Bi(OTf)3-catalyzed intramolecular amination of triazenylaryl allylic alcohols: A stereoselective, high-yield synthesis of (E)-3-Alkenyl 2 H -indazoles

Article abstract of DOI:10.1080/00397911.2014.903423

GRAPHICAL ABSTRACT An efficient Bi(OTf)₃-catalyzed synthesis of 3-alkenyl-2-pyrrolidine-2H-indazoles from triazenylaryl allylic alcohols via the intramolecular direct amination process is reported. Compared with the dodecyl benzenesulfonic acid (DBSA)-catalyzed method, the new method is more efficient and gives greater yields and functionality tolerance. Additionally, the 3-alkenyl-2-pyrrolidine-2H-indazoles can be transformed to a series of new products under different reaction conditions.

Full text of DOI:10.1080/00397911.2014.903423

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Bi(OTf)3-Catalyzed Intramolecular
Amination of Triazenylaryl Allylic
Alcohols: A Stereoselective, High-Yield
Synthesis of (E)-3-Alkenyl 2H-indazoles
Weijun Yang a , Zhigen Yang b , Lijun Xu b , Lili Zhang b , Xin Xu b ,
Maozhong Miao b & Hongjun Ren b
a Department of Chemistry , Zhejiang University , Hangzhou , China
b Department of Chemistry , Zhejiang Sci-Tech University ,
Hangzhou , China
Accepted author version posted online: 06 May 2014.Published
online: 01 Jul 2014.
To cite this article: Weijun Yang , Zhigen Yang , Lijun Xu , Lili Zhang , Xin Xu , Maozhong Miao &
Hongjun Ren (2014) Bi(OTf)3-Catalyzed Intramolecular Amination of Triazenylaryl Allylic Alcohols: A
Stereoselective, High-Yield Synthesis of (E)-3-Alkenyl 2H-indazoles, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:17, 2478-2487,
DOI: 10.1080/00397911.2014.903423
To link to this article: http://dx.doi.org/10.1080/00397911.2014.903423
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Synthetic Communications1, 44: 2478–2487, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.903423
Bi(OTf)3-CATALYZED INTRAMOLECULAR AMINATION
OF TRIAZENYLARYL ALLYLIC ALCOHOLS:
A STEREOSELECTIVE, HIGH-YIELD SYNTHESIS
OF (E)-3-ALKENYL 2H-INDAZOLES
Weijun Yang,1 Zhigen Yang,2 Lijun Xu,2 Lili Zhang,2 Xin Xu,2
Maozhong Miao,2 and Hongjun Ren2
1Department of Chemistry, Zhejiang University, Hangzhou, China
2Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, China
GRAPHICAL ABSTRACT
Abstract An efficient Bi(OTf)3-catalyzed synthesis of 3-alkenyl-2-pyrrolidine-2H-indazoles
from triazenylaryl allylic alcohols via the intramolecular direct amination process is reported.
Compared with the dodecyl benzenesulfonic acid (DBSA)-catalyzed method, the new
method is more efficient and gives greater yields and functionality tolerance. Additionally,
the 3-alkenyl-2-pyrrolidine-2H-indazoles can be transformed to a series of new products
under different reaction conditions.
Keywords Alkenyl 2H-indazole; amination; bismuth triflate; triazenylaryl allylic
alcohols
INTRODUCTION
Recently there has been a surge of renewed interest in the construction of
indazoles, especially 2H-indazoles, because these compounds have been reported
to have diverse applications[1] including antifungal agents,[2] anthelmintic agents,[3]
estrogen receptor b ligands,[4] c-secretase modulators,[5] and liver X receptor modu-
lators.[6] Compared with the thermodynamically favored 1H-indazoles, 2H-indazoles
Received February 16, 2014.
Address correspondence to Hongjun Ren, Department of Chemistry, Zhejiang Sci-Tech University,
Hangzhou 310018, China. E-mail: renhj@zstu.edu.cn
2478

Bi(OTf)3-CATALYZED SYNTHESIS OF 2H-INDAZOLES
2479
are much less explored. Recently, we reported a dodecyl benzenesulfonic acid
(DBSA)-catalyzed route to the efficent synthesis of trans-3-alkenyl 2H-indazoles
from triazenylaryl allylic alcohols in water.[7] A variety of triazenylaryl allylic
alcohols were converted into the corresponding 2H-indazoles, but the substrates
with the nonsubstituted benzyl alcohol and with Cl or Br on the aryl triazene motif
are not adaptive (Scheme 1). Although various reaction conditions including
increase of the DBSA concentration, extension of reaction time, and elevation of
temperature were tested, the yield could not be improved. Thus, to overcome this
problem a brand new and more compatible reaction system is needed.
Recently, with increasing environmental concerns, the need for more efficient,
sustainable, and environmentally friendly organic synthesis has gained increased
importance. Bismuth(III) salts, as typical green reagents and catalysts, have been
widely applied to the synthesis of natural products or important pharmaceuticals.[8]
Catalytic quantities of Bi(III) compounds are effective in promoting different type
of transformations[9] including epoxide opening,[10] Claisen rearrangement,[11]
protection=deprotection,[12] Mannich-type reactions,[13] Mukaiyama aldol reaction,[14]
and Diels–Alder reaction.[15] Among them, bismuth-catalyzed direct substitution of
alcohols has emerged as a desirable area of research because this approach does
not need preactivation of the alcohols. Recent elegant examples include tandem
ene-reaction=hydroamination of amino-olefin and amino-allene compounds with
some enophiles,[16] intramolecular alkynylcyclopropanation of olefins by Takaki
and coworkers,[17] benzylation of arenes or 2,4-pentanediones, allylic alkylation
of 2,4-pentanediones by Rueping,[18,19] and bismuth-catalyzed direct amination of
allylic, propargylic, and benzylic alcohols with amides by Shibasaki and cowor-
kers.[20] In addition, the intramolecular 1,3-chirality transfer reaction catalyzed by
Bi(OTf)3 for the formation of chiral 1-substituted tetrahydroisoquinolines from
the corresponding chiral allylic alcohols was also developed.[21] As a new
approach to direct substitutition of alcohols, herein we present a useful Bi(OTf)3-
catalyzed intramolecular amination strategy for the direct formation of
3-alkenyl-2-pyrrolidine-2H-indazoles from triazenylaryl allylic alcohols.[22]
Scheme 1. DBSA-catalyzed amination and limitation.

2480
W. YANG ET AL.
RESULTS AND DISCUSSION
Initially, we treated the substrate 1a with BF3.OEt2(5 mol%) in CH2Cl2,
leading to the formation of the desired 2H-indazoles 2a in 64% yield (Table 1, entry
1). Upon treating the substrate 1a with CuBr (5 mol%) in CH2Cl2, the starting material
was recovered in 95% yield and a trace of the desired 2H-indazole compound was
obtained (entry 2). To our delight, further examination on InCl3 and FeCl3 resulted
in improved yields of 71% and 74%, respectively (entries 3 and 4). Using BiCl3 as
the Lewis acid catalyst, the yield of the desired product 2a decreased to 13% (entry
5). When the reaction was carried out using Bi(OTf)3 as the catalyst in CH2Cl2, the
yield increased to 94%. Thus the best result was obtained when the reaction was
carried out in CH2Cl2 at room temperature using Bi(OTf)3 as the catalyst (entry 6).
With the optimized conditions in hand, a variety of triazenylaryl allylic alco-
hols 1 were examined, and the results are summarized in Table 2. Compared with
catalyst DBSA, Bi(OTf)3 gave greater yields in most cases, as indicated by the last
two columns of Table 2. Particularly, the formerly inadaptable substrates 1c and
1d were successfully converted to the corresponding 2H-indazoles (Table 2,
66–68%, entries 2 and 3). The electron property at the aryl triazene moiety signifi-
cantly influenced the outcome of the reaction: The yields of the 2H-indazoles
increased sharply from trace to 95% as stronger electron-withdrawing groups (entries
1–4, Table 2) were directed to the aryl triazene motif. The electronic identity of the
benzyl alcohol substituents also influenced the yield of the transformation: The yield
surged to 90%, when using OCH3 (entry 5) instead of H (entry 4) on the benzyl alco-
hol moiety for the substrates 1e and 1f. Substrates with electron-donating substitu-
ents such as CH3 (90%, entry 12), OCH3 (92–96%, entries 13 and 14), and dimethyl
Table 1. Optimization of the reaction conditionsa
Entry
Cat. (mol%)
Yieldb (%)
1
2
3
4
5
6
BF3 ꢀ OEt2 (5)
CuBr (5)
InCl3 (5)
64
4
71
74
13
94
FeCl3 (5)
BiCl3 (5)
Bi(OTf)3 (5)
aAll reactions were carried out with 3-aryl triazenylaryl allylic alcohol 1a
(0.5 mmol) and catalyst in DCM (2 mL) at room temperature for 12 h.
bYield of the isolated product after flash column chromatography.

Bi(OTf)3-CATALYZED SYNTHESIS OF 2H-INDAZOLES
2481
Table 2. Scope of synthesis of the functionalized 3-alkenyl-2-pyrrolidine-2H-indazoles from triazenylaryl
allylic alcohol 1a
Yield Yield
(%)a,b (%)c,d
Entry
Substrate
Product
=
1b: R CN
=
1c: R Cl
=
1d: R Br
=
1e: R CH3
=
2b: R CN
=
2c: R Cl
=
2d: R Br
=
2e: R CH3 Trace Trace
1
2
3
4
95
68
66
99
Trace
Trace
=
1f: R CH3
=
2f: R CH3
5
6
90
91
52
78
=
H
=
H
1 g: R
2 g: R
7
1 h
2 h
74
79
=
=
F
=
2j: R Cl
=
2k: R Br
8
1i: R
F
1j: R Cl
2i: R
85
86
90
81
90
92
83
77
59
57
92
95
=
9
=
1k: R Br
10
11
12
13
=
=
2 l: R I
1 l: R
I
1 m: R CH3
=
=
2 m: R CH3
=
2n: R OCH3
=
1n: R OCH3
14
1o
2o
96
83
15
1p
2p
94
79
(Continued )

2482
Entry
16
W. YANG ET AL.
Table 2. Continued
Yield Yield
(%)a,b (%)c,d
Substrate
Product
1q
2q
99
99
17
1r
2r
88
84
18
1 s
2 s
95
98
19
1 t
2 t
92
92
aBi(OTf)3-catalyzed reaction: All reactions were carried out with triazenylaryl allylic alcohol 1
(0.5 mmol), and Bi(OTf)3 (0.025 mmol, 5 mol%) in DCM (2 mL) at room temperature for 12 h.
bYield of the isolated product of Bi(OTf)3-catalyzed reaction after flash column chromatography.
cDBSA-catalyzed reaction: All reactions were carried out with triazenylaryl allylic alcohol 1 (0.5 mmol)
and DBSA (0.1 mmol, 20 mol%) in water (2 mL) at 35 ꢁC for 24 h to 4 d.
dYield of the isolated product of DBSA-catalyzed reaction after flash column chromatography from
Ref. 7.
groups (94–99%, entries 15 and 16) on the benzyl alcohol moiety showed greater
reactivity than electron-withdrawing ones (81–90%, entries 8–11) for the substrates
1i–q. All of these results suggest that enhancement of either the electron-withdrawing
ability on triazene moiety or the electron-donating ability on the benzyl alcohol unit
may facilitate the cyclization progress. Aromatic motifs including naphthyl and
biphenyl groups and cyclopropyl group were successfully incorporated into the
3-alkenyl-2-pyrrolidine-2H-indazole products in 88–95% yield, as shown in the
formation of the products 2r–t (entries 17–19).
Under the same conditions, the substrates 1-aryl triazene cinnamyl alcohols
were further investigated. Upon treating the 1-aryl triazene cinnamyl alcohols 3 with

Bi(OTf)3-CATALYZED SYNTHESIS OF 2H-INDAZOLES
2483
Scheme 2. Synthesis of 3-alkenyl-2-pyrrolidine-2H-indazoles from 1-aryl triazene cinnamyl alcohols 3.
All reactions were carried out with 1-aryl triazene cinnamyl alcohol 3 (0.5 mmol) and Bi(OTf)3
(0.025 mmol, 5 mol%) in DCM (2 mL) at room temperature for 12 h. Yields shown are of the isolated
product after flash column chromatography.
Bi(OTf)3 in dicholoromethane (DCM) for 12 h, substrates with ester, cyano, and
naphthyl groups were all converted to the corresponding 3-alkenyl-2-pyrrolidine-
2H-indazoles in good to excellent yields (Scheme 2).
On the basis of these mentioned results, a plausible reaction mechanism for this
process is depicted in Scheme 3. The interaction between the hydroxy group of tria-
zenylaryl allylic alcohol 1=triazene cinnamyl alcohol 3 and the Lewis acid Bi(OTf)3
led to the activation of the hydroxy and formed the intermediate cation 4=5, which
lost HOBi(OTf)2 to form allylic carbocation 6. Intramolecular nucleophilic attack
of allylic carbocation 6 by nitrogen on the triazene motif provided the intermediate
cation 7, which underwent deprotonation to generate the product 3-alkenyl-2-
pyrrolidine-2H-indazole 2.
The 3-alkenyl-2-pyrrolidine-2H-indazole 2a can be converted to various
products under different reaction conditions (Scheme 4). Upon cleavage of pyr-
rolidine from 3-alkenyl-2-pyrrolidine-2H-indazole 2a with Zn in CH3COOH,
3-alkenyl-1H-indazole 8a was obtained in 75% yield. By treating pyrrolidine-
substituted 2H-indazole 2a with Pd=C-H2 in EtOH at room temperature, the
alkene reduction product 8b was formed in 99% yield. Further cleavage pyrroli-
dine from 8b with Zn=CH3COOH led to the 1H-indazole 8e in 91% yield. Selective
oxidation can also be achieved by switching oxidants. When KMnO4 was used,
the 3-alkenyl-2-pyrrolidine-2H-indazole 2a could be oxidized to 3-formyl-2-
pyrrolidine-2H-indazole (8d) in 60% yield, while dihydroxylation product 8c was
obtained in 90% yield with OsO4 and 4-methylmorpholine N-oxide.
In conclusion, a convenient and facile protocol for the synthesis of
3-alkenyl-2-pyrrolidine-2H-indazoles from 3-aryl triazene allylic alcohols via the

2484
W. YANG ET AL.
Scheme 3. Plausible reaction mechanism for this reaction.
intramolecular direct amination process using Bi(OTf)3 catalysis in DCM has been
developed. Additionally, the 3-alkenyl-2-pyrrolidine-2H-indazoles can be further
transformed to a series of new products under different reaction conditions.
Scheme 4. Transformations of the 3-alkenyl-2-pyrrolidine-2H-indazole 2a.

Bi(OTf)3-CATALYZED SYNTHESIS OF 2H-INDAZOLES
2485
Further investigations of the reaction mechanism and synthetic applications are
under way.
EXPERIMENTAL
Typical Procedure for the Synthesis of 3-Alkenyl-2-pyrrolidine-
2H-indazoles 2 from Triazenylaryl Allylic Alcohols 1
Bi(OTf)3 (17 mg, 0.025 mmol) was added to a solution of triazenylaryl allylic
alcohol 1 (0.5 mmol) in DCM (2 mL). The reaction mixture was stirred at room
temperature for 12 h. Ethyl acetate and water were added, and the resulting mixture
was extracted with ethyl acetate. The organic fractions were dried over Na2SO4,
concentrated in vacuo, and purified by silica-gel chromatography to provide the
3-alkenyl-2-pyrrolidine-2H-indazoles 2.
Representative Analytical Data
(E)-5-Chloro-2-(pyrrolidin-1-yl)-3-styryl-2H-indazole (2c). Rf ¼ 0.44 (10:1
1
petroleum ether=EtOAc); mp ¼ 128–130 ꢁC; H NMR (400 MHz, CDCl3): d 7.94
(d, J ¼ 1.2 Hz, 1 H), 7.64–7.59 (m, 3 H), 7.56 (d, J ¼ 17.2 Hz, 1 H), 7.45–7.23
(m, 5 H), 3.51–3.38 (m, 4 H), 2.15–2.02 (m, 4 H); 13C NMR (100 MHz, CDCl3):
d 144.2, 137.0, 131.6, 131.0, 128.8, 128.2, 127.6, 127.5, 126.6, 119.6, 119.1, 115.5,
55.2, 23.1; IR (ATR-FTIR): 1942, 1394, 1211, 1110, 956, 805, 685 cmꢂ1; MS (EI):
m=z (%): 325 (Mþ, Cl37, 18), 323 (Mþ, Cl35, 45), 253 (100), 218 (75), 189 (58), 163
(12), 91 (14), 70 (40). HRMS (EI-TOF) calcd. for C19H18ClN3(Mþ): 323.1189;
found: 323.1187.
(E)-5-Bromo-2-(pyrrolidin-1-yl)-3-styryl-2H-indazole (2d). Rf ¼ 0.45 (10:1
1
petroleum ether=EtOAc); mp ¼ 134–136 ꢁC; H NMR (400 MHz, CDCl3): d 8.10
(s, 1 H), 7.60 (d, J ¼ 7.2 Hz, 2 H), 7.58–7.50 (m, 2 H), 7.50–7.23 (m, 5 H),
3.55–3.33 (m, 4 H), 2.15–2.00 (m, 4 H); 13C NMR (100 MHz, CDCl3): d 144.2,
137.0, 131.4, 131.1, 129.7, 128.8, 128.2, 126.6, 122.9, 119.3, 118.8, 115.4, 55.2,
23.1; IR (ATR-FTIR): 1206, 1106, 948, 807, 739, 681 cmꢂ1; MS (EI): m=z (%):
369 (Mþ, Br81, 6), 367 (Mþ, Br79, 6), 300 (100), 218 (45), 189 (49), 70 (10). HRMS
(EI-TOF) calcd. for C19H18BrN3(Mþ): 367.0684; found: 367.0681.
(E)-Ethyl 3-(2-Cyclopropylvinyl)-2-(pyrrolidin-1-yl)-2H-indazole-5-carboxylate
1
(2t). Rf ¼ 0.60 (5:1 petroleum ether=EtOAc); mp ¼ 72–74 ꢁC; H NMR (400 MHz,
CDCl3): d 8.55 (s, 1 H), 7.89 (dd, J ¼ 9.2, 1.2 Hz, 1 H), 7.58 (d, J ¼ 9.2 Hz, 1 H),
6.97 (d, J ¼ 16.4 Hz, 1 H), 6.22 (dd, J ¼ 16.0, 1.2 Hz, 1 H), 4.39 (q, J ¼ 7.1 Hz, 2
H), 3.49–3.30 (m, 4 H), 2.14–1.96 (m, 4 H), 1.76–1.61 (m, 1 H), 1.40 (t, J ¼ 7.2 Hz,
3 H), 0.97–0.92 (m, 2 H), 0.69–0.65 (m, 2 H); 13C NMR (100 MHz, CDCl3):
d 167.2, 147.2, 140.3, 134.4, 125.9, 125.3, 123.2, 116.81, 116.77, 114.6, 60.7, 54.9,
23.1, 15.7, 14.4, 8.1; IR (ATR-FTIR): 1698, 1277, 1230, 950, 771 cmꢂ1; MS (EI):
m=z (%): 325 (Mþ, 18), 280 (10), 256 (100), 241 (86), 227 (50), 213 (45), 183 (45),
168 (30), 153 (29), 70 (84). HRMS (EI-TOF) calcd. for C19H23N3O2 (Mþ):
325.1790; found: 325.1790.

2486
W. YANG ET AL.
FUNDING
We are thankful for financial support from the National Nature Science Foun-
dation of China (21272003), as well as the Science Foundation of Zhejiang Sci-Tech
University (Grant Nos. 1206820-Y and 1206821-Y).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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