Chiral N,N′-Dioxide/ScΙΙΙ Complex-Catalyzed Asymmetric Ring-Opening Reaction of Cyclopropyl Ketones with Indoles

Article abstract of DOI:10.1002/adsc.201800312

The asymmetric ring-opening reaction of cyclopropyl ketones with indoles has been realized by using a N,N′-dioxide/scandium(ΙΙΙ) complex as catalyst. The corresponding 3-alkylated indole derivatives were obtained in moderate to excellent yields with good ee values. Moreover, a possible transition state was proposed on the basis of experimental studies and X-ray structure of product. (Figure presented.).

Full text of DOI:10.1002/adsc.201800312

Title: Chiral N,N'-Dioxide/ScΙΙΙ Complex-Catalyzed Asymmetric Ring-
Opening Reaction of Cyclopropyl Ketones with Indoles
Authors: Fenzhen Chang, Lili Lin, Yong Xia, Hang Zhang, Shunxi
Dong, Xiaohua Liu, and Xiaoming Feng
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800312
Link to VoR: http://dx.doi.org/10.1002/adsc.201800312

10.1002/adsc.201800312
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Chiral N,N'-Dioxide/ScΙΙΙ Complex-Catalyzed Asymmetric
Ring-Opening Reaction of Cyclopropyl Ketones with Indoles
Fenzhen Chang,a Lili Lin,a* Yong Xia,a Hang Zhang,a Shunxi Dong,a Xiaohua Liua and
Xiaoming Fenga*
a
Key Laboratory of Green Chemistry＆Technology, Ministry of Education, College of Chemistry, Sichuan University
Chengdu 610064, P.R.China
Fax: (+86)28-85418249; phone: (+86)28-85418249; e-mail: lililin@scu.edu.cn; xmfeng@scu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
using a N,N'-dioxide/scandium(ΙΙΙ) complex as the
Abstract: The asymmetric ring-opening reaction of
cyclopropyl ketones with indoles has been realized by
using a N,N'-dioxide/scandium(ΙΙΙ) complex as catalyst.
The corresponding 3-alkylated indole derivatives were
obtained in moderate to excellent yields with good ee
values. Moreover, a possible transition state was proposed
on the basis of experimental studies and X-ray of product.
chiral catalyst.[10b]
Keywords: asymmetric catalysis; ring-opening reaction;
D-A cyclopropanes; indoles; heterocycles
Donor-Acceptor (D-A ) cyclopropanes are a kind
of highly active cyclopropanes with both electron-
withdrawing group and electron-donating group.[1]
The ring-opening reaction of D-A cyclopropanes can
provide a rapid access to a variety of cyclic or acyclic
compounds by annulation or addition reactions.[2] The
catalytic asymmetric ring-opening reactions of D-A
cyclopropanes with heteroatom nucleophiles, such as
nitrogen-,[3] sulfur-[4] and oxygen-containing[4a,5]
nucleophiles, have been realized by using the chiral
N,N'-dioxide/metal
complexes[6]
or
chiral
oxazoline/metal complexes. However, for the ring-
opening reaction of D-A cyclopropanes with carbon
nucleophiles, more challenges exist due to their
weaker nucleophilic ability. The enol ethers are
proved to be efficient carbon nucleophiles in the
asymmetric [3+2] and [4+3] cycloaddition reactions
of D-A cyclopropanes in the presence of
oxazoline/Cu(II) complexes by Tang's and Waser's
groups.[7-9] Besides, 2-naphthols were found to be
another type of suitable C-nucleophiles to proceed
Scheme 1. Catalytic asymmetric ring-opening reaction of
cyclopropanes with indoles.
On the other hand, indoles, as one kind of the most
important heterocyclic compounds emerging in many
research areas,[11] can also act as carbon nucleophiles
in the ring-opening reaction of D-A cyclopropanes.[12]
In the catalytic asymmetric version, Tang's group
developed a BOX/CuII complex-catalyzed highly
diastereo- and enantioselective annulation of indoles
with D-A cyclopropanes.[13] The BOX/CuII
complexes were also utilized to promote the [3+3]
[3+2]
cyclopentannulation
reaction
with
cyclopropanes in the presence of Bi(OTf)3.
Alternatively, a Friedel–Crafts-type addition to
cyclopropanes took place when Sc(OTf)3 was
employed as the Lewis acid.[10a] Very recently, we
accomplished the catalytic asymmetric ring-opening
reaction of cyclopropyl ketones with β-naphthols
annulation
of
2-alkynylindoles
with
D-A
cyclopropanes.[14] As for the Friedel-Crafts alkylation
of indoles with cyclopropanes to achieve ring-opened
1
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10.1002/adsc.201800312
Advanced Synthesis & Catalysis
adduct, only Johnson described an asymmetric
dynamic kinetic Friedel-Crafts alkylation promoted
by pybox-MgI2 complex in 2013.[15] The alkylation
products were obtained in good yields with moderate
to high enantioselectivities. To date, chiral
oxazoline/metal (especially Cu (II)) complexes were
the only efficient catalytic system for the ring-
opening reaction of D-A cyclopropanes with carbon
nucleophiles. As a consequence, developing new
catalytic systems for the ring-opening reaction is
meaningful. Herein, we reported our efforts in
the 3-alkylated indole product 3aa with 70% ee but
only in 19% yield (Table 1, entry 7). Fortunately, the
of MgCl2 to the reaction system could dramatically
increase the yield to 84% and the enantioselectivity to
94% ee (Table 1, entry 8). Prolonging the reaction
time to 5 days and increasing
the concentration of indole to 0.5 mol/mL could
effectively enhance the yield to 99% with the
maintained enantioselectivity (94% ee, Table 1, entry
9). Therefore, the optimized conditions entail the use
of the L-PiPr3/Sc(OTf)3 complex (10 mol%) as the
catalyst and the MgCl2 (1 equiv) as an additive in
CHCl3 at 35 oC.
With the optimized conditions in hand, the
substrate scope of the reaction was then evaluated.
Firstly, a variety of cyclopropyl ketones were
examined, and excellent level of enantioselectivity
was achieved (Table 2). Cyclopropyl ketones with
various substituents at different position of phenyl
group R1 converted efficiently to the corresponding
products (80-99% yields, 85-94% ee, Table 2, entries
1-9). In addition, naphthyl-substituted substrates 1k
and 1l were also tolerated, resulting the
corresponding products 3ka and 3la in excellent
yields and ee values (99% yield, 93-96% ee, Table 2,
entries 10-11). Futhermore, the vinyl-substituted
cyclopropyl ketone also transformed to the product
3ma in 52% yield with 73% ee (Table 2, entry 12).
Unfortunately, when aliphatic methyl substituted
cyclopropyl ketone 1n was applied in the reaction, no
developing
a
chiral N,N'-dioxide/scandium(ΙΙΙ)
complex catalytic system for the highly
enantioselective ring-opening reaction of cyclopropyl
ketones with indoles, achieving optically active 3-
alkylated indole derivatives in high yields with good
enantioselectivities.
Our study began with the ring-opening of
cyclopropyl ketone 1a with N-methylindole 2a as
model reaction to optimize the reaction conditions,
and the representative results were summarized in
Table 1. Firstly, various metal salts complexing with
(S)-pipecolic acid-derived N,N'-dioxide L-PiPr3 were
evaluated (Table 1, entries 1-6). Many metal salts,
such as Mg(OTf)2, MgCl2, Zn(OTf)2 and Ni(OTf)2,
can’t or faintly promote the reaction (Table 1, entries
1-4). Delightedly, rare earth metal salts exhibit better
catalytic abilities in this reaction, and scandium
complexes were proven to be competent catalysts.
Without additive, L-PiPr3/Sc(OTf)3 complex gave
Table 1. Optimization of the reaction conditions.[a]
Table 2. Scope of the cyclopropyl ketone.[a]
Entry
R1, R2
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
4-MeC6H4, Ph (1b)
4-FC6H4, Ph (1c)
4-ClC6H4, Ph (1d)
4-BrC6H4, Ph (1e)
3-MeC6H4, Ph (1f)
3-MeOC6H4, Ph (1g)
3-ClC6H4, Ph (1h)
3-BrC6H4, Ph (1i)
2-MeC6H4, Ph (1j)
1-Naphthyl, Ph (1k)
2-Naphthyl, Ph (1l)
Vinyl, Ph (1m)
99 (3ba)
99 (3ca)
94 (3da)
95 (3ea)
95 (3fa)
99 (3ga)
80 (3ha)
84 (3ia)
99 (3ja)
99 (3ka)
99 (3la)
52 (3ma)
0
91 (R)
89 (R)
93 (R)
89 (R)
86 (R)
85 (R)
93 (R)
94 (R)
91 (R)
96 (R)
93 (R)
73 (R)
--
Entry Metal salt Additive Yield [%][b]
ee [%][c]
1
2
3
4
5
6
Mg(OTf)2 --
NR
NR
trace
5
62
31
19
84
99
--
--
--
17
20
56
70
94
94
MgCl2
--
Zn(OTf)2
Ni(OTf)2
Yb(OTf)3
Y(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
--
--
--
--
--
MgCl2
MgCl2
9
10
11
12
13
14
15
7
8[d]
Me, Ph (1n)
Ph, 4-FC6H4 (1o)
Ph, 4-MeC6H4 (1p)
9[d,e]
97 (3oa)
86 (3pa)
94 (R)
86 (R)
92 (R)
[a]
Unless otherwise noted, all reactions were carried out
with 1a (0.22 mmol), 2a (0.10 mmol), L-PiPr3/metal (1:1,
10 mol%) in CHCl3 (0.5 mL) under nitrogen at 35°C for 3
16
Ph, 4-MeOC6H4 (1q) 77 (3qa)
[a]
Unless otherwise noted, the reactions were performed
with 1 (0.22 mmol), 2a (0.10 mmol), L-PiPr3/Sc(OTf)3
(1:1, 10 mol%), MgCl2 (1.0 equiv) in CHCl3 (0.2 mL)
days. [b] Yield of isolated product. [c] Determined by HPLC
[d]
analysis on a chiral stationary phase.
Add MgCl2 (1.0
equiv) to reaction system. [e] Prolong the reaction time to 5
days and decreasing the amount of CHCl3 to 0.2 mL. OTf
= trifluoromethansulfonate.
[b]
[c]
under nitrogen at 35 °C.
Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
2
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10.1002/adsc.201800312
Advanced Synthesis & Catalysis
product was detected (Table 2, entry 13).
Investigation of carbonyl substituent R2 revealed that
electron-withdrawing groups on the phenyl group had
little influence on the reaction, but the electron-
donating substituents would led to a reduced yield or
enantioselectivity (Table 2, entries 14-16).
the product 3aa in 92% ee (Scheme 2). Furthermore,
the product 3aa could be readily transformed to mono
carbonyl product 4a upon treatment with NaOH in a
mixed solvent of methanol and tetrahydrofuran.
When 2.0 equiv of cyclopropyl ketone 1a reacted
with 2a, the product 3aa was obtained in 48% yield
with 91% ee after 5 days, and the unreacted
cyclopropyl ketone 1a was recovered in 47% yield
with 87% ee, revealing that a kinetic resolution
process was invovled in our catalytical system.
Table 3. Scope of the indole substrate.[a]
Entry
R3, R4
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
H, H
H, TBS
2-Me, CH3
4-Me, CH3
4-Br, CH3
5-Me, CH3
5-MeO, CH3 80 (3aj)
5-Cl, CH3
5-F, CH3
6-Me, CH3
6-Cl, CH3
7-Me, CH3
7-Cl, CH3
67 (3ad)
56 (3ae)
90 (3af)
63 (3ag)
53 (3ah)
99 (3ai)
84 (R)
96 (R)
96 (R)
94 (R)
95 (R)
91 (R)
94 (R)
86 (R)
91 (R)
91 (R)
92 (R)
90 (R)
91 (R)
79 (3ak)
78 (3al)
95 (3am)
78 (3an)
99 (3ao)
88 (3ap)
9
10
11
12
13
[a]
Unless otherwise noted, the reactions were performed
with 1a (0.22 mmol), 2 (0.10 mmol), L-PiPr3/Sc(OTf)3
(1:1, 10 mol%), MgCl2 (1.0 equiv) in CHCl3 (0.2 mL)
Scheme 2. a) Scale-up reaction. b) Product transformation.
c) Kinetic resolution of 1a with 2a
[b]
[c]
under nitrogen at 35 °C.
Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
Table 4. Control experiment.[a]
Subsequently, various indoles 2 were investigated
by reacting with cyclopropyl ketone 1a (Table 3).
Indole 2d without protection and N-TBS-indole 2e
proceeded the reaction in lower activities but still
good enantioselectivities (Table 3, entries 1-2). When
the N-methyl-2-methy-indole 2f was subjected to the
reaction, the corresponding product 3af was obtained
in 90% yield with 96% ee (Table 3, entry 3). Indoles
with substitutions at C4-position converted to the
desired product with good enantioselectivities but in
lower yields(63-53% yields; 94-95% ee, Table 3,
entries 4-5), probably due to the steric hindrance
between the substrate and the ligand. In comparison,
indoles (2i-2p) bearing substituents attached on C5-,
C6-, and C7-position, mostly behaved well in the
current system (78-99% yields, 86-94% ee, Table 3,
entries 6-13), indicating that the electronic effect has
little effect on the reaction. The absolute
configuration of 3aa was determined to be (R) by X-
ray crystallography, and the configurations of others
were also assigned to be (R) by comparing the
circular dichroism spectrum with that of 3aa.
Entry Metal salt
Additive
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
ScCl3·6H2O
ScCl3·6H2O
Mg(OTf)2
MgSO4
nBu4NCl
NaCl
75
49
23
21
52
49
17
50
98
82
97
86
--
6
Mg(OTf)2
[a]
Unless otherwise noted, the reactions were performed
with 1a (0.22 mmol), 2 (0.10 mmol), L-PiPr3/metal salt
(1:1, 10 mol%), additive (1.0 equiv) in CHCl3 (0.2 mL)
[b]
[c]
under nitrogen at 35 °C.
Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
To evaluate the synthetic potential of this
methodology, a gram-scale synthesis of 3aa was
carried out. In the optimal conditions, 6.6 mmol
cyclopropyl ketone 1a reacted smoothly with 3 mmol
N-methylindole 2a, affording 1.32 g (96% yield) of
In order to explore the role of magnesium chloride
in this system, several control experiments and
HRMS analysis were conducted (Table 4). When the
3
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10.1002/adsc.201800312
Advanced Synthesis & Catalysis
In summary, a chiral N,N'-dioxide/scandium(ΙΙΙ)
complex was proved to be efficient for the highly
enantioselective ring-opening reaction of cyclopropyl
ketones with indoles. A series of 3-alkylated indole
derivatives were obtained in high to excellent yields
with high ee values. Further exploration of the
catalyst system for other asymmetric reactions is
currently under investigation.
Experimental Section
Scheme 3. Proposed transition model.
N,N'-Dioxide ligand L-PiPr3 (0.01 mmol), Sc(OTf)3 (0.01
mmol), MgCl2 (0.10 mmol) and cyclopropyl ketone 1a
(0.22 mmol) were stirred in CHCl3 (0.2 mL) at 35 °C under
N2 atmosphere for 0.5 h. Then, indole 2a (0.10 mmol) was
added and the mixture was stirred at 35 °C for 5 days. The
reaction mixture was purified by flash chromatography
(petroleum: ethyl acetate = 10:1) on silica gel to afford the
desired product as a yellow solid in 99% yield with 94% ee.
anion of additive was changed from Cl to OTf or
SO42, both yield and ee value of the product
decreased sharply (Table 4, entries 1-2). Switching
the cation of additive to nBu4N or Na+, the yield the
decreased but the enantioselectivity was still
excellent (Table 4, entries 3-4). These two control
experiments indicated that both magnesium cation
and chloride anion are critical for high yield and high
enantioselectivity in this reation[16]. Moreover, when
ScCl3·6H2O/L-PiPr3 was used as catalyst, the
corresponding product 3aa were obtained in 52%
yield with 97% ee value (Table 4, entry 5),
suggesting that counteranion exchange might exist
during the preparation of catalyst. HRMS analysis
confirmed this assumption. A mixture of ligand L-
PiPr3, Sc(OTf)3 and MgCl2 shown an ionic peak at
m/z 847.4476, 849.4464 {[L-PiPr3 + Sc3+ + 2Cl]+
m/z calcd 847.4484, 849.4455}. Besides, in the
mixture of ligand L-PiPr3, Sc(OTf)3, MgCl2, and 1a,
the characteristic signals of [L-PiPr3 + Sc3++ Cl +
1a]2+ at m/z 569.3047, 569.8102 (m/z calcd 569.3049,
569.8065) were also observed. Based on the above
studies, the absolute configuration of products, and
our previous work,[17] a possible transition state
model was proposed. As shown in Scheme 3, the
oxygen atoms of ligand and cyclopropyl ketone 1a
coordinate with the ScΙΙΙ to form an octahedral
complex, which could distinguish the two different
configurations of cyclopropyl ketone and selectively
activated (S)-1a via coordination (TS-1), then indole
attacked (S)-1a from the the direction with small
steric hindrance to deliver (R)-configured product. In
comparation, the coordination of (R)-1a with N,N'-
dioxide/scandium(ΙΙΙ) complex is much less efficient
due to the steric repulsion between the 2,4,6-
diisopropylphenyl group of the ligand and the phenyl
group of cyclopropyl ketone (in TS-2). Therefore,
(R)-1a was less active under optimized conditions
and could be recovered in high enantioselectivity.
With regards to the role of counteranion, we
envisoned that the Lewis acidity of metal species is
responsible for the observed reactivity and more
acidic ScCl3 exhibited a higher reactivity than
Sc(OTf)3 in the reaction. Moreover, the electrostatic
attraction between N,N'-dioxide-Sc(III) complex with
Cl provided a more suitable chiral enviroment for
recognizating (R)-1a, which was in consist with
experimental results.
Acknowledgements
We appreciate the National Natural Science Foundation of China
(No. 21432006) for financial support.
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