme 4. Kinetic isotope effect and oxygen labeling experiment.
cycle.
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Typical procedure for the decarboxylative annulation of a-
keto acids with internal alkynes
A 10 mL oven-dried reaction vessel equipped with a magnetic stir-
rer bar was charged with 2-oxo-2-phenylacetic acid (1a,
0.30 mmol), 1,2-diphenylethyne (2a, 0.20 mmol), [Ru(p-cymene)Cl2]2
(6.0 mg, 0.01 mmol), Cu(OAc)2 (36.2 mg, 0.20 mmol), PivOH
(20.4 mg, 0.20 mmol), and DMF (0.50 mL). The reaction vessel was
placed in an oil bath. After the reaction was carried out at 1208C
for 12 h, it was cooled to room temperature and extracted with
EtOAc (3ꢁ5.0 mL). The organic layers were combined, dried over
MgSO4, and concentrated to yield the crude product, which was
further purified by flash chromatography (silica gel, petroleum
ether/ethyl acetate=15:1 to 30:1) to give the desired product 3a
in 69% yield (41.1 mg).
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 21172092, 21402060) for financial support.
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Keywords: alkynes · alpha-oxocarboxylic acids · annulation ·
decarboxylation · ruthenium catalysis
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Received: October 18, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
&
Catalysis
H. Tan, H. Li,* J. Wang, L. Wang*
&& – &&
Ru-Catalyzed Decarboxylative
Annulations of a-Keto Acids with
Internal Alkynes: Dual Roles of COOH
as Directing Group and Leaving Group
Ru experienced: Carboxylic acid groups
serving as both directing and leaving
groups were discovered in Ru-catalyzed
decarboxylative annulations of a-keto
acids with alkynes. The well-established
protocol showed high functional group
tolerance, which provided an efficient
and alternative route to the coumarone
backbone.
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ