Palladium-catalyzed acyloxylation of 2-substituted 1,2,3-triazoles via direct sp2 C-H bond activation

Article abstract of DOI:10.1002/adsc.201400096

The palladium-catalyzed acyloxylation of 2-substituted 1,2,3-triazoles with acids via direct sp² C-H bond activation is described. This reaction is applied to a wide variety of 2-substituted 1,2,3-triazoles and exhibits excellent functional group tolerance. The route is found to be efficient for the preparation of 2-substituted 1,2,3-triazole derivatives.

Full text of DOI:10.1002/adsc.201400096

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DOI: 10.1002/adsc.201400096
Palladium-Catalyzed Acyloxylation of 2-Substituted
2
À
1,2,3-Triazoles via Direct sp C H Bond Activation
Zechao Wanga and Chunxiang Kuanga,b,*
a
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, Peopleꢀs Republic of China
E-mail: kuangcx@tongji.edu.cn
Key Laboratory of Yangze River Water Environment, Ministry of Education, Siping Road 1239, Shanghai 200092,
b
Peopleꢀs Republic of China
Received: January 5, 2014; Revised: April 9, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400096.
Abstract: The palladium-catalyzed acyloxylation of
2-substituted 1,2,3-triazoles with acids via direct sp2
C H bond activation is described. This reaction is
applied to a wide variety of 2-substituted 1,2,3-tri-
ACHTUNGTRENNUNGazoles and exhibits excellent functional group toler-
ance. The route is found to be efficient for the prep-
aration of 2-substituted 1,2,3-triazole derivatives.
To the best of our knowledge, 1,2,3-triazoles have
become one of the most relevant heterocycles in cur-
rent chemistry research.[11] In a broad scope of biolog-
ical active products, 1,2,3-triazoles have been exten-
sively investigated and applied in material science
and medicinal chemistry.[12] Recently various methods
for the direct catalytic functionalization of triazoles
have also been proven.[13] However, only few methods
have been reported for the direct functionalization of
2-substituted 1,2,3-triazoles.[14] In 2013, we reported
À
À
Keywords: acyloxylation; C H activation; palladi-
um; 2-substituted 1,2,3-triazoles
that a highly regioselective halogenation of 2-substi-
2
À
tuted 1,2,3-triazoles was developed via sp C H acti-
vation (Scheme 1).[15] Our team has recently reported
the palladium-catalyzed acylation of 2-aryl-1,2,3-tri-
À
Transition metal-catalyzed C H activation reactions
azoles with aldehydes (Scheme 2).[16] In our effort to
ACHTUNGTRENNUNG
are currently attracting interest.[1] The selective oxida- enhance the efficiency and practicality of C H activa-
À
À
tion of unactivated C H bonds is a significant re- tion reactions, we herein present the direct ortho-acyl-
search goal because of its importance in synthesis and
ACHTUNGTERNoNUGN xylation of 2-substituted 1,2,3-triazoles through pal-
the chemical industry.[2] The direct conversion of C H ladium-catalyzed sp C H bond oxidative activation
2
À
À
À
À
À
À
À
bonds into C C, C N, C S and C O bonds is a very to form the C O bond.
powerful methodology for building complex com-
Our initial investigations started with the reaction
pounds.[3] Importantly, most natural products, agro- of 2-phenyl-2H-1,2,3-triazole (1a) and benzoic acid
chemicals and pharmaceuticals contain suitable di- (2a). The reaction was performed with 2-phenyl-2H-
recting groups for this chemistry.[4] As such, these 1,2,3-triazole (1a, 1 equiv.), benzoic acid (2a,
transformations could be valuable for the derivatiza-
tion and analogue generation of such important
classes of molecules.[5] Over the past years, a wide
scope of metal catalysts, including Pd, Ru and Rh, has
been exploited with varying degrees of success.[6] Re-
À
cently, the development of regioselective C O bond
formation via C H bond functionalization has attract-
À
Scheme 1. Halogenation of 2-substituted 1,2,3-triazoles.
ed much attention.[7] In 2006, Yu described an elegant
example of CuACHTUNGTRENNUNG(OAc)2-catalyzed oxidative acetoxyla-
À
tion of arene C H bonds in HOAc/Ac2O using
oxygen as a clean oxidant at an elevated tempera-
ture.[8] In 2008, Sanford reported a detailed investiga-
tion of factors controlling the dominance of a directing
group in palladium-catalyzed ligand-directed arene
acetoxylation.[9] However, most reports on such trans-
formations are limited to acetoxylation.[10]
Scheme 2. Acylation of 2-substituted 1,2,3-triazoles.
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Table 1. Selected optimization of the reaction conditions.[a,b]
was increased to 65% when the temperature was in-
creased up to 1208C (Table 1, entry 15). Changing the
amount of PdACHTNUTRGNE(NUG OAc)2 to 5 mol% or 15 mol% disfa-
vored this reaction and gave yields of 45% and 20%,
respectively (Table 1, entries 16 and 17). The best
result was obtained using 10 mol% of PdACTHNURGTNEUNG(OAc)2 and
2 equiv. of K2S2O8 in DCE at 1208C for 20 h to afford
3a in high yield (65%), as shown in entry 15.
With the optimized conditions in hand, the scope of
carboxylic acids was investigated next (Table 2). At
the beginning of the experiment, we used aliphatic
carboxylic acids in this reaction (3a and 3b). The acyl-
AHCTUNGTREGoNNNU xyation of 2-substituted 1,2,3-triazoles by aromatic
carboxylic acids was also tested. Various substrates
worked well under the reaction conditions. A wide
range of functional groups, such as halogen, methoxyl
and trifluoromethyl groups, was tolerated in this pro-
cedure. For substituents R1, substrates with the elec-
tron-withdrawing group (CF3) showed slightly higher
reactivity than those with the electron-donating group
(OMe). However, 2-nitrobenzoic acid failed to deliver
the acyloxyation product via this procedure. In addi-
tion, 1-naphthoic acid was found to be favored in the
reaction, affording the desired product 3f with a slight-
ly decreased reactivity. The halogen moieties on ben-
zoic acids were all tolerated under these reaction con-
ditions and generated 3g to 3i in 53% to 73% yields.
A slight steric effect was observed when ortho-substi-
tuted benzoic acid (2g) reacted with 2-phenyl-2H-
1,2,3-triazole (1a). Also, we found that when cinnamic
[a]
Reaction conditions: 1a (0.4 mmol), 2a (0.48 mmol,
1.2 equiv.), catalyst (quantity noted), oxidant (0.8 mmol,
2 equiv.), solvent (2 mL) for 20 h in pressure tubes.
Isolated yield.
[b]
1.2 equiv.), 10 mol% of PdACTHNUTRGNENUG(OAc)2 and 2 equiv. of acid was subjected to the reaction, it afforded the
K2S2O8 in DCE at 1008C for 20 h, and generated the product 3j in 55% yield. 4-Trifluoromethylcinnamic
product 2-(2H-1,2,3-triazol-2-yl)phenyl benzoate (3a) acid reacted with the 2-phenyl-2H-1,2,3-triazole (1a)
in 50% yield (Table 1, entry 1). Based on this initial smoothly and yielded the corresponding product 3k in
finding, further investigations on the effect of various 71% yield. Halogen-substituted cinnamic acids were
reaction conditions were performed, as shown in converted to the corresponding products 3l and 3m in
Table 1. The solvent was found to function critically 66% and 65% yields, respectively. As shown in
in the reaction. When toluene or DMF was added as Table 2, aliphatic carboxylic acids exhibited higher re-
the solvent, the yields were decreased to 25% and activity than most of the aromatic carboxylic acids.
15%, respectively (Table 1, entries 2 and 5). The de- The tolerance of the reaction to these functional
sired product was not observed when using other sol- groups in substrates provided the possibility for fur-
vents, such as 1,4-dioxane and MeCN (Table 1, en- ther useful transformation of the products.
tries 3 and 4). Among the various oxidants used,
Inspired by the promising results above, we then fo-
K2S2O8 showed the best results, whereas TBHP cused on the effect of the 1,2,3-triazole directing
À
(70 wt% in water), H2O2 (40 wt% in water), Ag2CO3 group on the ortho C H acyloxylation of arenes. The
and
2,3-dichloro-5,6-dicyano-para-benzoquinone results showed that the 1,2,3-triazole group-directed
À
(DDQ) were all found to be inferior (Table 1, en- C H bond functionalization protocol is broadly appli-
tries 6 to 9).
cable to various substitutes affording the correspond-
Several other catalysts, such as PdCl2, AgOAc and ing acyloxylation products in 56% to 85% yields
Cu(OAc)2 were also screened. However, the yields (Table 3). Substrates with either electron-donating or
ACHTUNGTRENNUNG
were far from satisfactory in this transformation electron-withdrawing groups on the 2-substituted
(Table 1, entries 10 to 12). The results showed that 1,2,3-triazoles could be applied to yield the desired
temperature significantly affected this transformation. products 4a to 4l. However, electron-withdrawing sub-
No reaction took place at room temperature (Table 1, stituents were beneficial for the transformation,
entry 13). When the temperature was changed from whereas the electron-donating group decreased the
room temperature to 608C, a low product yield of efficiency (4c vs. 4d, 4i vs. 4j, 4k vs. 4l). When 2-sub-
40% of 3a was observed (Table 1, entry 14). The yield stituted 1,2,3-triazoles and acids were both present
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Palladium-Catalyzed Acyloxylation of 2-Substituted 1,2,3-Triazoles
Table 3. Scope of 2-substituted 1,2,3-triazoles.[a,b,c]
À
Table 2. Scope of aldehydes for Pd-catalyzed C H acyla-
ACHTUNGTRENNUNG
tion.[a,b,c]
[a]
Reaction conditions: 1b–d (0.4 mmol), 2a, b (2 mL),
PdACTHUNTGRENUNG(OAc)2, (10 mol%), K2S2O8 (0.8 mmol, 2 equiv.), DCE
(2 mL) for 20 h in pressure tubes (4a–4d).
Reaction conditions: 1c–f (0.4 mmol), 2c–e (0.48 mmol,
1.2 equiv.), PdACTHNUTRGNEUNG(OAc)2, (10 mol%), K2S2O8 (0.8 mmol,
2 equiv.), DCE (2 mL) for 20 h in pressure tubes (4e–4l).
[b]
[c]
[a]
Reaction conditions: 1a (0.4 mmol), 2a,
b (2 mL),
Pd(OAc)2, (10 mol%), K2S2O8 (0.8 mmol, 2 equiv.), DCE
ACHTUNGTRENNUNG
(2 mL) for 20 h in pressure tubes. The reaction failed
when the amount of 2a or 2b was 1.2 equiv.
Reaction conditions: 1a (0.4 mmol), 2c–m (0.48 mmol,
1.2 equiv.), PdACHTUNGTRENNUNG(OAc)2, (10 mol%), K2S2O8 (0.8 mmol,
2 equiv.), DCE (2 mL) for 20 h in pressure tubes.
Isolated yields.
Isolated yield.
[b]
[c]
have been made to investigate the acyloxylation of
other electron-donating groups. As for substrates con-
taining a meta-substituent on the 2-substituted 1,2,3-
with the electron-donating groups (OMe) in the reac- triazolesACTHNUGRTENUNG(4g–4j), modest selectivity was observed for
tion, no acyloxyation products were observed. Be- the acyloxylation of the less sterically hindered ortho-
À
cause of the unsatisfactory results of 4e, no efforts C H bond. Substitution of the 3,5-positions of the
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Scheme 5. Different directing groups
Scheme 3. Intermolecular kinetic isotope effect.
Scheme 6. Acyloxylation of 1-phenyl-1H-pyrazole.
active Pd(II) species for the next catalytic cycle.[21] To
prove this Pd(II)/Pd(IV) mechanism, we ran an ex-
periment without K2S2O8. In this reaction, 1a
(1 equiv.), PdACHTNUTRGNE(NUG OAc)2 (1 equiv.) and DCE (2 mL) were
added in a sealed tube. The reaction mixture was
stirred at 1208C for 10 h. Indeed, the intermediate A
was formed. Then benzoic acid was added in the
sealed tube. However, the desired product 3c was not
observed after 20 h. In this reaction, Pd(II) intermedi-
ate A was observed and could be isolated in 95%
yield. Therefore we can prove the mechanistic possi-
bility through a Pd(II)/Pd(IV) pathway in the present
acyloxylation reaction, but not the Pd(0)/Pd(II) path-
way.
With the developed protocol in hand, we tested
other common directing groups beside triazoles
(Scheme 5). This reaction was not limited to 2-substi-
Scheme 4. Possible reaction mechanism.
phenyl ring on 2-substituted 1,2,3-triazoles with tuted 1,2,3-triazoles. 1-phenyl-1H-pyrazole (5b) was
a methyl group resulted in failure of the reaction, also found to be favored in this reaction and generat-
which can probably be ascribed to the steric hin- ed the corresponding product 6b in 46% yield
drance.
(Scheme 6). However, other directing groups (5a, 5c,
The intermolecular kinetic isotope effects of 2-sub- 5d) failed to deliver the acyloxyation products in this
stituted 1,2,3-triazoles were studied. As shown in reaction.
Scheme 3, the kH/kD was found to be 3.0. It is consis-
tent with the C H cleavage being the rate-limiting ladium-catalyzed acyloxylation of 2-substituted 1,2,3-
step.
In conclusion, we have successfully developed a pal-
À
2
À
triazoles with acids via direct sp C H bond activa-
The Pd(II)/Pd(IV) mechanism has been reported tion. This method has afforded acyloxylation products
before.[17,18] Based on the well-documented literature in moderate to good yields. This protocol was conven-
for the reaction of various types of compounds via ient, efficient and applicable for various 2-substituted
a Pd(II)/Pd(IV) mechanism,[18] a possible mechanism 1,2,3-triazoles. Additionally, they generally exhibit
for the ortho-acyloxylation is outlined in Scheme 4. high levels of regioselectivity for functionalization at
À
The first step involves chelate-directed C H activa- the ortho-position. A wide substrate scope with good
tion of the substrate to afford the Pd(II) intermediate functional group tolerance has been demonstrated.
A.[7b,19,20] This Pd(II) intermediate A was oxidized by Current studies are focused on further exploration of
K2S2O8 in the presence of acids to afford the Pd(IV) the substrate scope and synthetic utility of this meth-
intermediate B.[19] Subsequently, reductive elimination odology as well as on probing the mechanism of this
would afford the desired product and regenerate the transformation.
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Palladium-Catalyzed Acyloxylation of 2-Substituted 1,2,3-Triazoles
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Typical Procedure for the Acyloxylation of 2-
Substituted 1,2,3-Triazoles
To a stirred solution of 2-phenyl-2H-1,2,3-triazole (1a)
(58 mg, 0.4 mmol, 1 equiv.), PdACHTUNRGTNEUNG(OAc)2 (9 mg, 0.04 mmol,
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10 mol%) and K2S2O8 (216 mg, 0.8 mmol, 2 equiv.) in DCE
(2 mL) at 1208C, acid (2) (2 mL) was added. The reaction
mixture was stirred for 20 h. Upon completion of the reac-
tion (as monitored by TLC), the mixture was cooled to
room temperature. The solvent was then evaporated under
vacuum. The resulting residue was purified by preparative
silica gel TLC to yield the product (3a, 3b, 4a to 4d).
Acid (2) (0.44 mmol, 1.2 equiv.) was added to an oven-
dried, sealed tube charged with 2-substituted 1,2,3-triazole
(1) (0.4 mmol, 1 equiv.), PdACHTNUGTRENUNG(OAc)2 (9 mg, 0.04 mmol,
10 mol%), and K2S2O8 (216 mg, 0.8 mmol, 2 equiv.) in DCE
(2 mL). The reaction mixture was stirred at 1208C for 20 h.
Upon completionof the reaction (as monitored by TLC), the
mixture was cooled to room temperature. The solvent was
then evaporated under vacuum. The resulting residue was
purified by preparative silica gel TLC to yield the product
(3c to 3m, 4e to 4l).
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Acknowledgements
The present work was supported by the Natural Science
Foundation of China (No. 21272174), the Key Projects of
Shanghai in Biomedicine (No.08431902700), and the Scientif-
ic Research Foundation of the State Education Ministry for
Returned Overseas Chinese Scholars. We also thank the
Center for Instrumental Analysis, Tongji University, China.
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Palladium-Catalyzed Acyloxylation of 2-Substituted 1,2,3-
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2
À
Triazoles via Direct sp C H Bond Activation
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Zechao Wang, Chunxiang Kuang*
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