Samarium(III)-catalyzed C(sp3)-H Bond activation: Synthesis of Indolizines via C-C and C-N coupling between 2-Alkylazaarenes and Propargylic Alcohols

Article abstract of DOI:10.1021/ol4034513

A new rare earth metal and samarium-catalyzed C(sp³)-H bond activation is reported in which 2-alkylazaarenes and propargylic alcohols were converted to indolizines. This process operates under mild conditions and solvent-free conditions. A broad scope of coupling partners has been established, and a likely mechanism has also been suggested.

Full text of DOI:10.1021/ol4034513

Letter
pubs.acs.org/OrgLett
Samarium(III)-Catalyzed C(sp3)−H Bond Activation: Synthesis of
Indolizines via C−C and C−N Coupling between 2‑Alkylazaarenes
and Propargylic Alcohols
Xu Wang, Shen-yan Li, Ying-ming Pan,* Heng-shan Wang, Hong Liang, Zhen-feng Chen,*
and Xiao-huan Qin
Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), School of
Chemistry & Chemical Engineering of Guangxi Normal University, Guilin 541004, People’s Republic of China
S
* Supporting Information
ABSTRACT: A new rare earth metal and samarium-catalyzed
C(sp3)−H bond activation is reported in which 2-alkylazaar-
enes and propargylic alcohols were converted to indolizines.
This process operates under mild conditions and solvent-free
conditions. A broad scope of coupling partners has been established, and a likely mechanism has also been suggested.
Table 1. Optimization of the Formation of Substituted
Indolizine
ransition metal catalyzed activation of C−H bonds has
become a powerful strategy for organic synthesis in an
a
T
energy-efficient and step-economic fashion.1·2 Among the many
elegant examples of transition metal catalyzed C−H activation,
chelation-assisted protocols have recently attracted increasing
attention, because this is an advantageous process in that no
prefunctionalization of C−H bonds is necessary.3 In recent
decades, impressive achievements have been made in enhancing
the efficiency of direct C−H bond activation of heteroaromatics
using various transition metal catalysts such as Pd,4 Ru,5 Rh,6
Cu,7 Ir,8 and others.9 It has been realized that transition metal
catalysts stand out in the area of C−C coupling that proceed via
a C−H activation pathway owing to their high functional group
tolerance and a wide range of synthetic utility. However,
relatively little work has focused on the unreactive C(sp3)−H
bonds, because the C(sp3)−H bonds are less acidic and lack
proximal empty low-energy or filled high-energy orbitals that
readily interact with orbitals of the metal. For these reasons, the
efficient and atom-economic activation of C(sp3)−H bonds
have emerged as an attractive and challenging goal. On the
other hand, the use of rare earth metal catalyzed reactions has
emerged as a versatile tool for developing syntheses due to their
numerous advantages, namely, their relatively high efficiency,
water compatibility, mild reaction conditions, and eco-friendly
catalytic reactions.10 To the best of our knowledge, there have
been no reports in the literature concerning the rare earth metal
catalyzed activation of C(sp3)−H bonds. As a result of our
efforts in the development of transition-metal-catalyzed C−H
functionalization,11 herein, we describe a new samarium-
catalyzed C(sp3)−H bond activation of 2-alkylazaarenes with
propargylic alcohols for the formation of indolizines12,13 under
mild conditions with minimal waste production.
b
entry
catalyst
Sc(OTf)3
time (h)
yield (%)
1
24
24
24
24
24
24
24
24
24
24
24
24
24
6
20
0
2
In(OTf)3
Zn(OTf)2
Er(OTf)3
Ce(OTf)3
Cu(OTf)2
Y(OTf)3
3
0
4
0
5
0
6
0
7
0
8
Pr(OTf)3
AuBr3
0
9
0
10
11
12
13
14
[RhCp*Cl2]2
AgOTf
0
0
Pd(MeCN)2Cl2
Bi(OTf)3
Sm(OTf)3
Sm(OTf)3
Sm(OTf)3
0
15
90
45
91
c
15
d
6
16
6
a
Reaction conditions: 1a (0.3 mmol), 2a (1.5 mmol), catalyst (10 mol
% to 1a), solvent-free, 120 °C, sealed tube. Isolated yield of pure
product based on 1a. The reaction was carried out using 5 mol %
b
c
d
catalyst. The reaction was carried out using 15 mol % catalyst.
In(OTf)3, Zn(OTf)2, Er(OTf)3, Ce(OTf)3, Cu(OTf)2, Y-
(OTf)3, Pr(OTf)3, AuBr3, [RhCp*Cl2]2, AgOTf, and Pd-
(MeCN)2Cl2, the desired product 3aa was not obtained at all
(Table 1, entries 2−12). However, when Bi(OTf)3 and
To identify suitable conditions for the reaction, a series of
catalysts were screened as shown in Table 1. Initially, the
desired product 3aa was obtained in 20% yield in the presence
of 10 mol % Sc(OTf)3 in a solvent-free and sealed tube at 120
°C for 24 h (Table 1, entry 1). With other catalysts, including
Received: November 28, 2013
© XXXX American Chemical Society
A
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Organic Letters
Letter
Sm(OTf)3 were used as the catalysts, the desired product 3aa
was obtained in 15% and 90% yields, respectively (Table 1,
entries 13 and 14). Notably, the Sm(OTf)3-catalyzed reaction
provided a higher yield and required a shorter reaction time. A
very slow reaction rate and low yield were also observed when
the catalytic amount of Sm(OTf)3 decreased from 10 to 5 mol
% (Table 1, entry 15 vs 14), but no obvious improvement in
the yield could be obtained as the amount of Sm(OTf)3 was
increased to 15 mol % (Table 1, entry 16 vs 14). Hence, 1a (0.3
mmol), 2a (1.5 mmol), Sm(OTf)3 (10 mol %), solvent-free,
and in a sealed tube at 120 °C for 6 h were chosen as the
optimized conditions.
reaction has features of high yields with water as the only
byproduct, suggesting good prospects toward commercializa-
tion.
Additionally, all the secondary propargylic alcohols 1 bearing
not only terminal alkyne groups but also internal alkyne groups
participated well in the C−H functionalization/cyclization,
producing the products in good yields and showing good
functional group tolerance. The propargyl alcohols 1c
possessing an electron-donating group on the aryl ring (R1 =
4-MeC6H4) afforded the desired products 3ca−3ce in 91%−
96% yields (Table 2, entries 13−15). Substrates 1b and 1e
possessing an electron-withdrawing group (R1 = 4-FC6H4 and
2-ClC6H4) on the benzene ring also reacted smoothly and
afforded the desired products in high yields (Table 2, entries
6−12). We also have observed some variation in yields as a
function of electronic effects: that is, aromatic propargyl
alcohols with an electron-donating group at the benzene ring
gave the corresponding products in higher yields than propargyl
alcohols which possessed an electron-withdrawing group on the
benzene ring (Table 2, entries 13−15 vs entries 6−12). The
electron-donating group presumably facilitated the process that
converted propargyl alcohols into propargylic cations. The
crystallization of compound 3ee from ethanol gave a single
crystal suitable for X-ray analysis. Figure 1 illustrates the
With these optimal reaction conditions in hand, we started to
investigate the scope and limitation of this reaction, and the
results are summarized in Table 2. To our delight, various
Table 2. Synthesis of Substituted Indolizines from Propargyl
a
Alcohols and 2-Alkylazaarenes
yield
b
entry
propargyl alcohol
1a: R1 = R2 = Ph
1a: R1 = R2 = Ph
1a: R1 = R2 = Ph
1a: R1 = R2 = Ph
1a: R1 = R2 = Ph
2-alkylazaarene product
(%)
1
2a: R3 = H
3aa
3ab
3ac
3ad
3ae
3ba
3ea
3bb
3eb
3ec
3ed
3ee
3ca
90
85
95
92
93
79
80
81
82
92
86
89
91
2
2b: R3 = CH3
2c: R3 = COOEt
2d: R3 = Ph
2e: R3 = CN
3
4
5
6
1b: R1 = 4-FC6H4; R2 = Ph 2a: R3 = H
1e: R1 = 2-ClC6H4; R2 = Ph 2a: R3 = H
1b: R1 = 4-FC6H4; R2 = Ph 2b: R3 = CH3
1e: R1 = 2-ClC6H4; R2 = Ph 2b: R3 = CH3
1e: R1 = 2-ClC6H4; R2 = Ph 2c: R3 = COOEt
1e: R1 = 2-ClC6H4; R2 = Ph 2d: R3 = Ph
1e: R1 = 2-ClC6H4; R2 = Ph 2e: R3 = CN
7
8
9
10
11
12
13
1c: R1 = 4-MeC6H4; R2 =
2a: R3 = H
Ph
Figure 1. X-ray crystal structure of indolizine 3ee.
14
15
1c: R1 = 4-MeC6H4; R2 =
2c: R3 = COOEt
2e: R3 = CN
3cc
3ce
96
95
Ph
molecular structure of the substituted indolizine 3ee. In
addition, a propargyl alcohol bearing a heterocyclic substituent
such as 1d (R1 = 2-thienyl) gave the desired product 3db in
83% yield (Table 2, entry 16). Interestingly, an unactivated
terminal propargyl alcohol such as 1-phenyl-prop-2-yn-1-ol 1f
(R2 = H) was efficient, coupling with pyridin-2-yl-acetic acid
ethyl ester 2c to form the substituted indolizine 4fc in 80%
yield (Table 2, entry 17). The reaction of propargylic alcohol
1g (R2 = TMS) with 2c also led to the coupled product 4gc in
81% yield (Table 2, entry 18). However, when the propargylic
aliphatic ether (e.g., benzyl propargyl ether) and the propargylic
alcohols with alkyl groups (R1 or R2 = alkyl) were used, the
reaction failed to afford the desired products.
Fortunately, the reaction of 2-methyl-quinoline 4a with
propargyl alcohol 1a under the standard conditions produced
the desired product 2-benzyl-1,3-diphenyl-pyrrolo[1,2-a]-
quinoline 4aa in 78% yield (Scheme 1). The structure of 4aa
was confirmed by extensive spectroscopic analysis (see
Suporting Information pp S27, S33). The pyrrolo[1,2-a]-
quinoline derivatives have significant antibacterial and anti-
fungal effects. Of particular note is that pyrrolo[1,2-a]quinoline
derivatives are important synthetic targets for developing new
pharmaceuticals.14 Herein, we have provided a rapid method
for the synthesis of the pyrrolo[1,2-a]quinoline derivatives.
1c: R1 = 4-MeC6H4; R2 =
Ph
16
17
18
1d: R1 = 2-Thienyl; R2 = Ph 2b: R3 = CH3
3db
4 fc
4gc
83
80
81
1f: R1 = Ph; R2 = H
1g: R1 = Ph; R2 = TMS
2c: R3 = COOEt
2c: R3 = COOEt
a
Reaction conditions: 1 (0.3 mmol), 2 (1.5 mmol), Sm(OTf)3 (10
b
mol % to 1), solvent-free, at 120 °C, sealed tube, 6 h. Isolated yield of
pure product based on 1.
pyridines with C-2 functional groups such as CH2CN,
CH2COOEt, CH2Ph, and CH2CH3 were compatible with the
reaction conditions, resulting in the formation of the desired
products in high yields. The reactions of 2-alkylazaarenes
having an electron-withdrawing group at R3, such as CN,
COOEt, and Ph groups, furnished the corresponding products
3ac−3ae in 92−95% yields (Table 2, entries 3−5). The 2-
alkylazaarenes bearing an electron-donating group at R3, such as
the CH3 group, also gave the desired product 3ab in high yield
(Table 2, entry 2). Obviously, electron-poor pyridines with C-2
functional groups provided the desired products in higher
yields than electron-rich pyridines with C-2 functional groups
(Table 2, entries 3−5 vs entry 2). The C6 substituted pyridine
such as 2,6-lutidine has also been examined in the reaction.
However, the desired product was not obtained at all. This
B
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Organic Letters
Letter
and propargylic alcohols. The reaction proceeds under solvent-
free conditions and displays wide functional group compati-
bility. The ability to incorporate halogen substituents, as well as
cyano and ester groups, into the product makes this process
potentially valuable for further synthetic transformations.
Scheme 1. Synthesis of 2-Benzyl-1,3-diphenyl-pyrrolo[1,2-
a]quinoline 4aa from 2-Methyl-quinoline and Propargyl
Alcohol
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterization of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
On the basis of these data, we propose the process detailed in
Scheme 2 as the most likely mechanism for this transformation.
Scheme 2. Possible Reaction Mechanism
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: panym2013@hotmail.com.
*E-mail: chenzfubc@yahoo.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the project 973 (2011CB512005), Ministry of
Education of China (IRT1225), the National Natural Science
Foundation of China (21362002, 41206077, and 81260472),
Guangxi Natural Science Foundation of China
(2012GXNSFAA053027 and 2011GXNSFD018010), and
Bagui Scholar Program of Guangxi for financial support.
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