Iridium-catalyzed intermolecular azide-alkyne cycloaddition of internal thioalkynes under mild conditions

Article abstract of DOI:10.1002/anie.201309855

An iridium-catalyzed azide-alkyne cycloaddition reaction (IrAAC) of electron-rich internal alkynes is described. It is the first efficient intermolecular AAC of internal thioalkynes. The reaction exhibits remarkable features, such as high efficiency and r

Full text of DOI:10.1002/anie.201309855

Angewandte
Chemie
DOI: 10.1002/anie.201309855
Click Chemistry
Iridium-Catalyzed Intermolecular Azide–Alkyne Cycloaddition of
Internal Thioalkynes under Mild Conditions**
Shengtao Ding, Guochen Jia,* and Jianwei Sun*
Abstract: An iridium-catalyzed azide–alkyne cycloaddition
reaction (IrAAC) of electron-rich internal alkynes is described.
It is the first efficient intermolecular AAC of internal thio-
alkynes. The reaction exhibits remarkable features, such as
high efficiency and regioselectivity, mild reaction conditions,
easy operation, and excellent compatibility with air and
a broad spectrum of organic and aqueous solvents. It comple-
ments the well-known CuAAC and RuAAC click reactions.
a mild intermolecular AAC of internal thioalkynes has been
realized for the first time.[14]
Fully substituted 1,2,3-triazoles decorated with a 5-sulfur
substituent represent a family of useful molecules. For
example, triazoles I are potential herbicides with antifungal
activity,[15a] and triazole II can serve as an excellent chiral
ligand in organic synthesis (Figure 1).[15b] However, despite
H
uisgen 1,3-dipolar azide–alkyne cycloaddition (AAC) is
the most straightforward and atom-economical synthesis of
1,2,3-triazoles.[1] However, the traditional thermal conditions
typically require high temperatures and proceed with limited
regioselectivity. In 2002, the groups of Sharpless and Meldal
independently reported their pioneering studies on copper-
catalyzed AAC, thus providing a mild and efficient synthesis
of 1,4-disubstituted 1,2,3-triazoles.[2] Owing to its excellent
fidelity and compatibility in a broad context, CuAAC has
evolved into a powerful tool with wide applications in organic
synthesis, molecular biology, and materials science.[3–6]
Figure 1. Selected useful 1,2,3-triazoles with a 5-sulfur substituent.
the utility of this core structure, the current strategies for its
synthesis are indirect and require multiple steps.[15,16] To date,
no mild and direct synthesis by intermolecular AAC between
thioalkynes and azides is known.
In contrast to the widely successful use of terminal alkynes
(including metal acetylides) in catalyzed AACs,[3] the corre-
sponding reactions of internal alkynes for the synthesis of
fully substituted 1,2,3-triazoles remain a challenge[7] owing to
the increased energy barrier and difficulty in regiocontrol,
particularly for intermolecular reactions. As a result, most
intermolecular AACs of internal alkynes require high tem-
peratures[8] and/or activated substrates, such as electron-
deficient alkynes (e.g., haloalkynes)[9,10] or strained alkynes
(e.g., cyclooctynes).[11] Mild and regioselective AACs of
electron-rich internal alkynes are scarce. While the advent
of ruthenium-based catalytic systems (RuAAC) has
addressed the challenge to some extent,[12] additional robust
catalytic systems complementary to CuAAC and RuAAC
remain in high demand. In a continuation of our studies of
electron-rich alkynes,[13] we report herein the first iridium-
catalyzed AACs of electron-rich internal alkynes. Specifically,
We started our investigation with thioalkyne 1a and
benzyl azide (2a) by using dichloromethane as the solvent. In
contrast to the established CuAACs of terminal alkynes,
internal alkyne 1a did not undergo the expected [3+2]
cycloaddition in the presence of either CuI or CuSO4 (Table 1,
entries 1 and 2). The previous catalysts of choice in RuAAC,
[Cp*Ru(PPh3)2Cl] and [Cp*Ru(cod)Cl],[12] catalyzed the
desired reaction with excellent conversion but with mediocre
regioselectivity (Table 1, entries 3 and 4). Cationic ruthenium
complexes were not effective (Table 1, entries 5 and 6).
Further extensive screening of a wide range of metal
complexes led us to identify [{Ir(cod)Cl}2] as a superb catalyst
for the desired transformation, furnishing adduct 3a not only
in essentially quantitative yield but also with absolute
regioselectivity (Table 1, entry 7). Other iridium and rhodium
complexes gave inferior results (Table 1, entries 8–16). The
reaction catalyzed by [{Ir(cod)Cl}2] works equally well in
various polar and nonpolar solvents, including THF, MeCN,
MeNO2, toluene, DMF, iPr2O, and EtOH (Table 1, entry 18).
The mild reaction conditions (room temperature) combined
with the observed compatibility with a broad spectrum of
solvents may lead to the widespread application of this
process.
[*] S. Ding, Prof. G. Jia, Prof. J. Sun
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR (China)
E-mail: chjiag@ust.hk
sunjw@ust.hk
With the standard conditions established, we evaluated
the generality of our IrAAC reaction. A wide range of
thioalkynes with aryl and alkyl substituents on both termini
can participate in the cycloaddition to give the fully sub-
stituted 5-thio-1,2,3-triazoles in good to excellent yields
[**] We thank Prof. Zhenyang Lin for insightful discussion. We also
thank Dr. Herman Sung, Prof. Ian Williams, and Dr. Weimin Hu for
assistance with structure elucidation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309855.
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Table 1: Optimization of the reaction conditions.[a]
Table 2: Alkyne scope: thioalkynes.
R1
R2
3
Yield [%][a]
Entry Cat.
Yield (3a+3a’) [%][b] 3a/3a’[c]
Entry
1
2
3
4
5
6
7
8
CuI
CuSO4
0
<5
90
>95
0
0
>95
10
<5
0
<5
30
75
<5
0
–
–
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
nBu
nOct
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
72
91
92
77
85
82
94
60
98
99
94
96
54[b]
91
[Cp*Ru(PPh3)2Cl]
[Cp*Ru(cod)Cl]
[Cp*Ru(MeCN)3]PF6
[CpRu(MeCN)3]PF6
[{Ir(cod)Cl}2]
[(cod)Ir(acac)]
[(CO)2Ir(acac)]
[(cod)Ir(Me-Cp)]
[(cod)Ir(h5-indenyl)]
[(cod)Ir(acac-F6)]
[(cod)Ir]BF4
4.4:1
4.5:1
–
cyclopropyl
C2H4OTBS
C2H4OTHP
C2H4OMs
C2H4OAc
C2H4OH
Ph
nBu
nBu
nBu
nBu
–
1:0
1:0
–
–
–
1:0
1:0
–
–
–
9
9
Ph
10
11
12
13
14
15
16
17
18
10
11
12
13
14
(p-Me)C6H4
(p-OMe)C6H4
(p-Cl)C6H4
(p-NO2)C6H4
(p-Me)C6H4
[{(coe)2IrCl}2]
[{Cp*RhCl2}2]
[{Rh(cod)Cl}2]
[{(CO)2RhCl}2]
tBu
15
nBu
3o
61[b]
0
<10
–
1:0
16
17
18
19
20
21
22
23
24
25
26
Me
nBu
nBu
nBu
nBu
nBu
nBu
Bn
nBu
nBu
cyclopropyl
C2H4OTBS
C2H4OTHP
Ph
TMS
nBu
nBu
nBu
3p
3q
3r
3s
3t
3u
3v
3w
3x
3y
3z
97
82
86
86
89
84
61
89
41
93
22[c]
[{Ir(cod)Cl}2] (other solvents)[d] >95
[a] 1a (0.10 mmol), 2a (0.15 mmol), cat. (2 mol%), CH2Cl2 (1 mL),
under N2 at RT overnight. [b] Yield determined by GC with n-decane as
the internal standard. For all entries, recovered starting material
accounts for the mass balance. [c] Determined by NMR analysis of the
crude mixture. [d] Other solvents evaluated (including THF, MeCN,
MeNO2, toluene, DMF, iPr2O, EtOH) all give almost quantitative yield.
Cp*=pentamethylcyclopentadiene, cod=cycloocta-1,5-diene, coe=cy-
clooctene, acac=acetylacetonate, DMF=dimethylformamide.
(3-furyl)CH2
iPr
(p-OMe)C6H4
H
[a] Yield of isolated product. [b] Incomplete conversion. [c] Approxi-
mately 35% conversion by GC analysis. TBS=tert-butyldimethylsilyl,
THP=tetrahydropyranyl, Ms=methanesulfonyl.
(Table 2). Increased steric hindrance on the alkyne does not
affect the reaction efficiency (3n and 3y). The reaction is also
efficient with various alkyl and aryl azides (Table 3). The
product structure, in particular with regard to regiochemistry,
was confirmed by 2D NMR spectroscopy and X-ray crystal-
lography (see the Supporting Information). The mild reaction
conditions mean that the reaction tolerates a diverse set of
functional groups, such as silyl- and THP-protected alcohols,
ethers, aryl halides, esters, Boc- and phthalimide-protected
amines, esters, mesylates, and even free alcohols. Although all
the reactions were run overnight (about 10 h) for simplicity,
a time-dependence study shows that the reaction of 1a and 2a
can reach full conversion within one hour. Terminal thio-
alkyne 1z gave 1,5-disubstituted triazole 3z, but with low
conversion (Table 2, entry 26).
Table 3: Azide scope.
Entry
1
RN3
3
Yield [%][a]
87
3aa
2
3
3ab
3ac
89
86
We also examined other internal alkynes with different
electronic properties (Table 4). Most of these electron-rich
and normal alkynes show low reactivity, although promising
results were observed for aryloxy alkyne 6b and selenyl
alkyne 6c (low yield but excellent regioselectivity). By
contrast, electron-deficient alkynes 6h and 6i can participate
in the [3+2] cycloaddition with moderate to good efficiency
(Table 4, entries 8 and 9). Of note is the opposite regioselec-
tivity of the reaction of sulfonyl alkyne 6h to form 4-sulfonyl
triazole 7h.[17]
4
5
3ad
3ae
92
56
[a] Yield of isolated product. Boc=tert-butoxycarbonyl, TMS=trimeth-
ylsilyl.
We were interested in knowing the sensitivity of the
IrAAC toward moisture and air. To our delight, the reaction
of 1a and 2a (Table 1, entry 7) exhibited excellent efficiency
(> 95% yield as determined by GC) when it was run in an
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Table 4: Alkyne scope: other alkynes.
Entry
1
6
7
Yield [%][a]
–
<5[b]
2
3
42
19[c]
Scheme 1. Synthesis of 5-sulfonyl triazoles.
4
–
<5[b]
5
6
7
–
–
–
<5[b]
<5[b]
–
[c]
8
9
77[e]
56
[a] Yield of isolated product. [b]<5% conversion as determined by GC
analysis. [c] Low conversion, recovered starting alkyne accounts for the
mass balance. [d] No desired product, but a complicated reaction
mixture was observed. [e] Only one regioisomer. Ts=toluene-4-sulfonyl,
PMP=para-methoxyphenyl..
Scheme 2. Proposed mechanism.
open flask with water additive (10 equiv). Moreover, we
carried out a randomly selected reaction (Table 2, entry 25) in
water. Notably, although not all the starting materials were
fully dissolved, the reaction went smoothly to completion
within 2 h without significant loss of efficiency [Eq. (1)].
These results suggest that our IrAAC process is operationally
robust.
Scheme 2 depicts a proposed reaction mechanism for the
IrAAC of thioalkynes. We propose that the catalytic cycle
begins with the complexation of Ir by both substrates to give
A, in which the azide coordinates through the internal
nitrogen atom.[19] Next, oxidative cyclization results in irido
heterocycle B. The sulfur atom may coordinate to the iridium
center to provide additional stabilization.[20] Subsequent
reductive elimination of B leads to the formation of triazole
3 via C. It is unclear whether the coordination of sulfur
contributes to the observed regioselectivity. Detailed mech-
anistic study is required to substantiate the proposed catalytic
cycle.[21]
In summary, the first iridium-catalyzed azide–alkyne
cycloaddition reaction (IrAAC) of electron-rich internal
alkynes is presented. This reaction also represents the first
efficient intermolecular AAC of internal thioalkynes. With
the proper choice of an iridium catalyst, various internal
thioalkynes and azides can participate in this efficient
cyclization to furnish a wide range of fully substituted
triazoles under mild reaction conditions with complete
regioselectivity, thereby complementing the well-known
copper- and ruthenium-based catalytic AAC systems. The
combination of many remarkable features, such as high
efficiency and regioselectivity, mild reaction conditions, and
easy operation, as well as excellent compatibility with air and
a broad spectrum of organic and aqueous solvents, paves the
To further demonstrate the utility of our process, we have
performed derivatization of a triazole product to synthesize 5-
sulfonyl triazoles. As shown in Scheme 1A, triazole 3j could
be easily oxidized with m-chloroperbenzoic acid (MCPBA) at
room temperature to give 5-sulfonyltriazole 4 as a single
isomer in good yield. By contrast, the literature synthesis of
this compound starting from alkynyl sulfone 5 requires high
temperature and a long reaction time but gives low regiose-
lectivity (Scheme 1B).[18] These results (combined with those
in Table 4, entry 8) clearly demonstrate the exceptional
advantages of our approach, which enables the synthesis of
both 4- and 5-sulfonyltriazoles efficiently and exclusively.
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Angew. Chem. 2009, 121, 8162 – 8165; Angew. Chem. Int. Ed.
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Org. Chem. 2012, 77, 6443 – 6455; d) During the preparation of
our manuscript, an Ir-catalyzed AAC of bromoalkynes was
reported (with mostly moderate efficiency): E. Rasolofonjatovo,
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F. Taran, Org. Lett. 2013, 15, 4698 – 4701.
way for subsequent applications in various contexts. Further
detailed investigations on IrAAC are underway.
Received: November 13, 2013
Keywords: click chemistry · cycloaddition · heterocycles ·
.
iridium · triazoles
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