Palladium-Catalyzed Primary Amine-Directed Decarboxylative Annulation of α-Oxocarboxylic Acids: Access to Indolo[1,2-a]quinazolines

Article abstract of DOI:10.1002/adsc.201900001

An efficient protocol for the preparation of indolo[1,2-a]quinazolines via palladium-catalyzed decarboxylative annulation of indols with α-oxocarboxylic acids has been realized by using primary amine as a directing group (DG). This transformation proceeds smoothly with exclusive regioselectivity and represents an one-pot Domino synthesis of indo-lo[1,2-a]quinazolines from α-oxocarboxylic acids. (Figure presented.).

Full text of DOI:10.1002/adsc.201900001

Title: Palladium-Catalyzed Primary Amine-directed Decarboxylative
Annulation of α-Oxocarboxylic Acids : Access to
Indolo[1,2‑a]quinazolines
Authors: Guangbin Jiang, Shoucai Wang, Jun Zhang, Jianwen Yu,
Ziang Zhang, and Fanghua Ji
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900001
Link to VoR: http://dx.doi.org/10.1002/adsc.201900001

10.1002/adsc.201900001
Advanced Synthesis & Catalysis
Very Important Publication
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium-Catalyzed Primary Amine-directed Decarboxylative
Annulation of α-Oxocarboxylic Acids : Access to Indolo[1,2-
a]quinazolines
Guangbin Jianga,*, Shoucai Wanga, Jun Zhanga, Jianwen Yua, Ziang Zhanga, and
Fanghua Jia,*
a
Guangxi Key Laboratory of Electrochemical and Magneto-chemical Function Materia, College of Chemistry and
Bioengineering, Guilin University of Technology, Guilin 541004, China.
E-mail: jianggb@glut.edu.cn, fanghuaji@glut.edu.cn; Fax: 86-773-899-1304
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. An efficient protocol for the preparation of such as ketones,[8] indoles,[9] pyrimidines,[10] etc., have
indolo[1,2‑ a]quinazolines
via
palladium-catalyzed
been involved in the decarboxylative coupling
reactions with α-oxocarboxylic acids.[11] Despite such
great achievements, however, in a striking contrast to
the abundant protocols for the synthesis of acylated
products by employing directing group strategy
(Scheme 1, a), the access to functionalized
heterocyclic compounds from α-oxocarboxylic acids
is still limited (Scheme 1, b).[7a]
decarboxylative annulation of indols with α-oxocarboxylic
acids has been realized by using primary amine as a
directing group (DG). This transformation proceeds
smoothly with exclusive regioselectivity and represents an
one-pot Domino synthesis of indo-lo[1,2‑ a]quinazolines
from α-oxocarboxylic acids.
Keywords: palladium-catalyzed, , α-oxocarboxylic acids,
decarboxylative annulation, directing group
Scheme 1. Directing Group-Assisted Decarboxylative
Cross-Coupling of α-Oxocarboxylic Acids
Transition-metal-catalyzed decarboxylative cross-
coupling is an efficient and robust synthetic protocol
for carbon-carbon bond formation since it does not
rely on the use of stoichiometric metallorganics and
produces CO2 instead of often toxic metal by-
products.[1]
Over
decarboxylative cross-coupling reactions have
achieved unprecedented α-
development.[2]
the
past
decades,
the
Oxocarboxylic acids have been recognized as
valuable coupling reagents in the field of
decarboxylation coupling reactions.[3] Therefore, to
date, transition-metal-catalyzed decarboxylative cross
coupling reactions involving α-oxocarboxylic acids
have developed a widespread attention.[4] Among
them, directing groups assisted decarboxylative
coupling of α-oxocarboxylic acids are particularly
attractive due to their excellent regioselectivity and
reactivity.[5] For instance, Ge et al. disclosed an
elegant procedure for the construction of diverse o-
acyl acetanilides via palladium-catalyzed acylation
reaction of α-oxocarboxylic acids and acetanilides.[6]
Very recently, wang and co-workers reported that α-
amino acids and pyridines could be used successfully
as a directing group in decarboxylative coupling
reaction.[7] In addition, a variety of chelating group,
Nitrogen-containing heterocycles are important
components in numerous pharmaceutical compounds
and natural products compounds and natural products
that exhibit a wide range of biological activities,[12]
such as antitumor, anti-parasitic and antifungal.
Among them, indolo[1,2‑ a]quinazolines are one of
the most significant N-heterocycles found in
functional molecules.[13] In this context, considerable
efforts have been devoted to develop practical
strategies
for
the
synthesis
of
indolo[1,2‑ a]quinazolines.[14] Unfortunately, the
efficient synthesis methods of functionalized
indolo[1,2‑ a]quinazolines is rarely reported. Herein,
we described a palladium catalyzed decarboxylative
annulation of indoles and α-oxocarboxylic acids,
1
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10.1002/adsc.201900001
Advanced Synthesis & Catalysis
providing the indolo[1,2‑ a]quinazolines in excellent toluene had no better results on the reaction efficiency
yields under mild conditions (Scheme 1, b). The (Table 1, entries 11-13). To our delight, 3a was
transformation is probably initiated by primary detected in 81% isolated yield by replacing 1,4-
amine-directed C(sp2)-H palladation, followed by dioxane with diglyme (Table 1, entry 10 vs 14).
decarboxylative annulation of the α-oxocarboxylic Finally, decreasing or increasing the reaction
acids.
temperature would erode the yield of 3a slightly
(Table 1, entries 15-16).
Table 1. Optimization of Reaction Conditionsa
Scheme 2. Scope of 2-(1H-indol-1-yl)anilinesa, b
entry
1
catalyst
PdCl2
oxidant
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Cu(OAc)2
AgOAc
p-BQc
solvent
yieldb (%)
43
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
2
Pd(OAc)2
Pd(TFA)2
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
61
3
47
4
n.d.
26
5
6
trace
trace
trace
69
7
8
9
(NH4)2S2O8 1,4-dioxane
10
11
12
13
14
15e
16f
Na2S2O8
1,4-dioxane
DMSO
52
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
n.d.
n.d.
n.d.
86 (81)
75
DMF
toluene
diglymed
diglyme
diglyme
72
a) Reaction Conditions: 1a (0.2 mmol), 2a (0.4 mmol), catalyst (5
mol %), oxidant (2 equiv) and diglyme (2.0 mL) were added to a
test tube at 78 oC under air for 12 h.
b) Determined by GC and dodecane as internal standard; Numbers
in parentheses are yields of isolated; n.d. = not detected.
c) p-BQ = p-Benzoquinone.
d) diglyme = 1-Methoxy-2-(2-methoxyethoxy)ethane.
e) At 70 oC.
f) At 85 oC.
a) Reaction conditions: a mixture of 1 (0.2 mmol), 2a (0.4 mmol),
(NH4)2S2O8 (0.4 mmol), Pd(OAc)2 (5 mol %), and diglyme (2.0
mL) were added to a test tube at 78 °C for 12 h.
b) Isolated yields.
c) 4 equiv of 2m were used.
To demonstrate the efficiency and generality of this
palladium-catalyzed
decarboxylative
annulation
reaction, we have tested the transformation of 2-oxo-
2-phenylacetic acid with a variety of 2-(1H-indol-1-
yl)anilines under the optimized reaction conditions
and the representative examples are summarized in
Scheme 2. Pleasingly, various 2-(1H-indol-1-
yl)anilines with different substituent groups on the
aromatic ring underwent decarboxylative annulation
with 2-oxo-2-phenylacetic acid to furnish the
corresponding products 3a-3r in moderate to
excellent yields. Substrates 1b and 1c with methyl-
substituted gave the corresponding products 3b and
3c in 79% and 76% yields, respectively. The indole
rings bearing either halogen (-F, -Cl), or electron-
donating groups (-methoxy, -benzyloxy-), at C4, C5,
C6 position were able to transfer to the desired
annulation products 3d-3g and 3i-3l in moderate to
excellent yields (72%-82%). Cyano-substituted
substrate was compatible with our standard reaction
We initiated our investigation on the model
reaction of 2-(1H-indol-1-yl)aniline (1a) with 2-oxo-
2-phenylacetic acid (2a) to screen various reaction
parameters. After intensive screening of the catalytic
systems (Table 1), we are pleased to find the
corresponding product 3a could be obtained in 43%
GC yield with PdCl2 (5 mol %) as the catalyst,
K2S2O8 as the oxidant in 1,4-dioxane at 110 oC for 12
h (Table 1, entry 1). Palladium catalysts such as
Pd(OAc)2, Pd(TFA)2, Pd(PPh3)4, and Pd(PPh3)2Cl2
were also screened, and the results demonstrated that
Pd(OAc)2 was the best catalyst (Table 1, entries 2-5).
Next an oxidant screening was carried out to improve
the yield (Table 1, entries 6-10), and it was found that
the GC yield of desired product could be improved to
69% with (NH4)2S2O8 as the oxidant (Table 1, entry
9). Switching the solvents to DMSO, DMF, and
2
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10.1002/adsc.201900001
Advanced Synthesis & Catalysis
conditions, albeit in 54% yield (3h). Remarkably, 2- 83% yields. Notably, 2-(naphthalen-1-yl)-2-oxoacetic
(7-fluoro-1H-indol-1-yl)aniline 1m was found to acid was also suitable for this transformation and
yield corresponding product 3m in an inferior yield transferred to the annulation product 3ag in 73% yield.
(35%), albeit 4 equiv of 2m were introduced Unfortunately, 2-oxohexanoic acid failed to produce
incrementally. Finally, substrates with electron- the annulation product under the current reaction
donating and -withdrawing groups on the aniline rings conditions (3ah). Moreover, several heterocycles,
could undergo this reaction smoothly, delivering the related to medical chemistry,[15] including furan and
annulation products with good yields (3n-3r).
benzothiophene, were also suitable substrates in this
reaction conditions (3ai-3aj).
Scheme 3. Scope of α-oxocarboxylic acidsa, b
Scheme 4. Control Experiments
Several control experiments were subsequently
conducted to develop a deeper understanding of the
reaction mechanism (Scheme 4). When 3.0 equiv of
radical scavengers TEMPO (2,2,6,6-tetramethyl-1-
piperidinyloxy) and BHT (3,5-di-tert-butyl-4-
hydroxytoluene) were added under the current
reaction conditions, the corresponding annulation
product 3a was also obtained in 76% and 80%
isolated yields, respectively (Scheme 4, a). This
observation revealed that the reaction process should
not be a radical mechanism. In addition, parallel
competition reactions between 2-(1H-indol-1-
yl)aniline (1a) and 2-(1H-indol-1-yl-2-d)aniline (1a-
d1) were carried out under the standard reaction
conditions and revealed a kinetic isotope effect (kH/kD
= 2.4, Scheme 4, b), indicating that the cleavage of C-
H bond of 2-(1H-indol-1-yl)anilines is likely to be the
rate-limiting step in the overall process.
a)
Reaction conditions: a mixture of 1a (0.2 mmol), 2 (0.4 mmol),
(NH4)2S2O8 (0.4 mmol), Pd(OAc)2 (5 mol %), and diglyme (2.0
mL) were added to a test tube at 78 °C for 12 h
b) Isolated yields.
Scheme 5. Proposed Mechanism
Subsequently, we examined a variety of α-
oxocarboxylic acids and pleased to find that this
decarboxylative annulation strategy was very efficient.
The results were summarized in Scheme 3. Under the
standard reaction conditions, a variety of 2-oxo-2-
arylacetic acids with o-Me, o-F, and o-Br substituents
were
tested,
delivering
the
expected
indolo[1,2‑ a]quinazolines 3s-3u in 63-83% yields.
The annulation reaction was compatible with
disubstituted 2-oxo-2-arylacetic acid to give the
corresponding product in 61% yield (3v). To our
delight,
a
series of meta-substituted 2-oxo-2-
arylacetic acids, such as -F, -Br, -OCH3 substitutions,
were transformed to annulation products in acceptable
yields (3w-3y). Similarly, the decarboxylative
annulation could tolerate with either electron-
donating or electron-poor groups including the CH3,
Et, i-Bu, t-Bu, OCH3, Cl, and Br at the para-position,
affording the corresponding products 3z-3af in 67-
A plausible mechanism for this palladium-catalyzed
decarboxylative annulation reaction is outlined in
Scheme 5.[16][5] First, the palladacycle intermediate
3
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10.1002/adsc.201900001
Advanced Synthesis & Catalysis
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Int-2 was formed via coordination of the palladium
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α-oxocarboxylic acids (2) to generate
a
cyclopalladated complex Int-3 along with release of
HOAc. Decarboxylation of Int-3 followed by
reductive elimination provides acylated compounds
Int-4. Simultaneously, the Pd(0) can be reoxidized to
the active Pd(II) species with (NH4)2S2O8. This
acylated products then cyclized onto the carbonyl
groups followed by dehydration to lead to the desired
product indolo[1,2‑ a]quinazolines.
In summary, an unique and efficient decarboxylative
annulation reaction of 2-(1H-indol-1-yl)anilines with
α-oxocarboxylic acids has been reported via a primary
amine directed C(sp2)-H bond functionalization
process. This transformation is the representative
example of decarboxylative annulation of α-
oxocarboxylic acids by employing directing group
strategy. A wide range of indolo[1,2‑ a]quinazolines
were obtained in moderate to excellent yield.
Significantly, this protocol features exclusive
regioselectivity, step economy and mild reaction
conditions. We anticipate that this strategy may be
applicable for other C-H functionalization/annulation
reactions.
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Experimental Section
Typical Experimental Procedure for Product 3
A mixture of 0.2 mmol of 2-(1H-indol-1-yl)anilines (1), 0.4
mmol of α-oxocarboxylic acids (2), 0.4 mmol (NH4)2S2O8,
5 mol % of Pd(OAc)2, and 2.0 mL of diglyme were added
to a test tube equipped with a magnetic stirring bar. The
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o
mixture was then stirred at 78 C under air for 12 h. After
the reaction was completed (monitored by TLC), the
resulting mixture were cooled to room temperature and
extracted with ethyl acetate. The combined organic layers
were evaporated under vacuum. The desired products 3
were obtained in the corresponding yields after purified by
column chromatography on silica gel with mixture of
petroleum ether and ethyl acetate.
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Acknowledgements
This work was supported by the Ph. D. Scientific Research
Foundation of Guilin University of Technology and Guangxi
Natural Science Fundation (2017GXNSFBA198224 and
2018GXNSFAA281203) and Key Laboratory of Electrochemical
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5
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10.1002/adsc.201900001
Advanced Synthesis & Catalysis
COMMUNICATION
Palladium-Catalyzed Primary Amine-directed
Decarboxylative Annulation of α-Oxocarboxylic
Acids : Access to Indolo[1,2‑ a]quinazolines
Adv. Synth. Catal. 2018, Volume, Page-Page
Guangbin Jianga,*, Shoucai Wanga, Jun Zhanga,
Jianwen Yua, Ziang Zhanga, and Fanghua Jia,*
6
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