Palladium/Copper-Catalyzed Denitrogenative Alkylidenation and ortho-Alkynylation Reaction of 1,2,3-Benzotriazin-4(3 H)-ones

Article abstract of DOI:10.1021/acs.orglett.9b04297

An efficient palladium-catalyzed approach to access various functionalized (Z)-3-benzylidene-isoindoline-1-ones and (Z)-3-benzylidene(imino)isobenzofuranones via a denitrogenative tandem alkynylation/cyclization reaction of 1,2,3-benzotriazin-4(3H)-ones w

Full text of DOI:10.1021/acs.orglett.9b04297

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium/Copper-Catalyzed Denitrogenative Alkylidenation and
ortho-Alkynylation Reaction of 1,2,3-Benzotriazin-4(3H)‑ones
Madasamy Hari Balakrishnan† and Subramaniyan Mannathan*,‡
†Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, India
‡Department of Chemistry, SRM University AP, Amaravati, Andhra Pradesh 522502, India
S
* Supporting Information
ABSTRACT: An efficient palladium-catalyzed approach to access various functionalized (Z)-3-benzylidene-isoindoline-1-ones
and (Z)-3-benzylidene(imino)isobenzofuranones via a denitrogenative tandem alkynylation/cyclization reaction of 1,2,3-
benzotriazin-4(3H)-ones with aromatic terminal alkynes is described. Furthermore, a denitrogenative ortho-alkynylation
reaction of 1,2,3-benzotriazinones with aliphatic terminal alkynes is also developed. The reaction proceeds through a five-
membered azapalladacyclic intermediate with the extrusion of a nitrogen molecule. The significance of the reaction is also
demonstrated by a one-pot synthesis of (Z)-3-benzylideneisobenzofuran-1(3H)-one derivatives in good to high yields.
aged by this result, we continued our interest in search of new
ransition-metal-catalyzed denitrogenative transannulation
Treactions of 1,2,3-triazoles such as pyridotriazoles, N- coupling reactions of 1,2,3-benzotriazinones. Herein, we report
a Pd/Cu-catalyzed denitrogenative tandem alkynylation/
cyclization reaction of 1,2,3-benzotriazinones with aromatic
terminal alkynes to synthesize highly stereoselective (Z)-3-
aryl(alkyl)idene isoindolin-1-one and (Z)-3-benzylidene-
(imino)isobenzofuranone derivatives. As compared to iso-
quinolone synthesis, isoindolinones from 1,2,3-benzotriazi-
nones are less commonly studied and notably by employing
terminal alkynes as the coupling partner with 1,2,3-
benzotriazinones are yet to be developed (Figure 1). In
addition to this cyclization reaction, a method is also
developed with aliphatic terminal alkynes to access ortho-
alkynylated benzamides in good yields (Figure 1).
3-Substituted isoindolin-1-ones are one of the important
heterocyclic compounds which are present in numerous drugs,
natural products, and materials.8 Particularly, 3-arylmethylene
isoindolin-1-one has been recognized as a vital scaffold not
only because of its remarkable biological activities but also as a
useful intermediate for the synthesis of various alkaloids.9 The
literature report reveals that transition-metal-catalyzed tandem
alkynylation/cyclization reaction was perhaps one of the most
sulfonyl-1,2,3-triazoles, 1,2,3-benzotriazones, and N-aroylben-
zotriozoles have emerged as a powerful method in the
preparation of various N-heterocycles relevant to natural
products and pharmaceuticals.1,2 In this context, the research
groups of Murakami,3 Liu,4 and Cheng5 studied a variety of
transannulation reactions of 1,2,3-benzotriazinones with
various π-components such as alkynes, alkenes, allenes,
isocyanides, and benzynes to prepare functionalized isoquino-
lones and isoindolinone derivatives in a highly effective
manner. Mechanistically, the reaction proceeds through a
five-membered aza-metallacycle intermediate, which is in sharp
contrast to other 1,2,3-triazoles that proceed via a metal-
locarbene intermediate,1 and extrude only molecular nitrogen
as effluent which is eco-friendly in nature. Recently, a visible-
light-promoted regioselective denitrogenative insertion of
terminal alkynes into 1,2,3-benzotriazinones to synthesize
substituted isoquinolones was also described by Yu and Wang.6
Despite these significant contributions on annulation reactions,
denitrogenative cross-coupling reactions involving 1,2,3-
benzotriazones are scarcely studied and relatively unknown.
Very recently, our group7a and Cheng et al.7b independently
reported a nickel-catalyzed denitrogenative biaryl synthesis
using 1,2,3-benzotriazinones and organoboronic acids. Encour-
Received: November 29, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
b
yield (%)
entry
ligand
copper salts
solvent
3aa
4aa
c
1
2
3
4
5
6
7
8
Xantphos
PPh3
dppe
CuCN
CuCN
CuCN
CuCN
CuCN
CuCN
CuI
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
97 (92)
3
9
14
10
52
40
38
Figure 1. Metal-catalyzed denitrogenative reactions of 1,2,3-
benzotriazinones with terminal alkynes.
dppp
e
phen
bpy
f
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
48
10
74
49
40
5
7
frequently utilized strategies to synthesize 3-arylmethylene
isoindolin-1-one derivatives in a highly stereoselective
manner.10 In 2000, Kundu and his co-workers reported a
palladium-catalyzed stereoselective synthesis of (Z)-3-aryl-
(alkyl)idene isoindolin-1-ones using ortho-halo benzamides
and terminal alkynes as substrates.10a Since then, more efficient
catalyst systems involving palladium10b−d and copper10e−i were
developed to improve its scope and versatility. To avoid the
use of ortho-halo benzamides in the reaction, the directing
group assisted C−H activation strategy has been developed as
an excellent alternative for the synthesis of isoindolinone
derivatives.11 However, to perform these transformations,
special directing groups such as 2-aminoquinoline,11a−c
pyridine oxides,11d and 2-aminophenyl-1H-pyrazole11e and, in
most cases, a higher reaction temperature are always necessary.
Despite these notable efforts, the development of mild,
efficient, and environmentally friendly strategies toward the
synthesis of isoindolinone molecules is still in high demand.
The reaction of 1,2,3-benzotriazin-4-(3H)-one (1a) with
phenyl acetylene (2a) in the presence of Pd(OAc)2 (10 mol
%) as the catalyst, Xantphos (20 mol %) as the ligand, and
CuCN (10 mol %) as an additive in DMSO at 100 °C for 12 h
under a nitrogen atmosphere afforded (Z)-3-benzylidene-
isoindoline-1-one (3aa) in 92% isolated yield (Table 1, entry
1). The reaction is highly stereoselective, where the desired
product formed as a Z-isomer. Control experiments revealed
that, in the absence of a palladium catalyst, no reaction
occurred. Changing the ligand from Xantphos to other
phosphine ligands such as PPh3, dppe, and dppp, the desired
product was obtained only in 38−52% yield. Nitrogen ligands
such as 2,2′-bipyridine and 1,10-phenanthroline are not
compatible with the present catalytic reaction. Other copper(I)
salts like CuI and CuBr were also active but furnished the
product 3aa in 48 and 10% yields, respectively. However, in
the absence of copper salt, the reaction yielded the desired
product in a very low yield (entry 16). The choice of solvent is
essential in obtaining the desired product in high yield. Of all
of the solvents that we screened, DMSO appeared to be the
best, and other common solvents such as DMF, 1,2-
dichloroethane (DCE), and toluene were relatively less
effective. Finally, decreasing the reaction temperature lowered
the product yield to 85%. As shown in Table 1, (Z)-N-((Z)-3-
benzylideneisobenzofuran-1(3H)-ylidene)-4-methoxyaniline
(4aa) was observed as the competitive side product; however,
when the reaction was carried out in CH3CN solvent, 4aa was
obtained as a primary product in 75% yield (entry 13).
CuBr
9
CuCN
CuCN
CuCN
CuCN
CuCN
CuCN
CuCN
10
11
12
13
14
15
NMP
DCE
3
toluene
CH3CN
DMSO
DMSO
DMSO
9
13
86
85
5
75
2
3
d
g
16
a
All of the reactions were carried out using 1,2,3-benzotriazin-4-(3H)-
one (1a) (0.40 mmol), phenyl acetylene (2a) (0.60 mmol),
Pd(OAc)2 (0.04 mmol), ligand (0.04−0.08 mmol), additive (0.04
mmol), and solvent (dry), 2 mL at the mentioned temperature for 12
b
c
d
h under N2. GC yields. Isolated yield. The reaction was carried out
e
f
g
at 80 °C. phen = 1,10-phenanthroline. bpy = 2,2′-bipyridine. The
reaction was carried out at 80 °C for 24 h.
benzotriazinones (1b−h). As shown in Scheme 1, the reaction
is compatible with N-aryl-substituted 1,2,3-benzotriazinones
affording the corresponding products 3ba−ea in good to high
Scheme 1. Synthesis of (Z)-3-Benzylidene-isoindoline-1-
ones
a b
,
a
All of the reactions were carried out using 1,2,3-benzotriazin-4-(3H)-
ones 1 (0.40 mmol), aromatic terminal alkynes 2 (0.60 mmol),
Pd(OAc)2 (0.04 mmol), Xantphos (0.04 mmol), and CuCN (0.04
Under optimized reaction conditions, we have investigated
the scope of the cyclization reaction with a range of 1,2,3-
b
mmol) for 12 h. Isolated yields.
B
DOI: 10.1021/acs.orglett.9b04297
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Organic Letters
Letter
yields. On the other hand, reactions involving N-alkyl-
substituted 1,2,3-benzotriazinones were unsuccessful. Benzo-
triazinones having fluoro (1f), methyl (1g), and carbonyl (1h)
substituents on the benzene ring also participated in the
reaction, giving the desired products 3fa−ha in 81, 95, and
84% yields, respectively. Similarly, the reaction of 1a with a
variety of terminal alkynes (2b−j) afforded the desired (Z)-3-
aryl methylene isoindolin-1-ones in good to high yields. For
instance, substituted phenyl acetylenes (2b−i) containing
electron donating and neutral groups both at the para- and
meta-positions gave the corresponding products 3ab−ai in
75−89% yields. Single-crystal X-ray analysis of 3ab further
confirms the structure and the stereoselectivity.12 Hetero-
aromatic alkyne, 2-ethynylpyridine (2j), also furnished the
product 3aj in 71% yield. Unfortunately, aliphatic alkynes such
as 1-hexyne (2k), ethynylcyclohexane (2l), ethynylcyclopro-
pane (2m), and ethynylcyclopentane (2n) failed to deliver the
desired product 3. However, tri-isopropyl silylacetylene (2o)
underwent a coupling reaction smoothly with 1a even in the
absence of CuCN to give ortho-alkynlated benzamides 5ao in
95% yield. In a similar fashion, 2o also underwent coupling
with 1b, 1g, 1i, and 1j to give the desired products in 81−97%
yields (Scheme 2), albeit TMS acetylene failed to participate in
the reaction.
isobenzofuranone synthesis with other benzotriazinones 1 and
substituted terminal alkynes 2. Aromatic alkynes bearing
methyl- (2b), ethyl- (2c), n-pentyl- (2d), t-butyl- (2e),
methoxy- (2f), and fluoro- (2g) substituents at the para-
position underwent a cyclization reaction, giving 4ab−ag in
good yields. meta-Substituted aromatic terminal alkynes 2h and
2i also gave 4ah and 4ai in 74 and 75% yield, respectively.
Pleasingly, 3-ethynyl thiophene (2p) also furnished the desired
products in good yields. Correspondingly, 1f, 1g, and 1k also
successfully participated in the reaction, giving 4fa, 4ga, and
4ka in 80, 82, and 85% yields, respectively. It is important to
mention that ortho-substituted aromatic terminal alkynes are
not suitable for the present denitrogenative tandem alkynyla-
tion/cyclization reaction. Moreover, in all of the reactions, as
shown in Scheme 3, isoindoline-1-ones 3 were observed in 5−
15% yield.
Scheme 3. Synthesis of (Z)-3-Benzylidine
a b
,
(Imino)isobenzofuranones
a b
,
Scheme 2. Synthesis of ortho-Alkynylated Benzamides
a
a
All of the reactions were carried out using 1,2,3-benzotriazin-4-(3H)-
ones 1 (0.40 mmol), terminal alkynes 2k−o (0.60 mmol), Pd(OAc)2
All of the reactions were carried out using 1,2,3-benzotriazin-4-(3H)-
ones 1 (0.40 mmol), aromatic terminal alkynes 2 (0.60 mmol),
Pd(OAc)2 (0.04 mmol), Xantphos (0.04 mmol), and CuCN (0.04
b
(0.04 mmol), and Xantphos (0.04 mmol) for 12 h. Isolated yields.
b
mmol) for 12 h. Isolated yields.
Excited by the formation of substituted alkynes 5ao−jo, we
next turned our attention to find a suitable reaction condition
for the ortho-alkynylation reaction of 1 with other aliphatic
alkynes 2k−n (see the Supporting Information). Interestingly,
by changing the solvent from DMSO to a binary solvent
comprised of CH3CN and THF in a ratio of 1:1, the desired
ortho-alkynylated products 5ak−an were obtained in good to
high yields (Scheme 2). To our surprise, when the reaction was
carried out with phenylacetylene (2a) under similar reaction
conditions, (Z)-3-benzylidine (imino)isobenzofuranone (4aa)
was selectively obtained in 85% isolated yield. This result is
highly interesting because, in the reported tandem alkynyla-
tion/cyclization reactions, the examples are limited only to the
preparation of isoindolin-1-one derivatives, whereas the
synthesis of (Z)-3-(imino)isobenzofuranones13 has yet to be
explored. Thus, we also studied the scope of the (imino)-
A gram scale reaction of 1a with 2a was performed.
Gratifyingly, the reaction proceeded smoothly in the presence
of 5 mol % Pd(OAc)2 and afforded 3a in a 90% yield.
Subsequently, the significance of the present catalytic reaction
is also demonstrated by achieving a one-pot synthesis of
biologically relevant (Z)-3-benzylideneisobenzofuran-1(3H)-
one derivatives14 6a−d from 1a and 2a via a tandem
alkynylation/cyclization and hydrolysis process (Scheme 4a).
In addition, a Pd/C-catalyzed reduction reaction of 3 and 4
was also carried out in acetic acid at rt for 2 h to obtain 3-
benzylated isoindolinones15 7a−b and ortho-alkylated benza-
mides 8a−b in good to high yields (Scheme 4c).
To understand the role of CuCN in the tandem
alkynylation/cyclization process, we have performed a
controlled experiment by treating N-(4-methoxyphenyl)-2-
C
DOI: 10.1021/acs.orglett.9b04297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Synthetic Applications
Scheme 6. Proposed Mechanism
(phenylethynyl)benzamide (5aa) in the presence of CuCN
(10 mol %) in DMSO solvent at 100 °C (Scheme 5). After 12
(OAc−) to give intermediate 11. Subsequent reductive
elimination of 11 affords 5 and regenerates the Pd(0) for
the further catalytic cycle. If the alkyne is aromatic, the
resultant 5 may undergo facile N- and O-cyclization under the
suitable standard reaction conditions to give 3 and 4,
respectively. In the case of nonaryl acetylenes, the cyclization
of 5 is not favorable under our present reaction conditions
possibly due to the poor coordination of alkyne to metal
complex because of the bulkier TIPS substituent. Similarly, the
presence of an aliphatic moiety at another end of acetylene also
might have left alkyne 5 less activated for the cyclization. This
was further confirmed by the results shown in Schemes 1 and
3.
Scheme 5. Mechanistic Studies
The exact reasons for the chemoselective O- versus N-
cyclization reaction by changing the solvents are unclear at the
moment. However, we assume that the metalation of amide
oxygen16a,17 and the change in solvent polarity might have
favored the O-cyclization reaction leading to 3-(imino)-
isobenzofuranones. During the course of studies, we did not
observe any isoquinolinone derivatives, which is in contrast to
the previously reported nickel-catalyzed reaction3a with the
same set of starting materials. This is probably due to the slow
reactivity of azapalladacycle 9 toward alkyne insertion.
Moreover, the generation of copper acetylides during the
reaction might have led to the facile formation of intermediate
11.
In conclusion, we have successfully developed the first
denitrogenative alkylidenation reaction of 1,2,3-benzotriazin-
4(3H)-ones with aromatic terminal alkynes using a palladium
complex as the catalyst. By fine-tuning the solvents, we can
selectively prepare (Z)-3-benzylidene-isoindoline-1-ones and
(Z)-3-(imino)isobenzofuranones in good to high yields.
Furthermore, synthetically useful ortho-alkynylated benzamides
were also synthesized in good to high yields using aliphatic
alkynes as a coupling partner. The significance of the reaction
is also demonstrated by a one-pot synthesis of (Z)-3-
benzylideneisobenzofuran-1(3H)-one derivatives in good to
high yields.
h, the reaction afforded 3aa only in 45% yield10e due to poor
conversion of 5aa. When the same reaction was carried out
with Pd(OAc)2 (10 mol %) and Xantphos (10 mol %), the
desired product 3aa was obtained in 72% yield. The yield of
3aa was improved to 95% when 5aa was treated in the
presence of both Pd(OAc)2 and CuCN. Similarly, cyclo-
merization of 5aa also yielded 4aa16 in high yield by changing
the solvent from DMSO to the binary solvent comprising
CH3CN and THF. These results show that both palladium and
copper are necessary for the cyclomerization process to obtain
3aa and 4aa in high yield. Moreover, it also appears that
CuCN played a vital role not only in the transmetalation step
to obtain 11 but also in the cycloisomerization process in
improving the product yields of 3 and 4.
A plausible catalytic reaction mechanism is proposed, as
shown in Scheme 6. Oxidative addition of palladium(0) to
1,2,3-benzotriazin-4(3H)-ones affords five-membered aza-
nickelacyclic intermediate 9.3d The reaction of 9 with terminal
alkyne 2 in the presence of CuCN affords intermediate 11. On
the other hand, the reaction of TIPS acetylenes proceeds
through an intermediate 10 which then reacts with base
D
DOI: 10.1021/acs.orglett.9b04297
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04297.
■
S
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Experimental procedures and characterization of all
products (PDF)
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Adv. Synth. Catal. 2018, 360, 284−289.
Accession Codes
CCDC 1963275−1963276 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mannathan.s@srmap.edu.in.
ORCID
Subramaniyan Mannathan: 0000-0002-3114-8265
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Science and Technology (DST),
New Delhi, India (DST/INSPIRE/04/2015/002987), for
support of this research under a DST-INSPIRE faculty
scheme. We also extend our sincere thanks to Prof. M.
Sasidharan, SRM Institute of Science and Technology,
Chennai, for providing laboratory facilities.
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DOI: 10.1021/acs.orglett.9b04297
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