Rhodium(II)-Catalyzed Formal [4+1]-Cycloaddition of Pyridotriazoles and Propargyl Alcohols: Synthesis of 2,5-Dihydrofurans

Article abstract of DOI:10.1002/adsc.201801497

A rhodium-catalyzed formal [4+1]-cycloaddition of pyridotriazoles and aryl propargyl alcohols is reported, providing an effective access to 2-pyridyl-substituted 2,5-dihydrofuran derivatives in moderate to high yields. Mechanistically, the proposed oxonium ylide intermediate in this catalytic alkyne carbocyclization transformation is verified by an interception experiment for the first time. (Figure presented.).

Full text of DOI:10.1002/adsc.201801497

Title: Rhodium(II)-Catalyzed Formal [4 + 1]-Cycloaddition of
Pyridotriazoles and Propargyl Alcohols: Synthesis of 2,5-
Dihydrofurans
Authors: Lv Xinxin, Yang Haijian, Shi Taoda, Dong Xing, Xinfang Xu,
and Wenhao Hu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801497
Link to VoR: http://dx.doi.org/10.1002/adsc.201801497

10.1002/adsc.201801497
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Rhodium(II)-Catalyzed Formal [4 + 1]-Cycloaddition of
Pyridotriazoles and Propargyl Alcohols: Synthesis of 2,5-
Dihydrofurans
Xinxin Lv,a Haijian Yang,a Taoda Shi,a Dong Xing,a Xinfang Xub* and Wenhao Hua,b*
a Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry
and Chemical Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P.R. of
China
b School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, P.R. of China
E-mail: xinfangxu@suda.edu.cn; huwh9@mail.sysu.edu.cn.
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))
Our group has reported
a
Pd(II)-catalyzed
Abstract.
A
rhodium-catalyzed
formal
[4+1]-
cycloaddition of pyridotriazoles and aryl propargyl
alcohols is reported, providing an effective access to 2-
pyridyl-substituted 2,5-dihydrofuran derivatives in
moderate to high yields. Mechanistically, the proposed
oxonium ylide intermediate in this catalytic alkyne
carbocyclization transformation is verified by an
interception experiment for the first time.
cycloaddition of a diazo compound with phenyl
propargyl alcohol through an ylide intermediate (path
c).[14] Notably, in these advances, the compatibility of
the catalysts with corresponding substrates is essential
in the development of selective catalytic
transformations. Especially in the gold- and
palladium-catalyzed reactions, poisoning of these
catalysts by N-heterocycles occurs regularly.[15]
Keywords: [4+1]-Cycloaddition; Pyridotriazoles; 2,5-
Dihydrofuran; Oxonium ylide; Rhodium Carbene
In the last decade, Gevorgyan and others have
reported a variety of cycloaddition reactions with
pyridotriazole as α-pyridyl carbene precursor.[16-20] In
most of these transformations, pyridotriazole served
as a three-element "N-C-C" synthon in [3+2]-
cycloaddition with alkynes (Scheme 1b),[16]
alkenes,[18] nitriles,[19] amides.[20] However, no
cyclization reaction with this species as a "one-C"
building block has been disclosed.[21] Inspired by
these advances, and in continuation of our interest in
catalytic metal carbene transformations,[9,14] we
Metal carbene reactions represent a powerful tool in
modern organic synthesis, allowing for the rapid
assembly of complex structures that cannot be readily
realized with other methods.[1] A variety of metal
catalysts have been reported to mediate these
reactions with diazo compounds as the carbene
precursor,
including
rhodium,[2]
copper,[3]
palladium,[4] gold,[5] silver,[6] iron,[7] etc.[8] In
comparison with other catalysts, dirhodium catalysts
show advantages in both the reaction efficiency and
selectivity controls. Recently, combined with other
co-catalysts,
phosphoric
asymmetric
acid cooperatively
rhodium(II)/chiral
catalyzed
multicomponent reactions through the trapping of
ylide/zwitterionic intermediates; this has been
extensively studied by us[9] and others.[10]
On the other hand, the different transition-metal
catalysts exhibit distinct reactivities and selectivities
in modulating the decomposition chemistry of diazo
compounds, even for reactions with the same starting
material(s).[11] For example, in the reaction of diazo
compounds with phenyl propargyl alcohol, Davies
and co-workers reported a rhodium-catalyzed tandem
ylide formation/[2,3]-sigmatropic rearrangement for
the enantioselective synthesis of allenes[12] (Scheme
1a, path a); however, in the presence of a gold catalyst,
the α-hydroxy allene underwent a 5-endo-trig
cyclization, resulting in 2,5-dihydrofuran (path b).[13]
Scheme 1. Divergent catalytic diazo-yne carbocyclization
1
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10.1002/adsc.201801497
Advanced Synthesis & Catalysis
envisioned that
a
formal [4+1]-cycloaddition of 2,3- sigmatropic rearrangement product 5 in 21% and
pyridotriazoles and aryl propargyl alcohols in the 10% yields, respectively (entry 7). Although high
presence of a compatible dirhodium catalyst could be yields were obtained in the analogous [4+1]-
realized for the synthesis of 2-pyridyl 2,5- cycloaddition of aryl diazoacetates with aryl
dihydrofuran. Herein, we describe our recent results propargyl alcohols in the presence of
in this context, a Rh2(esp)2-catalyzed formal [4+1]- [PdCl(C3H5)]2,[14] the reaction gave complex mixture
cycloaddition through a 5-endo-dig carbocyclization when promoted by the same catalyst. Reaction in
of oxonium ylide intermediate (Scheme 1c, path d). In other solvents (entries 9-11) or in the absence of 4Å
addition, the proposed ylide intermediate was molecular sieves (entry 13) led to inferior results, and
intercepted experimentally (path e).
the best results were obtained in DCE at 25 °C in the
To probe the feasibility of our assumption, we presence of Rh2(esp)2 to give 3a in 75% isolated yield
initially chose 7-chloro-pyridotriazole (1a) and 3- (entry 12). The structure of product 3a was confirmed
phenylpropargyl alcohol (2a) as the standard by X-ray crystallographic analysis,[22] and these
substrates, and dirhodium catalysts were examined in generated 2,5-dihydrofuran derivatives serve as
1,2-dichloroethane (DCE) at 25 °C. Although high important structural units in many bioactive natural
reactivity was observed for these tested rhodium products.[23]
catalysts, low selectivity was obtained in the cases
with Rh2(OAc)4 or Rh2(Oct)4 catalyst (Table 1, entries
With the optimized reaction conditions in hand,
1 & 2). Notably, Rh2(esp)2 was found to improve the Table 2. Substrate Scope a).
yield of 3a dramatically (Table 1, entry 3). Other
transition-metal catalysts such as Ag, Pd and Cu were
also examined, but poor results were obtained (entries
4-8). For example, the reaction catalyzed by Cu(OTf)2
in 1,2-dichloroethane at 80 °C yielded 3a in 36%
yield concomitant with O-H insertion product 4 and
Table 1. Optimization of the Reaction Conditions [a]
.
cat
(x mol %)
T
(°C)
yield (%)[b]
entry
solvent
3/4/5
1
2
3
4
Rh2(OAc)4 (2)
Rh2(Oct)4 (2)
Rh2(esp)2 (2)[g]
AgSbF6 (10)
DCE
DCE
DCE
DCE
DCE
25
30/13/4
25 19/12/13
25
25
80 <5/<5/<5
25 <5/8/<5
80 36/21/10
80 <5/<5/<5
76/8/16
25/6/<5
5[c] [PdCl(C3H5)]2 (5)
6[d] Pd(CH3CN)2Cl2 (5) DCE
Cu(OTf)2 (10) DCE
8[d] Cu(MeCN)4PF6 (10) DCE
7
9
10
11
12
13[f]
Rh2(esp)2 (2)[g]
Rh2(esp)2 (2)[g]
Rh2(esp)2 (2)[g]
Rh2(esp)2 (2)[g]
Rh2(esp)2 (2)[g]
Toluene 25 16/11/<5
PhCF3 25
30/5/<5
49/7/42
75[e]/-/-
46/-/-
DCM
DCE
DCE
25
25
25
[a]
Reaction conditions: A solution of 1a (30.4 mg, 0.12
mmol) and 2a (13.2 mg, 0.1 mmol) in 0.5 mL of indicated
solvent was added to a suspension of corresponding metal
catalyst and activated 4Å molecular sieves (50 mg) in 0.5
[b]
mL of the same solvent slowly at indicated temperature.
Determined by H NMR of the crude reaction mixture.
1
[c]
[d]
a) Reaction conditions: a solution of 1 (0.36 mmol) and 2
(0.3 mmol) in DCE (1.5 mL) was added to a suspension of
Rh2(esp)2 (4.36 mg, 2.0 mol %) and activated 4Å molecular
sieves (150 mg) in DCE (1.5 mL) dropwise at 25 °C, and
the yields were given in isolated yields. b) The reaction was
Material 1a was decomposed into a complex mixture.
[e]
Most of material 1a was recovered. The reaction was
carried out on a 0.3 mmol scale and the yields are given in
[f]
isolated yields. The reaction was carried out in the
[g]
absence of 4Å molecular sieves.
Rh2(esp)2 =
c)
conducted at 115 °C. The reaction was conducted at
140 °C.
bis[rhodium(α,α,α',α'-tetramethyl-1,3-benzenedipropionic
acid)].
2
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10.1002/adsc.201801497
Advanced Synthesis & Catalysis
different substituted phenyl propargyl alcohols and
pyridotriazoles for this rhodium catalyzed [4+1]-
cycloaddition were tested (Table 2). A variety of
substituents on the aromatic ring of propargyl
alcohols were well tolerated in this reaction (3a-3h),
especially substrates with electron-withdrawing
groups, giving the desired products 3d and 3e in 90%
and 85% yield, respectively. The reaction showed
obvious influence of the steric effect; moderate yield
was obtained with ortho-substituted material (3j, 49%
yield), while high yields were obtained in the cases of
propargyl alcohols with para and meta substituents
(3g and 3i, >70% yield). Next, the scope with regard
to
the
pyridotriazoles
was
investigated.
Pyridotriazoles bearing chloro, bromo, and fluoro
groups on the pyridine moiety produced the expected
products 3k–3m in good to high yields. The non-
substituted ones (3n and 3o), and 4-, 5-, or 6-
substituted substrates (3q-3t) were also applicable to
the reaction, delivering the corresponding products at
elevated temperature in moderate to high yields due to
the lower reactivity of these pyridotriazoles.[24]
Moreover, the oxazolyl group was also tolerated in
this rhodium catalysis to afford 3p in high yield at
115 °C according to the proton NMR of the crude
reaction mixture; however, 3p was isolated in 35%
yield, which may due to the slow decomposition of
3p during column chromatography. The reaction of
terminal propargylic alcohol or 3-alkyl propargylic
alcohol gave a complex mixture under current
conditions. In the case of the secondary and tertiary
propargylic alcohols, instead of forming the cyclized
product, corresponding allene products 5b and 5c
were generated in 82% and 51% yields, respectively,
which is consistent with previously reported results.[13]
In order to gain insight into the mechanism of this
Rh(II)-catalyzed formal [4+1]-cyclization, several
control experiments were conducted (Scheme 2). The
Scheme 2. Control experiments
O-H insertion product
4
and 2,3-sigmatropic
rearrangement product 5 were isolated and subjected
to the optimized conditions, and no cyclization
product 3a was observed (eq 1), indicating that the
isolated insertion product 4 and 2,3-sigmatropic
rearrangement product 5 were not the intermediate in
our reaction system. In the presence of
JohnPhosAuSbF6, 3a was isolated in 29% yield at
room temperature (eq 2), which is consistent with
previous results in gold-catalyzed carbocyclization
(Scheme 1, path b).[13] When aryl diazoacetate 6 was
used instead of pyridotriazole 1, only insertion
product 7 and α-hydroxy allene 8 were produced in
46% and 51% yields, respectively (eq 3), and no
cyclization product 9 was obtained even when
conducting the reaction with 7 and 8 under the
standard conditions (eq 4). These different catalytic
outcomes indicate the substrate-catalyst matching
effect in controlling the reaction selectivity.
Cyclopropene 10 was prepared from 6 and 2a in the
presence of Cu(OTf)2, and this material couldn't
produce 9 under the standard conditions (eq 5),
therefore excluded the reaction pathway involving a
cyclopropene route.[17] In addition, the Mannich type
addition product 12 was isolated in 78% yield with
76:24 dr when the template reaction was carried out
in the presence of imine 11 (1.0 equiv.) with
phosphoric acid as co-catalyst (eq 6). No reaction
occurred between insertion product 4 and imine 11
under the standard conditions in the presence of
phosphoric acid (eq 7). These results provide a direct
evidence for the formation of the proposed oxonium
ylide or corresponding enolate intermediate in this
catalytic alkyne carbocyclization reaction.
Based on the above observation and previous
studies,[12-14] plausible reaction mechanisms were
depicted in Scheme 3. Pyridotriazole 1 decomposed in
the presence of Rh2(esp)2 to form rhodium carbenoid
A, followed by the formation of oxonium ylide B (or
in equilibrium with its enolate form C) with propargyl
alcohol 2, and terminated by intramolecular
nucleophilic attack of the alkyne species to deliver the
cyclization product
3
via
a
formal [4+1]-
cycloaddition (Scheme 3, path a). As concluded by
3
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10.1002/adsc.201801497
Advanced Synthesis & Catalysis
investigations revealed that the oxonium ylide is the
key intermediate in this transformation, which is
trapped successfully through a Mannich-type addition
with external imine.
Experimental Section
General Procedure for Rh(II)-catalyzed Formal [4 + 1]-
Cycloaddition:
Procedure A:
Under a nitrogen atmosphere, to the solution of Rh2(esp)2
(4.36 mg, 0.006 mmol, 2.0 mol%) and 4 Å MS (150 mg) in
DCE (1.0 mL) was added the solution of pyridotriazoles 1
(0.36 mmol, 1.2 equiv) and aryl propargyl alcohols 2 (0.3
mmol, 1.0 equiv) in DCE (2.0 mL) via a syringe pump over
2 h at 25 °C. After completion of the addition, the resultant
solution was stirred overnight under these conditions. The
solvent was removed and the crude residue was purified by
flash chromatography on silica gel (eluent: light petroleum
ether/EtOAc =50:1~10:1) to afford pure products 3.
Scheme 3. Proposed mechanism for the [4+1]-
cycloaddition
Procedure B:
Under a nitrogen atmosphere, to the solution of Rh2(esp)2
(4.36 mg, 0.006 mmol, 2.0 mol %) and 4Å MS (150 mg) in
DCE (1.0 mL) was added the solution of pyridotriazoles 1
(0.36 mmol, 1.2 equiv) and aryl propargyl alcohols 2 (0.3
mmol, 1.0 equiv) in DCE (2.0 mL) via a syringe pump in
or 10 minutes at 115 °C. After completion of the addition, the
resultant solution was stirred 2 h under these conditions.
The solvent was removed and the crude residue was
purified by flash chromatography on silica gel (eluent: light
petroleum ether/EtOAc =50:1~10:1) to afford pure
products 3.
the control experiments, other pathways, involving O-
H insertion (Scheme 3, path b), 2,3-sigmatropic
rearrangement
(Scheme
3,
path
c),
cyclopropenation (Scheme 3, path d) could be
excluded for this transformation.
The synthetic versatility of 2,5-dihydrofurans 3
was then explored (Scheme 4). The Suzuki coupling
reaction of 3a with phenylboronic acid provided the
corresponding 6-aryl-substituted 13 in 89% yield.
Oxidation of 3a with PCC lead to the furan-2-ketone
14 in 55% yield. 3n was converted into alcohol 15 by
reduction with LiAlH4 in 42% yield.
Procedure C:
Under a nitrogen atmosphere, to the solution of Rh2(esp)2
(4.36 mg, 0.006 mmol, 2.0 mol %) and 4Å MS (150 mg) in
DCE (1.0 mL) was added the solution of pyridotriazoles 1
(0.36 mmol, 1.2 equiv) and aryl propargyl alcohols 2 (0.3
mmol, 1.0 equiv) in DCE (2.0 mL). The resultant solution
was stirred at 140 °C for 6 h. The solvent was removed and
the crude residue was purified by flash chromatography on
silica gel (eluent: light petroleum ether/EtOAc =10:1~5:1)
to afford pure products 3.
Acknowledgements
We are grateful for financial support from NSF of China
(21332003, 21402051, 21772043). We also thank the
financial supported by Guangdong Innovative and
Entrepreneurial
Research
Team
Program
(No.
2016ZT06Y337), and National Science & Technology
Major Project "Key New Drug Creation and Manufacturing
Program" (2018ZX09711002).
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COMMUNICATION
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Xinxin Lv,a Haijian Yang,a Taoda Shi,a Dong
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