Palladium-Catalyzed Annulation of Aryltriazoles and Arylisoxazoles with Alkynes

Article abstract of DOI:10.1002/adsc.201900435

We developed herein a palladium-catalyzed annulation of aryltriazoles and arylisoxazoles with internal alkynes via C?H bond activation process. 4,5-disubstituted-3H-naphtho[1,2-d][1,2,3]triazoles and 4,5-disubstituted-naphtho[2,1-d]isoxazoles could be afforded in good yields, respectively. The starting materials are readily available and the scope and applications of this transformation were explored. The reaction offers a practical approach to naphthalene fused heterocycles. (Figure presented.).

Full text of DOI:10.1002/adsc.201900435

Title: Palladium-Catalyzed Annulation of Aryltriazoles and
Arylisoxazoles with Alkynes
Authors: Hairui Yuan, Min Wang, Zhenghu Xu, and Hongyin Gao
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900435
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Advanced Synthesis & Catalysis
10.1002/adsc.201900435
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium-Catalyzed Annulation of Aryltriazoles and Arylisoxazoles with Alkynes
Hairui Yuan, Min Wang, Zhenghu Xu and Hongyin Gaoa*
a
a
a,b*
a
Key Lab for Colloid and Interface Chemistry of Education Ministry, Department of Chemistry, Shandong University
No. 27 South Shanda Road, Jinan, 250100 (China), E-mail: xuzh@sdu.edu.cn, hygao@sdu.edu.cn.
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, Shanghai, 200032, China
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######
Abstract. We developed herein a palladium-catalyzed
annulation of aryltriazoles and arylisoxazoles with internal
alkynes via C-H bond activation process. 4,5-disubstituted-
3
H-naphtho[1,2-d][1,2,3]triazoles and 4,5-disubstituted-
naphtho[2,1-d]isoxazoles could be afforded in good yields,
respectively. The starting materials are readily available
and the scope and applications of this transformation were
explored. The reaction offers a practical approach to
naphthalene fused heterocycles.
Keywords: Triazole; Polycylces; Isoxazole; Palladium;
Annulation
Figure 1. Selected biologically active 1,2,3-triazole fused
polycycles.
As the rapid development and application of click
reaction in many areas, especially, copper(I)-
catalyzed cycloaddition of azides with alkynes
(
CuAAC), triazole chemistry has gained considerable
[1]
attentions in the past few dacades. The 1,2,3-
triazole moiety is not only connecting linkers, but
also privileged scaffolds used as important
[2]
pharmacophores in medicinal chemsitry. A large
number of triazole derivatives show interesting
therapeutic acitivites and have been used as potential
drug candidates for the treatment of cancer, HIV, etc.
[
2]
1
,2,3-triazoles fused polycycles, a subclass of
triazoles, possess a wide range of biological activities
Figure 1). For instance, triazole fused polycycles I, II
(
[3]
and III showed important anticancer activity. The
sugar derived fused triazoles IV possed an significant
glycosidase, galactosidase and SGLT2 inhibitory
activities.[ Morpholine fused triazoles V has been
4]
Scheme 1. Synthesis of triazoles fused polycycles by
CuAAC/C-H activation sequence.
found to have potential against Alzheimer disease
activities.[
5]
Consequently, the development of
intramolecular azide alkyne cycloaddition could
generate polycyclic triazoles, however relative
complicated cycloaddition precursors need to be
efficient synthetic methodology to access these
triazole fused polycycles is of great importance.
Various synthetic methods of polycyclic triazole
fused derivatives have been developed in the past few
decades. Traditionally, building the azide and alkyne
group in one molecular, followed by an
[6]
prepared. Other methods such as organo-catalyzed
cycloaddition of cyclic ketone with azides have also
[7]
been reported. Direct C-H activation/annulation of
simple triazoles, which are relatively easily obtained
via the well-established CuAAC reaction, would be
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201900435
one of the most efficient strategies to make more
complex polycyclic triazoles (Scheme 1). However,
to realize the selective cleavage of the undirected C-
H bond of triazole in the presence of other aromatic
C-H bonds is extremely challenging. An alternative
strategy is the application of triazole iodide
Diazabicyclo[4.3.0]non-5-ene) was found to be a
more effective base to furnish 3a in 62% isolated
yield (Table 1, entry 11). The yields decreased when
the reaction was run at lower temperature or reduced
the amount of alkyne 2a and catalyst (Table 1, entries
12-14).
[8]
intermediate in an intramolecular arylation reaction
8a]
(
Scheme 1A). [ In 2016, Lautens group reported a
Table 1. Optimization of reaction conditions.[a]
Cu/Pd-catalyzed
alkyne
insertion/C-H
functionalization strategy to achieve seven-membered
pocyclic triazoles (Scheme 1B). [9] Naphthalene-fused
triazoles are an important type of polyaromatics and
their synthesis haven't been reported. Recently, Chen
and Wu tried to use a rhodium-catalyzed oxidative
annulation between triazole and internal alkynes to
construct naphthalene-fused triazoles via double C-H
activation, however due to the difficulty in cleaving
the inert triazole C-H bond, another mesoionic
triazolo[5,1-a] isoquinolium derivatives were
b
Entry Catalyst (mol %)
Base (equiv)
Yield (%)3a/3a'
N.R.
N.R.
0/34
0/60
52/0
51/0
N.R.
N.R.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
(20)
(20)
(20)
(20)
(20)
Cs
Ag
2
CO
3
(2.0)
(2.0)
2
CO
3
NaOAc (2.0)
Li CO (2.0)
2
3
DBU (2.0)
DBU (2.0)
DBU (2.0)
DBU (2.0)
DBU (2.0)
DBU (2.0)
DBN (2.0)
DBN (2.0)
DBN (2.0)
DBN (2.0)
Pd(CF
3
CO
2
)
2
(20)
obtained rather than expected naphthalene-fused
triazoles (Scheme 1C).[10] Transition-metal catalyzed
Pd(PPh ) (20)
3
4
Pd
2
(dba)
3
(20)
Cl (20)
direct C-H activation and subsequent annulations has
been widely exploited for the synthesis of various
Pd(PPh
3
)
2
2
2
2
2
2
2
N.R.
0c
1c
2
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
(20)
(20)
(20)
(10)
(20)
56/16
65(62) /9
50/9
45/9
55/22
heterocyclic aromatic compounds due to its atom-
d
and step-economic features.[
11]
In particular,
intramolecular
palladium-catalyzed
inter-
or
c
3
[12]
annulation of aryliodides with alkynes and alkenes.
c,e
4
Inspired by these above mentioned studies and
following our recent interests in construction of
multisubstituted triazoles,[ we envisioned that a
palladium-catalyzed annulation of 1,2,3-triazole
[a]Reaction conditions: 1a (0.1 mmol), 2a (0.4 mmol),
catalyst (20 mol%), base (2.0 equiv), PivOH (2.0 equiv) in
13]
o
DMF (1 mL) under N
2
at 140 C for 6 h, unless otherwise
[
b]
noted. NMR yield using trimethoxybenzene as the
[
c]
o
[d]
substituted aryliodides with internal alkynes may internal standard. The reaction was run at 160 C. The
occur to give naphthalene-fused polycyclic triazoles number in the parentheses is the isolated yield. [e]0.2 mmol
2
a was employed.
(
Scheme 1D).
We initiated our investigation by employing 1-
With optimal conditions established, we next
benzyl-4-(2-iodophenyl)-1H-1,2,3-triazole 1a and
,2-diphenylethyne 2a as model substrates in the
presence of 20 mol% of Pd(OAc) as catalyst, 2.0
equivalents of PivOH as additive, under N in DMF
investigated the substrate scope of the synthesis of
naphthalene-fused triazoles. The results were
summarized in Table 2. Various substituents on the
phenyl ring of aryltriazoles were found to be well
tolerated. For instance, various benzyl azide-derived
aryltriazoles, including electron-rich and electron-
deficient groups substituted benzyl aryltriazoles, can
be employed in this reaction to give the
corresponding naphthalene-fused triazoles in good
yields under standard conditions. (Table 2. entries 1-
1
2
2
o
at 140 C for 6 h and 2.0 equivalents of various bases
were firstly screened (Table 1, entries 1-5). As a
result, Cs
designed reaction (Table 1, entries 1-2). Using
NaOAc or Li CO as base, only the product 3a´,
2
CO
3
and Ag
2
CO
3
were not effective to the
2
3
which is confirmed by X-ray (See Supporting
Information), can be detected as product (Table 1,
entries 3-4). To our delight, the reaction proceeded
smoothly to afford the desired product 3a in good
yield when DBU (1,8-Diazabicyclo[5.4.0]undec-7-
ene) was employed as base (Table 1, entry 5).
5
).
Naphthyl, cinnamyl groups, phthalimide-
protected amines skeletons and long chain alkyl
group onto the nitrogen of the triazole were all well
tolerated under the standard conditions (Table 2,
entries 6-10). The variety of aryliodide was also
Various palladium catalysts including Pd(CF
3
CO ) ,
2
2
explored,
the
results
revealed
that
o-
Pd(PPh
3
)
4
, Pd
2
(dba)
3
and Pd(PPh
3
)
2
Cl
2 2
2
were then
led to the
iodophenylacetylene with different substituents
derived aryltriazoles were also amenable to this
reaction (Table 2, entries 11-12). We next
investigated the substrate scope of this reaction with
regard to alkynes. To our delight, diphenylacetylenes
tested, we found that only Pd(CF
3
CO
)
formation of the desired product 3a (Table 1, entries
-9). A higher NMR yield of product 3a was obtained
6
o
when the reaction was proceeded at 160 C (Table 1,
entry 10). We were pleased to find that DBN (1,5-
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201900435
bearing either an electron-donating group (3-Me, 4- gratefully found that 5-(2-iodophenyl)isoxazoles,
Me) or an electron-withdrawing group (4-F, 4-Cl, 4-
Br, 4-CF ) onto the phenyl ring, could also be applied
which were readily synthesized from 1-ethynyl-
-iodobenzene and oxime chloride through [3+2]
cycloaddition, can react with internal alkynes
to afford naphthalene-fused isoxazoles in good
yields under the similar reaction conditions
3
2
to this reaction to furnish the corresponding
naphthalene-fused triazoles products, albeit with
lower yields in some cases (Table 2, entries 13-18).
In addition, non-symmetrical alkyne 2i was
investigated under standard conditions and the
corresponding annulation products 3s and 3s′′ were
afforded with moderate regioselectivity and yields
[15]
(Table 3).
(
Table 2, entry 19). The aliphatic alkyne 2j was also
applicable to the current catalytic system albeit with
low yield of the annulation product 3t (Table 2, entry
20). The structures of the products were
unambiguously confirmed by the single crystal X-ray
[14]
diffraction of compounds 3a and 3s (Figure 2).
Table 2. Substrate scope for polycyclic triazoles.[a,b]
Figure 2.X-ray structure of 3a and 5m
Table 3. Substrate scope for polycyclic isoxazoles.[a,b]
[a]Reaction conditions: 4 (0.2 mmol), 2 (0.4 mmol),
Pd(OAc)
in DMF (2 mL) under N
otherwise noted. Isolated yields.
2
(20 mol%), DBN (2.0 equiv), PivOH (2.0 equiv)
o
2
at 140 C for 6-12 h, unless
[
b]
Further substrate scope investigation revealed that
5-(2-iodophenyl)isoxazoles with various substituents
onto the isoxazole and iodobenzene as well as diverse
alkynes with different functional groups were well
tolerated. It was noteworthy that this naphthalene-
fused isoxazoles synthesis protocol was relatively
more efficient than the triazoles system and resulted
in higher yields of the corresponding annulation
[a]Reaction conditions: 1 (0.2 mmol), 2 (0.8 mmol),
Pd(OAc) (20 mol%), DBN (2.0 equiv), PivOH (2.0 equiv)
in DMF (2 mL) under N
2
o
2
at 160 C for 6-12 h, unless
[
b]
otherwise noted. Isolated yields.
Encouraged by these aforementioned results,
we then attempt to expand the generality of this products (Table 3, entries 1, 6, 14 and 17 VS Table 2,
palladium-catalyzed annulation protocol to other entries 1, 7, 13 and 17). The structure of the
naphthalene-fused isoxazole product was also
heterocycle systems rather than triazoles. We
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201900435
confirmed by the single crystal X-ray diffraction of
compound 5m (Figure 2).
straightforward ring extension method for the
construction of polycyclic aromatic compounds.
Further study to evaluate the utilities of the products
and extend the scope of related heterocyclic
substrates are now in progress.
[
16]
Based on these observations and the previous
studies related to the palladium-catalyzed annulation
reactions,[ a plausible mechanism for the reaction
of aryltriazole with alkyne was proposed in Scheme 2.
The reaction starts with the oxidative addition of 1a
with Pd(0) to generate arylpalladium(II) intermediate
A, which subsequently react with alkyne 2a to furnish
the vinylpalladium species B. An electrophilic attack
of this vinylic palladium intermediate B to the C-5
position of triazole affords a seven-membered
palladacycle C, and the subsequent reductive
elimination to furnish the adduct 3a with the
generation of the active Pd(0) catalyst. With regard to
the formation of the adduct 3a', participation of
another route, which involves a successive insertion
of two molecules of alkyne 2a and the subsequent
cyclopalladation to generate a different seven-
membered palladacycle intermediate E. The
corresponding adduct 3a' was formed through the
subsequent reductive elimination of intermediate E. It
is noteworthy that the selection of base is critical to
17]
Experimental Section
General procedure: To a mixture of Pd(OAc)
mmol, 0.2 eq.), PivOH (0.4 mmol, 2.0 eq.), 1-benzyl-
-(2-iodophenyl)-1H-1,2,3-triazole 1a or 3-benzyl-5-
2-iodophenyl)isoxazole 4a (0.2 mmol, 1.0 eq.) and
1,2-diphenylethyne 2a (0.8 mmol, 4.0 eq.) in DMF (2
mL) under N atmosphere, DBN (0.2 mmol, 2.0 eq.)
2
(0.04
4
(
2
o
was added. The system was stirred at 160 C or 140
C until the complete consumption of 1-benzyl-4-(2-
o
iodophenyl)-1H-1,2,3-triazole 1a or 3-benzyl-5-(2-
iodophenyl)-isoxazole 4a detected by TLC analysis.
The resulting mixture was washed with water and
extracted with ethyl acetate. The organic layer was
filtered on celite and evaporated under reduced
pressure.
Purification
by
flash
column
selectively generate the adducts 3a or 3a'. Probably,
chromatography (PE:EA = 10:1) afforded the desired
product 3a or 5a.
the better solubility in organic solvent[
18]
and the
relatively steric hindrance of DBN resulted in the
favorable transformation from B to C and prevented
the successive insertion of the second molecular of
alkyne.
Acknowledgements
We gratefully acknowledge the financial support of
Shandong University, the National Natural Science
Foundation of China (21702122, 21572118) and the
Natural Science Foundation of Shandong Province
(
ZR2017MB002). We thank Prof. Di Sun at
Shandong University for the X-Ray diffraction and
data analysis.
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