Iridium(III)-Catalyzed Direct C-H Sulfonamidation of 2-Aryl-1,2,3-triazole N-Oxides with Sulfonyl Azides

Article abstract of DOI:10.1002/adsc.201501036

We have developed a method for the direct sulfonamidation of 2-aryl-1,2,3-triazole N-oxides using sulfonyl azides as the amino source to release molecular nitrogen as the sole by-product. This protocol exhibits excellent functional group tolerance and proceeds efficiently under external oxidant-free conditions. Various 2-(2-sulfonamidoaryl)-1,2,3-triazoles were prepared in up to 97% yields for 25 examples with excellent regioselectivity.

Full text of DOI:10.1002/adsc.201501036

UPDATES
DOI: 10.1002/adsc.201501036
À
Iridium(III)-Catalyzed Direct C H Sulfonamidation of 2-Aryl-
1,2,3-triazole N-Oxides with Sulfonyl Azides
Bingfeng Zhu,a Xiuling Cui,a,b,* Chao Pi,a Dong Chen,a and Yangjie Wua,*
a
Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of
Applied Chemistry of Henan Universities, Zhengzhou University, Zhengzhou 450052, Peopleꢀs Republic of China
Fax: (+86)-371-6776-7753; e-mail: cuixl@zzu.edu.cn or wyj@zzu.edu.cn
School of Biomedical Sciences, Engineering Research Center of Molecular Medicine of Chinese Education Ministry,
b
Xiamen Key Laboratory of Ocean and Gene Drugs, Institute of Molecular Medicine of Huaqiao University, Fujian,
Xiamen 361021, Peopleꢀs Republic of China
Received: November 11, 2015; Published online: January 12, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501036.
Abstract: We have developed a method for the
direct sulfonamidation of 2-aryl-1,2,3-triazole N-
oxides using sulfonyl azides as the amino source to
release molecular nitrogen as the sole by-product.
This protocol exhibits excellent functional group
tolerance and proceeds efficiently under external
oxidant-free conditions. Various 2-(2-sulfonamidoar-
yl)-1,2,3-triazoles were prepared in up to 97%
yields for 25 examples with excellent regioselectivi-
ty.
dants,[10,11] with N2 being released as the sole by-prod-
uct and with azide employed as an internal oxidant.
On the other hand, 2-substituted 1,2,3-triazoles
have attracted increasing attention due to their spe-
cial chemical and biological properties.[12] Especially,
2-(2-amidoaryl)-1,2,3-triazoles have been widely ap-
plied in agrochemicals, dyes, pharmaceuticals and ad-
vanced functional materials.[13] For example, com-
pounds A and B are key components in light-stable
polymers, and compound C is a potential candidate as
an antagonist of the natural peptides (Figure 1).[14]
Despite their importance, a procedure to build such
a structure in the literature is still rare.[15] Therefore,
it is desirable to efficiently realize the ortho-sulfona-
midation of 2-aryl-1,2,3-triazoles. Herein, we present
À
Keywords: 2-aryl-1,2,3-triazole N-oxides; C H
bond activation; iridium catalysis; sulfonamidation
À
an iridium-catalyzed direct C H sulfonamidation of
2-aryl-1,2,3-triazole N-oxides with excellent regiose-
À
Transition metal-catalyzed direct C H amination has lectivity using sulfonyl azide as an amino source to re-
attracted great attention because of its high atom lease N2 as the sole by-product. This reaction pro-
economy.[1] Thus, some amine sources have been suc- ceeded smoothly with low catalyst loading under ex-
cessfully explored as effective coupling partners for ternal oxidant-free reaction conditions.
this strategy, including N-benzoates of alkylamines,[2]
N-chloroamines,[3] sulfonamides,[4] amides,[5] parent
amines,[6] N-fluorobenzenesulfonimide,[7] and nitroso-
benzenes.[8] Stoichiometric or excess external oxidants
are necessary in most cases when parent amines are
employed as the starting materials. Although the use
of preactivated amine precursors could avoid the use
of external oxidants, an additional step is required to
install the mask, which might produce stoichiometric
by-products as waste. In 2012, Chang[9] reported the
amidation of 2-phenylpyridine with tosyl azide cata-
lyzed by a cationic Cp*Rh(III) species generated in
situ by treating [RhCp*Cl2]2 with a silver salt. Their
work inspired the development of the use of organic
À
azides as a benign nitrogen source for the direct C N
bond construction in the absence of external oxi-
Figure 1. Examples illustrating the importance of 2-aryl-
1,2,3-triazoles.
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UPDATES
Iridium(III)-Catalyzed Direct C H Sulfonamidation
808C (entry 16). It was noted that the reaction could
proceed at room temperature or with 1 mol% catalyst
loading. However, the yields were decreased to 56%
and 37%, respectively (entries 17 and 18). Other tran-
sition metals, such as ruthenium, rhodium or palladi-
um complexes, which are known to be effective in
À
direct C H bond functionalization, were ineffective in
this transformation (entries 19–22).
With the optimized reaction conditions in hand, the
scope of sulfonyl azides (4aa–4ap) for the reaction
with 2-phenyl-1,2,3-triazole N-oxide (3a) was investi-
gated. Both electron-rich (4aa–4ac) and electron-defi-
cient (4ae-4ai) arylsulfonyl azides could proceed
smoothly and afford the corresponding sulfonamidat-
Scheme 1. Sulfonamidation of 2-phenyl-2H-1,2,3-triazole.
We initially explored the sulfonamidation of 2- ed products in good to excellent yields. Arylsulfonyl
phenyl-2H-1,2,3-triazole (1a 0.2 mmol) with p-tolu- azides bearing electron-donating groups, such as 4-
enesulfonyl azide (2a 1.1 equiv.) in 1,2-dichloroethane methylphenylsulfonyl azide (4aa), 4-methoxyphenyl-
at 508C with a dimeric cyclopentadienyl iridium com- sulfonyl azide (4ab) and 4-(tert-butyl)phenylsulfonyl
plex [IrCp*Cl2]2 (2 mol%), silver bis(trifluorometh- azide (4ac) gave the target products in excellent
anesulfonimide)
(8 mol%)
and
pivalic
acid yields (89% to 95%). Phenylsulfonyl azide afforded
(0.5 equiv.) for 12 h (Scheme 1). The isolatable mono- the product 4ad in a yield of 93%. Halogens in aryl-
(4a’) and bis-sulfonamidated products (4a) were ob- sulfonyl azides were tolerated well. 4-Fluorophenyl-
tained in 29% and 20% yields, respectively. The regio- sulfonyl azide (4ae), 4-chlorophenylsulfonyl azide
selectivity was not obviously improved after screening (4af) and 4-bromophenylsulfonyl azide (4ag) provided
various reaction parameters. Recently, the introduc- 87%, 89% and 90% yields, respectively. This catalytic
tion of an N-oxide function into quinolines, pyridines, system was also applied to the arylsulfonyl azides sub-
and azobenzene by Fagnou,[16] Chang,[10h,17] Wang,[18] stituted with strong electron-withdrawing groups and
and our group[19] could overcome the poor regioselec- was well tolerated. For example, 4-(trifluoromethyl)-
tivity. Inspired by these results, we proposed that the phenylsulfonyl azide and 4-acetylphenylsulfonyl azide
À
presence of the N O motif in 2-aryl-1,2,3-triazoles gave the corresponding products 4ah and 4ai in yields
might improve the regioselectivity.
of 89% and 81%, respectively. These results showed
Then, the reaction of 2-phenyl-1,2,3-triazole N- that functional groups commonly used in organic syn-
oxide (3a) with p-toluenesulfonyl azide (2a) was thesis, such as halogens, acetyl and nitro groups, were
chosen as a model reaction to screen various reaction compatible with the present conditions, which made
parameters (Table 1). 4aa was obtained as a main this reaction particularly attractive for increasing the
product in a 46% yield at 508C in the presence of molecular complexity by various chemical transforma-
2 mol% [IrCp*Cl2]2 and 8 mol% AgNTf2 (entry 1). tions. This transformation was slightly affected by
The molecular structure of 4aa was confirmed by steric hindrance. The yield was slightly decreased
single crystal X-ray diffraction analysis,[20] and is when the ortho-position of the phenylsulfonyl azide
shown in Figure S1 in the Supporting Information. No was substituted by a methyl group. For example, 3-
desired product was obtained in the absence of silver chloro-2-methyl (4aj) and 2,4-dimethyl substituents
salt, which could help the neutral [IrCp*Cl2]2 to be (4ak) gave rise to 83% and 77% yields, respectively.
converted into its cationic species (entry 2).[21] Bases A nitro group at the meta position of phenylsulfonyl
would suppress this reaction, and only a trace amount azide gave the desired product 4al in a yield of 72%.
of the target product was observed in the presence of It was worthwhile to note that aliphatic sulfonyl
sodium acetate (entry 3). To our delight, the yield im- azides also worked well. Benzylsulfonyl azide and eth-
proved to 88% when 0.5 equiv. of PivOH was added ylsulfonyl azide gave the corresponding products 4am
(entry 4). Then, several silver salts, such as AgNTf2, and 4an in yields of 97% and 96%, respectively. In
AgSbF6, AgBF4 and AgNO3 were screened. AgNTf2 addition, we tested condensed aromatic and heterocy-
was observed to be most effective in the presence of clic sulfonyl azides. The corresponding products 4ao
0.5 equiv. of PivOH (entry 4 vs. entries 5-7). Among and 4ap were obtained in 89% and 96% yields for
the solvents tested (toluene, CH3CN, dioxane, THF, naphthyl-2-sulfonyl azide and thienyl-2-sulfonyl azide,
DMSO, DMF and methanol), DCE proved to be the respectively. Only a trace amount of the desired prod-
best for this transformation (entries 8–14). The excel- uct was obtained when benzoyl azide was used as the
lent yield of 95% was afforded when the reaction substrate.
temperature was 608C for 10 h (entry 15). The yield Next, the scope of 2-aryl-1,2,3-triazole N-oxides
decreased when the temperature was increased to was investigated. In general, the 2-phenyl-1,2,3-tri-
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UPDATES
Bingfeng Zhu et al.
Table 1. Optimization of the sulfonamidation of 2-phenyl-1,2,3-triazole N-oxide with p-toluenesulfonyl azide.[a]
Entry
Catalyst
Additive
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2
–
–
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene
CH3CN
dioxane
THF
DMF
DMSO
CH3OH
DCE
DCE
DCE
DCE
DCE
46
NR
trace
88
NR
80
trace
27
NR
trace
85
trace
trace
54
95
85
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgSbF6
[IrCp*Cl2]2/AgBF4
[IrCp*Cl2]2/AgNO3
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[IrCp*Cl2]2/AgNTf2
[RuCp*Cl2]2/AgNTf2
[RhCp*Cl2]2/AgSbF6
[RuCl2(p-cymene)]2/AgSbF6
Pd(OAc)2
NaOAc
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
Cu(OAc)2
PhI(OAc)2
9
10
11
12
13
14
15[c]
16[d]
17[e]
18[f]
19
20[g]
21[h]
22
56
37
NR
NR
62
DCE
DCE
DCE
NR
[a]
Reaction conditions: 3a (0.2 mmol), 2a (1.1 equiv.), catalyst (2 mol%), silver salt (8 mol%), additive (0.5 equiv.), solvent
(1 mL), under air, 508C, 12 h. NR=no reaction. DCE=1,2-dichloroethane, THF=tetrahydrofuran, DMF=N,N-dime-
thylformamide, DMSO=dimethyl sulfoxide.
Isolated yield.
608C, 10 h.
808C, 3 h.
258C, 12 h.
[IrCp*Cl2]2 (1 mol%), AgNTf2 (4 mol%).
808C, 12 h.
Cu(OAc)2 (30 mol%), 808C, 12 h.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
azole N-oxides bearing electron-donating groups ortho-sulfonamidation products and the reaction
(4ba–4ca) gave higher yields than those with electron- mainly occurred on the side with the smaller steric
withdrawing groups (4da–4fa). For example, those hindrance. 2-(3-Methoxyphenyl)-2H-1,2,3-triazole 1-
with 4-methyl (4ba) and 4-methoxy (4ca) substituents oxide gave two products 4ja and 4ka in yields of 60%
at the phenyl moiety gave 85% and 89% yields, re- and 15%, respectively.
À
spectively. Those substrates with weak electron-with-
As is well known, the iridium-catalyzed direct C H
drawing groups fluoro (4da) and chloro (4ea) provid- amidation system has a wide range of substrates that
ed the corresponding products in yields of 60% and could form 5- or 6-membered iridacycle intermedi-
70%, respectively. A yield of 52% was obtained for 2- ates.[10e] When 2-phenyl-2H-1,2,3-triazole was used as
(4-nitrophenyl)-1,2,3-triazole 1-oxide (4fa). An ortho- the substrate under the optimized conditions, 25%
substituent at the phenyl moiety decreased the reac- mono- and 31% bis-sulfonamidated products were ob-
tivity, presumably due to the steric hindrance. For in- tained, which suggested that a nitrogen atom could be
stance, fluoro (4ga), nitro (4ha), and methyl (4ia) employed as the directing atom. An ortho-substituent
groups substituted at the 2-position of phenyl gave at the phenyl moiety for 2-phenyl-2H-1,2,3-triazole N-
rise to diminished yields. In addition, a meta-substitu- oxides affected the transformation dramatically, such
ent on the phenyl moiety produced a mixture of as for 4ga, 4ha and 4ia in Table 2. Nevertheless, prod-
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UPDATES
Iridium(III)-Catalyzed Direct C H Sulfonamidation
Table 2. Scope of substrates.[a,b]
[a]
[b]
Reaction conditions: 3 (0.2 mmol), 2 (1.1 equiv.), [IrCp*Cl2]2 (2 mol%), AgNTf2 (8 mol%), PivOH
(0.5 equiv.) in DCE (1 mL), under air, 608C, 10 h.
Isolated yield.
ucts 6a–6c were achieved in good to excellent yields azide to the iridium atom in intermediate I initiated
when deoxygenated substrates 5a–5c were used as a subsequent insertion of a sulfonamido moiety to
substrates under the standard reaction conditions afford intermediate II. Then, the sulfonamido transfer
(Table 3). These results suggested that it was difficult provided Ir(III) complex III along with releasing an
À
to rotate the C N bond between the triazole and ben- N2 molecule. Finally, protonolysis of III afforded the
zene rings due to the steric hindrance of the ortho- Ir-coordinated product V, presumably passing through
À
substituents at the phenyl moiety with the N O a hypothetical transition state IV. We have tried to
group, which made it harder for C H bond activation synthesize five-membered iridacycle intermediate I to
À
to take place after Ir coordination with the N-atom. verify the plausible mechanism we proposed. Un-
In addition, the phenomenon that the reaction was fortunately, intermediate I has not been isolated and
sensitive to the acidity of additives implied that the more detailed mechanistic studies are currently un-
protodemetallation process could be facilitated by an derway.
acid additive, which might influence the bond strength
2-[2-(4-Methylphenylsulfonamido)phenyl]-2H-1,2,3-
triazole 1-oxide (4aa) was easily reduced by PCl3
[23]
between the metal and amido moieties.[10h,22]
Based on the results obtained and the litera- and delivered the corresponding deoxygenated prod-
ture,[9,10e–h,22] a plausible pathway was proposed and is uct (4a’) in an excellent yield (Scheme 3), showing
À
shown in Scheme 2. First, treatment of the [IrCp*Cl2]2 that this protocol through dehydrogenative C N cou-
precursor with AgNTf2 generated a cationic Ir(III) pling is practical for the construction of the 2-(2-ami-
species, which coordinated with the N atom of the tri- doaryl)-1,2,3-triazole skeleton.
azole, and was followed by electrophilic attack at the
In summary, we have successfully developed an iri-
2-carbon of the phenyl ring, leading to a five-mem- dium-catalyzed sulfonamidation of 2-aryl-1,2,3-tri-
À
bered iridacycle intermediate I. Coordination of the azole N-oxides with sulfonyl azides via direct C H
Adv. Synth. Catal. 2016, 358, 326 – 332
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UPDATES
Bingfeng Zhu et al.
Table 3. Sulfonamidation of deoxygenated substrates 5.[a]
Scheme 3. Deoxygenation of 1,2,3-triazole N-oxide.
bond activation. A wide range of substrates was toler-
ated and efficiently provided the desired products
with excellent regioselectivity. External oxidants were
avoided and N2 was released as the sole by-product.
This protocol is convenient, efficient and applicable
for the preparation of various 2-aryl-1,2,3-triazoles.
Experimental Section
Typical Procedure
2-Phenyl-1,2,3-triazole N-oxide (32 mg, 0.2 mmol), p-tolu-
enesulfonyl azide (44 mg, 0.22 mmol), [IrCp*Cl2]2 (3.2 mg,
0.004 mmol), AgNTf2 (6.2 mg, 0.016 mmol), PivOH
(10.2 mg, 0.1 mmol), and anhydrous DCE (1 mL) were
added to a 25-mL reaction tube. The mixture was stirred at
608C for 10 h and then cooled down to room temperature.
The resulting mixture was filtered and washed with 50 mL
of ethyl acetate. The solvent was concentrated under
vacuum, and the resulting residue was purified by prepara-
tive thin layer chromatography (silica gel, ethyl acetate pe-
troleum ether=1:1, v/v) to give the desired product 4aa as
a white solid; yield: 62.7 mg (95%).
[a]
Reaction conditions:
5
(0.2 mmol), 2a (1.1 equiv.),
[IrCp*Cl2]2 (2 mol%), AgNTf2 (8 mol%), PivOH
(0.5 equiv.) in DCE (1 mL), under air, 608C, 10 h.
Isolated yield.
[b]
[c]
808C, 20 h.
Acknowledgements
This work was supported by the NSF of China (21172200,
21572072), and Zhengzhou University.
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