Pd/C-catalyzed synthesis of N-aryl and N-alkyl isoquinolones via C[sbnd]H/N[sbnd]H activation

Article abstract of DOI:10.1016/j.cattod.2017.02.005

Pd/C-catalyzed direct synthesis of N-aryl and N-alkyl isoquinolones was developed via the annulation reactions of benzamides and alkynes in high yields (up to 99%) through the cleavage of C[sbnd]H/N[sbnd]H bonds. The reaction was ligand-free and air was used as oxidant. High regioselectivities were found when unsymmetrical alkynes or meta-benzamides were used as substrates. The heterocyclic carboxamide substrates, such as furan and thiophene derivatives, also afforded the corresponding products in high yields.

Full text of DOI:10.1016/j.cattod.2017.02.005

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Catalysis Today
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Pd/C-catalyzed synthesis of N-aryl and N-alkyl isoquinolones via
C H/N H activation
Zhen Shu, Yuntao Guo, Wei Li∗, Baiquan Wang∗
College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of
Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd/C-catalyzed direct synthesis of N-aryl and N-alkyl isoquinolones was developed via the annulation
reactions of benzamides and alkynes in high yields (up to 99%) through the cleavage of C H/N H bonds.
The reaction was ligand-free and air was used as oxidant. High regioselectivities were found when unsym-
metrical alkynes or meta-benzamides were used as substrates. The heterocyclic carboxamide substrates,
such as furan and thiophene derivatives, also afforded the corresponding products in high yields.
© 2017 Elsevier B.V. All rights reserved.
Received 11 September 2016
Received in revised form 24 January 2017
Accepted 5 February 2017
Available online xxx
Keywords:
C
H activation
Pd/C
Synthetic method
Isoquinolones
1. Introduction
and anti-cancer drugs, etc. [31–35] (Scheme 1). The direct syn-
thesis of isoquinolones via annulation reactions of benzamides
In the recent years, atom economy and environment friendly are
the two hot topics which claim simplifying the steps and decreasing
the waste in reactions. Transition-metal-catalyzed C H activation
is one of the most promising methods to access this goal and
has achieved a great progress in the past decade. Among them,
and alkynes has achieved by homogeneous Rh(III) [36–46], Ru(II)
[47–52], Pd(II) [53–55], Ni(II) [56], Co(II) [57], and I(III) [58,59] cata-
lysts in recent years. Different oxidant sources can result in different
products. The use of N O group as an internal oxidant will give
N
H isoquinolones and the use of an external oxidant will afford N-
homogeneous palladium is a common catalyst used for direct C
H
substituted isoquinolones [Scheme 2, Eqs. (1) & (2)]. Very recently,
our group successfully developed the direct synthesis of N-alkoxyl
isoquinolones with Pd/C as the heterogeneous catalyst [Scheme 2,
Eq. (3)] [30], but we couldn’t obtain the corresponding product of
low active N-aryl/N-alkyl benzamides. In this work, we successfully
achieved the direct synthesis of N-aryl/N-alkyl isoquinolones using
Pd/C as catalyst and air as external oxidant [Scheme 2, Eq. (4)].
activation as its high activity [1–5]. However, an obvious draw-
back of homogeneous palladium catalysts is hard to recover after
reactions. The requirement of precious palladium often hampers
commercial access to such methodology. One way to solve this
problem is the application of heterogeneous palladium, such as
Pd/C (palladium on carbon) [6–13]. As a commercial hydrogena-
tion catalyst, Pd/C has also been widely used in other organic
synthesis, including Heck, Suzuki, and Sonogashira reactions; the
construction of heterocyclic compound, and other coupling reac-
2. Results and discussion
tions [6–13]. In contrary, Pd/C has only limited applications in C
activation [14–30], and is a promising catalyst in the transition-
metal-catalyzed C H activation in future.
H
2.1. Optimization of reaction conditions
At the beginning of our study, we used N-phenylbenzamide (1a)
and diphenylacetylene (2a) as starting materials, Pd/C as catalyst,
air as oxidant, NaI as additive, Cs2CO3 and KOAc as base, DMAc
(dimethylacetamide) as solvent, and 120 ◦C as the temperature. The
product 3aa was obtained in 24% yield (Table 1, entry 1). Then,
several solvents were tested in this reaction. To our delight, when
DMF was used as the solvent, 3aa was obtained in nearly quan-
titative yield (Table 1, entry 2). In contrast, when EtOH, toluene,
CH2Cl2, H2O, hexane, CH3CN, THF, and dioxane were used as sol-
Isoquinolone skeleton is a basic heterocyclic structure and
widely exists in the natural alkaloids. Isoquinolone derivatives have
well biological activities used for lower blood pressure, antiviral,
∗
Corresponding authors.
E-mail addresses: weili@nankai.edu.cn (W. Li), bqwang@nankai.edu.cn
(B. Wang).
http://dx.doi.org/10.1016/j.cattod.2017.02.005
0920-5861/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Z. Shu, et al., Pd/C-catalyzed synthesis of N-aryl and N-alkyl isoquinolones via C H/N H activation,
Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.005

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2.2. Scope of N-aryl benzamides and alkynes
Reactions of substituted N-aryl benzamides with different
alkynes were carried out in the optimal conditions (Table 1, entry
15). The results revealed that the reaction had a good tolerance
with both electron-donating and electron-withdrawing groups.
High yields of products were obtained generally when electron-
rich benzamides were participated in the reaction (Table 2, 3ba–da,
3ga–ka, 3oa). Unexpected, when 4-methoxy-N-phenyl benzam-
ide and N-(4-methoxyphenyl) benzamide were used slight lower
yields were obtained (Table 2, 3ca, 3ka), probably due to the strong
electron-donating effect of methoxy group. In contrast, lower yields
were got if the benzamide had an electron-withdrawing group
(Table 2, 3ea–fa, 3la–na). No obviously steric effect was obtained
during the reaction scope (Table 2, 3ga, 3ia, 3oa, 3qa). Good yields
were also obtained when heterocyclic carboxamides were used
(Table 2, 3ra–sa). Then, different alkynes were tested. Substituted
diphenylacetylene showed same disciplinarian with substituted
benzamides and the electronic effect was more obviously. Dipheny-
lacetylene with an electron-donating group delivered higher yield
than that with an electron-withdrawing group (Table 2, 3ab–ac).
Comparable, alkyl phenyl alkynes showed low activities (Table 2,
3ad–ae). High regioselectivities were found when meta-substituted
benzamides and unsymmetrical alkynes were used as substrates
(Table 2, 3ha–ia, 3ad–ae).
Scheme 1. Some N-aryl/N-alkyl isoquinolones derivatives used for anti-cancer
drugs.
vents, no desired product was obtained (Table 1, entries 3–8, 11,
12), while DMSO and NMP (N-methyl-2-pyrrolidone) were used as
solvents the product was obtained in 67% and 27% yields, respec-
tively (Table 1, entries 9, 10). Lowering the temperature decreased
evidently the yield (Table 1, entries 13, 14). The yield was still quan-
titative when the ratio of 1a and 2a reduced to 1:2 (Table 1, entry
15). Further reduced the ratio, 1a could not be exhausted (Table 1,
entries 16, 17). The reaction could not occur in the absence of Pd/C
or NaI and the yield was apparently declined in the absence of
Cs2CO3 or KOAc (Table 1, entries 18–21). When 3.0 equiv. of KOAc
or Cs2CO3 was used instead of the mixed bases the yields of 3a
dropped to 73% and 48%, respectively (Table 1, entries 22–23). No
product was detected when the reaction underwent in the absence
of air (Table 1, entry 24).
Scheme 2. Direct synthesis of isoquinolones via annulations of benzamides and alkynes.
Please cite this article in press as: Z. Shu, et al., Pd/C-catalyzed synthesis of N-aryl and N-alkyl isoquinolones via C H/N H activation,
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Table 1
Optimization of reaction conditionsa.
Entry
Solvent
Temp (◦C)
1a:2a
Yield (%)b
1
2
3
4
5
6
7
8
DMAc
DMF
EtOH
toluene
CH2Cl2
H2O
hexane
CH3CN
DMSO
NMP
THF
dioxane
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
120
120
120
120
120
120
120
120
120
120
120
120
100
80
120
120
120
120
120
120
120
120
120
120
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:2
1:1.5
1:1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
24%
99%
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
67%
27%
N.R.
N.R.
89%
49%
99%
98%
88%
N.R.
N.R.
27%
35%
73%
48%
N.R.
9
10
11
12
13
14
15
16
17
18c
19d
20e
21f
22g
23h
24i
a
Reaction conditions: 1a (0.3 mmol), 10% Pd/C (10 mol%), NaI (0.6 mmol), Cs2CO3 (0.3 mmol), KOAc (0.6 mmol), solvent (1.0 mL), 36 h, air.
b
c
d
e
f
Isolated yields. N.R. = no reaction.
Without Pd/C.
Without NaI.
Without Cs2CO3.
Without KOAc.
g
h
i
3.0 equiv. of KOAc was used instead of Cs2CO3 (1.0 quiv.) and KOAc (2.0 equiv.) as base.
3.0 equiv. of Cs2CO3 was used instead of Cs2CO3 (1.0 quiv.) and KOAc (2.0 equiv.) as base.
Under an atmosphere of argon.
2.3. Scope of N-alkyl benzamides with alkynes
To extend the reaction scope, reactions of N-methyl benza-
mides with alkynes were carried out (Table 3). The reaction also
showed well tolerance with both electron-donating and electron-
withdrawing groups (Table 3, 5ba–fa, 5ab–ac, 5af–ah). But strong
electronic effect had a bad influence to generate the product. Benza-
mides and alkynes with OMe or CF3 group afforded relative low
yields (Table 3, 5ca, 5fa, 5ab, 5ah). The use of N-nBu benzamide also
gave desired product in high yield (Table 3, 5ga). When unsymmet-
rical alkynes were used, the corresponding products were obtained
in low yields, but with excellent regioselectivities (Table 3, 5ad,
5ae).
Scheme 3. Competition reaction of different substrates.
2.4. Competition study
palladium content was tested in reaction and after reaction. After
hot filtration the palladium content in hot solution was tested as
high as 2978 ppm, indicating that the true catalyst is homogeneous
palladium species which released into solution by aerobic oxida-
tion and stabilized by coordination of iodide and acetate anions.
However, the palladium content of the solution is only 26 ppm after
reaction. This indicated that the homogeneous Pd species could go
back to carbon after reaction. The rapid loss of activity of palla-
dium may be caused by the transformation of palladium into the
inactivated Pd-species, rather than running off into the solution. To
activate the recovered Pd/C catalyst, N2H4 aqueous solution was
added as reductant at the end of reaction. After treatment, Pd/C
The competition study of 1b and 1f in reaction with 2a was
done (Scheme 3). The results further proved that the electron-
rich benzamides had higher activity compared to the electron-poor
benzamides.
2.5. Recycle of Pd/C catalyst
At last, we carried out the recycle experiment of Pd/C catalyst
in the reaction of 1a and 2a under the standard conditions. Product
3aa was obtained in 99%, 65%, and 30% yields after the first, second,
and third runs. To investigate the deactivation of Pd/C catalyst, the
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Table 2
Scope of N-aryl benzamides and alkynes[a]
.
[a] Reaction conditions: 1 (0.3 mmol), 2 (0.6 mmol), 10% Pd/C (10 mol%), NaI (0.6 mmol), Cs2CO3 (0.3 mmol), KOAc (0.6 mmol), DMF (1.0 mL), air, 36 h. [b] Isolated yields.
displayed a better recycle activity. Product 3aa was obtained in
85%, 80%, and 78% yields after the first, second, and third runs,
respectively (Fig. 1).
product and Pd(0) after reductive elimination. The regenerated
Pd(0) species can be used for the next catalytic cycle (Scheme 4).
3. Experimental section
2.6. Possible mechanism
3.1. General information
A possible mechanism was proposed on the basis of literature
research [30,53–55]. Firstly, Pd(0)/C was oxidized to active homo-
geneous Pd(II) species A by the oxygen of air. Then, reaction of A
with benzamide in the assistance of NaI and base generated five-
membered palladacycle intermediate B. Coordination and insertion
of an alkyne into the Pd-C bond afforded seven-membered pal-
ladacycle intermediate C, which was easily converted into the final
1H NMR (400 MHz), 19F (376 M Hz), and 13C NMR (101 MHz)
were recorded on a NMR spectrometer with CDCl3 as solvent. Col-
umn chromatography was performed on silica gel 200–300 mesh
or alumina 200–300 mesh. IR spectra were recorded as KBr disks on
a FT-IR spectrometer. High-resolution mass spectrometry (HRMS)
was done on a FTICR-mass spectrometer. Palladium content was
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Table 3
Scope of N-alkyl benzamides and alkynes[a]
.
[a] Reaction conditions: 4 (0.3 mmol), 2 (0.9 mmol), 10% Pd/C (10 mol%), NaI (0.6 mmol), Cs2CO3 (0.3 mmol), KOAc (0.6 mmol), DMF (1.0 mL), air, 36 h. [b] Isolated yields. [c]
140 ◦C was used. [d] 2 equiv of CsOAc was used instead of KOAc.
tested by TAS-990 Atomic Absorption Spectrophotometer (AAS) (Pd
absorption wavelength = 244.8 nm). The N-aryl and N-alkyl benza-
mides [37–39,47,53] and substituted alkynes [60] were prepared
following the literature procedures. All other compounds are com-
mercially available without any further purification.
3.2. General procedure
A mixture of a substituted benzamide (1) (0.3 mmol, 1.0 equiv),
an alkyne (2) (0.6 mmol or 0.9 mmol, 2.0 equiv or 3.0 equiv),
10% Pd/C (0.03 mmol, 10 mol%, Alfa Aesar, No. 044696, eggshell,
Fig. 1. Recycle of Pd/C catalyst.
reduced), NaI·2H2O (0.6 mmol, 2.0 equiv), Cs2CO3 (0.3 mmol, 1.0
equiv), and KOAc (0.6 mmol, 2.0 equiv) was weighted in a Schlenk
tube equipped with a stir bar. DMF (1.0 mL) was added and the
mixture was stirred at 120 ◦C for 36 h under air. Afterwards, the
mixture was filtered and washed with H2O (30 mL) and extracted
with CH2Cl2 (3 × 30 mL). The combined organic phase was dried
with anhydrous Na2SO4. After removal of solvents under reduced
pressure, the residue was absorbed to small amounts of silica. The
purification was performed by flash column chromatography on
silica gel with EA:PE (Petroleum ether) = 1:5 or 1:10 as eluent.
3.3. Recycle of Pd/C catalyst
After reaction, the mixture was cooled down to 0 ◦C and 0.5 mL of
80% N2H4 aqueous solution was added dropwise. Stirred for 20 min,
the mixture was filtered. The residue (Pd/C and base) was washed
with ethanol (2*5 mL) and water (2*5 mL). All the organic phase
was combined. The purification was performed by flash column
Scheme 4. Proposed mechanism.
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Acknowledgment
We thank the National Natural Science Foundation of China
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Please cite this article in press as: Z. Shu, et al., Pd/C-catalyzed synthesis of N-aryl and N-alkyl isoquinolones via C H/N H activation,
Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.005