Enantioselective Synthesis of 2,2,3-Trisubstituted Indolines via Bimetallic Relay Catalysis of α-Diazoketones with Enones

Article abstract of DOI:10.1021/acs.orglett.8b01744

An efficient asymmetric intramolecular trapping of ammonium ylides of α-diazoketones with enones to synthesize indoline derivatives was realized. A Rh(II)/chiral N,N′-dioxide-Sc(III) complex bimetallic relay catalytic system was established. A series of optically active 2,2,3-trisubstituted indolines were obtained in high yields (up to 99%), good enantioselectivities (up to 99% ee), and excellent diastereoselectivities (up to >19:1 dr) under mild reaction conditions.

Full text of DOI:10.1021/acs.orglett.8b01744

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Synthesis of 2,2,3-Trisubstituted Indolines via
Bimetallic Relay Catalysis of α‑Diazoketones with Enones
Jian Yang, Chaoqi Ke, Dong Zhang, Xiaohua Liu,* and Xiaoming Feng*
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, China
S
* Supporting Information
ABSTRACT: An efficient asymmetric intramolecular trapping of ammonium
ylides of α-diazoketones with enones to synthesize indoline derivatives was
realized. A Rh(II)/chiral N,N′-dioxide−Sc(III) complex bimetallic relay
catalytic system was established. A series of optically active 2,2,3-trisubstituted
indolines were obtained in high yields (up to 99%), good enantioselectivities
(up to 99% ee), and excellent diastereoselectivities (up to >19:1 dr) under
mild reaction conditions.
Scheme 1. Intramolecular Trapping Ammonium Ylides with
Enones
hiral indoline skeletons are widespread in a number of
1
Cnatural products and biologically active compounds.
The development of a novel protocol for the efficient synthesis
of enantiomerically enriched indoline derivatives remains a hot
topic in synthetic organic chemistry. Significant progress has
been made through several strategies, such as kinetic resolution
of 2-substituted indoline,2 intramolecular coupling,3 hydro-
genation of substituted indole,4 and miscellaneous trans-
formations of indole.5 Nevertheless, highly diastereo- and
enantioselective construction of 2,2,3-trisubstituted indoline
derivatives is rare.
Asymmetric catalytic cascade reactions have become a
powerful tool to rapidly and efficiently construct complicated
molecules from simple reactants.6 To realize cascade trans-
formations, cooperative catalytic systems, including bimetallic
catalysis, metallic-organocatalysis, and organo-organocatalysis,
have been proven to be useful and efficient strategies. Each
catalyst component performs its own functions separately and
consonantly in each reaction step.7 The former two strategies
have been successfully applied in several asymmetric cascade
reactions via trapping of the active onium ylides and
zwitterions formed in situ from the reaction of α-diazoester
with a nucleophile.8 For example, Hu’s group has utilized the
intramolecular trapping of ammonium ylides with carbonyl
compounds or enones for diastereoselective synthesis of
trisubstituted indolines (Scheme 1, eq 1). Nevertheless, it
seems hard to achieve satisfactory reactivity and enantiose-
lectivity with combined common Rh(II)/chiral Brønsted acid
after much effort.9
Our group has engaged in developing asymmetric trans-
formations catalyzed by chiral N,N′-dioxide−metal com-
plexes.10 Enones can undergo conjugate addition with a variety
of nucleophiles in the presence of chiral N,N′-dioxide−metal
complexes.11 We became interested in developing efficient
bimetallic catalytic systems, which combine a transition metal
salt with a chiral Lewis acid complex,12 to realize the
asymmetric cascade reaction in Scheme 1, eq 1 for the
synthesis of trisubstituted indoline. However, previous screen-
ing of the cocatalysts9b showed that the general Lewis acids,
such as ZnCl2, Mg(OTf)2, and Sc(OTf)3, could not yield the
desired product. To realize the enantioselective version of this
cascade reaction via chiral Lewis acid cooperative catalysis, two
methods are available. One includes using a stronger chiral
Lewis acid, and the other one, increasing the nucleophilicity of
the ammonium ylides. We plan to use α-diazoketone instead of
α-diazoester, which bears a stronger electron-withdrawing
carbonyl group, benefiting the formation of a free ylide or enol
intermediate13 with less steric hindrance and stronger
nucleophilicity (Scheme 1, eq 2). In this context, we reported
our efforts in developing highly efficient Rh(II) and chiral
N,N′-dioxide−Sc(III) complex cocatalyzed intramolecular
trapping of ammonium ylides of α-diazoketones with enones.
Received: June 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01744
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Letter
The corresponding optically active 2,2,3-trisubstituted indo-
lines were obtained in high yields, good enantioselectivities,
and excellent diastereoselectivities under mild reaction
conditions.
We initially chose 2-aminophenyl-substituted enone 1a and
1-diazo-1-phenylpropan-2-one 2a as the model substrates to
optimize the reaction conditions (Table 1). However, we
L-PrPr3 with a three-carbon linkage (Table 1, entry 8 vs entry
7), affording the indoline 3a in 84% ee. Furthermore, the
addition of 5 Å molecular sieves and adjustment of the ratio of
Sc(OTf)3 to L2-PrPr3 (1:1.2) resulted in the optimal outcome,
and the product 3a could be obtained in 88% yield, 90% ee,
and >19:1 dr (Table 1, entries 9 and 10).
With the optimized reaction conditions in hand, we turned
to evaluating the substrate scope of this protocol. A variety of
2-aminophenyl-substituted enones 1 were investigated. As
shown in Table 2, enones 1 with different substituents on the
phenyl ring of R1 could be smoothly converted to the
corresponding products. The electron-donating substituted
enones 1f−1i gave slightly higher enantioselectivity than the
electron-withdrawing substituted enones 1b−1e. The desired
indolines 3b−3i were generated in 72−94% yields with 81−
a
Table 1. Optimization of the Reaction Conditions
Table 2. Substrates Scope of 2-Aminophenyl-Substituted
a
Enones
b
c
d
entry
metal salt
ligand
yield (%)
dr
ee (%)
1
2
3
4
5
6
7
8
−
−
trace
69
34
85
60
75
87
90
89
88
−
−
20
3
60
55
−7
83
84
88
90
Ni(OTf)2
Zn(OTf)2
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
L-PrPr2
L-PrPr2
L-PrPr2
L-RaPr2
L-PiPr2
L-PrPr3
L2-PrPr3
L2-PrPr3
L2-PrPr3
>19:1
>19:1
>19:1
>19:1
>19:1
>19:1
>19:1
>19:1
>19:1
e
9
10
f
a
Unless otherwise noted, all reactions were performed by addition of
2a (0.2 mmol) in THF (0.5 mL) via a syringe to the mixture of 1a
(0.1 mmol), Rh2(OAc)4 (1 mol %), and metal salt/ligand (1:1, 10
mol %) in THF (1.0 mL) at 25 °C for 10 min. After completion of the
b
addition, the reaction mixture was stirred at 25 °C for 2 h. Isolated
c
d
1
yield. Determined by chiral HPLC and H NMR. Determined by
e
f
chiral HPLC. 5 Å MS (20 mg) was added. Sc(OTf)3/L2-PrPr3
(1:1.2, 10 mol %).
found that the major product was the N−H insertion product
5a and only a trace amount of the target indoline product 3a
was detected in the presence of Rh2(OAc)4 without a Lewis
acid cocatalyst. It indicates that Rh2(OAc)4 could slightly
induce the unasymmetric nucleophilic addition step (Table1,
entry 1). Next, different metal salts coordinating with L-proline
derived ligand L-PrPr2 were investigated. Several metal salts
(see the Supporting Information for details), such as Ni(OTf)2
and Zn(OTf)2, could promote the reaction in moderate yield
with excellent diastereoselectivity, but the enantioselectivity
was poor (Table 1, entries 2 and 3). The desired product 3a
was obtained in 85% yield with 60% ee and >19:1 dr when
Sc(OTf)3/L-PrPr2 was used (Table 1, entry 4). Next, we used
Sc(OTf)3 to explore different chiral N,N′-dioxide ligands. A
survey of the amino acid backbone showed that L-proline
derived L-PrPr2 was more competent than L-ramipril derived
L-RaPr2 and L-pipecolic acid derived L-PiPr2 in terms of
enantioselectivity and reactivity (Table 1, entry 4 vs entries 5
and 6). To our delight, the ligand L-PrPr3 with more steric
hindrances significantly benefited the improvement of the
enantioselectivity (Table 1, entry 7 vs entry 4). Ligand L2-
PrPr3, which has a two-carbon linkage was slightly superior to
a
Sc(OTf)3/L2-PrPr3 (1:1.2, 10 mol %), Rh2(OAc)4 (1 mol %), 5 Å
MS (20 mg), 2 (0.2 mmol), and 1 (0.1 mmol) in THF (1.5 mL) at 25
°C for 2 h. Isolated yield. Determined by chiral HPLC analysis. All
dr value were up to >19:1 detected by H NMR.
b
c
1
B
DOI: 10.1021/acs.orglett.8b01744
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91% ee (Table 2, entries 1−9). Heteroaromatic substituted 1j
and 1k, as well as ring-condensed 1l and 1m, were also suitable
substrates, affording the related products 3j−3m in good
results (81−94% yields, 77−92% ee) (Table 2, entries 10−13).
It was noteworthy that enones 1n−1p bearing alkyl acyl
substituents also successfully underwent the reaction to
provide the desired products 3n−3p, albeit the enantiose-
lectivity decreased slightly (76−99% yields, 73−82% ee)
(Table 2, entries 14−16). Enones 1q−1v with a substituted
β-aryl group were all tolerated well, resulting in 66−98% yields
and 73−95% ee (Table 2, entries 17−22). To our delight, 1-
indanone derived substrate 1w could undergo this reaction
smoothly and afforded the desired product 3w containing three
adjacent stereocenters with excellent results (81% yield, 95%
ee) (Table 2, entry 23). It was worth noting that only one
diastereomer was detected in these cases. The absolute
configuration of the product 3r was determined to be
(2S,3R) according to X-ray crystal structural analysis (CCDC
1844834).
observation implied that this reaction did not conduct a
sequential N−H insertion/Michael addition pathway.14 The
generation of an ammonium ylide intermediate in situ is much
preferable for the intramolecular nucleophilic addition. When
we subjected methyl 2-diazo-2-phenylacetate to the standard
reaction conditions instead of α-diazoketone 2a, the
corresponding indoline product 6a was isolated in 40% yield
with 11% ee (Scheme 2). It implied that the coordination of
Scheme 2. Control Experiment
N,N′-dioxide L2-PrPr3 with Sc(OTf)3 increased the Lewis
acidity of the cocatalyst; thus, the reaction could be performed
smoothly. The decreased enantioselectivity might result from
the formation of a Rh(II)-bonded enolate intermediate
(Scheme 1, TS A), which brings steric hindrance and reduces
the nucleophilicity.
Next, we examined the reaction of 2-aminophenyl-
substituted enone 1t with various α-diazoketones 2, with
results listed in Table 3. It was found that the electronic nature
Based on the previous work,11 the X-ray crystal structure of
a
Table 3. Substrate Scope of α-Diazoketones
the product 3r, and the N,N′-dioxide−Sc(III) complex,10
a
possible enantioselective catalytic model for the cascade
reaction was proposed (Figure 1). Initially, the Rh(II) complex
Figure 1. Proposed enantioselective catalytic model.
promotes the decomposition of diazoketone 2a and the
formation of the free ammonium ylide intermediate.13a Next,
the intermediate coordinates to one site of the chiral L2-
PrPr3−Sc(III) catalyst with the oxygen of the enone unit. The
steric hindrance of one amide subunit of the ligand shields the
β-Si-face of the enone, and the phenyl group of the
diazoketone is far from the β-aryl group of the enone to
reduce the steric hindrance (TS B′ and TS B′′). Next,
intramolecular nucleophilic addition occurs via the transition
state as in Figure 1, followed by a proton transfer to give the
corresponding (2S,3R)-product 3r. In all these cases, only
trans-diastereomers were identified. The oxygen anion of the
enolate intermediate could not coordinate to Sc(III) due to the
steric hindrance shown in TS B′′; thus, the corresponding
(2R,3R)-diastereomer was not observed.
a
b
The reaction conditions were the same as those in Table 2. Isolated
c
yield. Determined by chiral HPLC analysis. All dr value were up to
>19:1 detected by H NMR.
1
and position of the substituents on the aryl group of α-
diazoketones 2b−2h had a limited influence on the
enantioselectivity (93−97% ee) but dramatically varied the
yield (78−96% yields) (Table 3, entries 1−7). The reaction
was also tolerable to diazoketones with different alkyl acyl
substituents, including ethyl, n-butyl, isobutyl, and cyclopropyl
groups. The corresponding products 4ti−4tl were obtained in
moderate to good yield (67−87% yields) with high
enantioselectivity (94−99% ee) (Table 3, entries 8−11).
To explore the mechanism of this catalytic reaction, we
carried out several control experiments. The reaction of the
N−H insertion product 5a could not be converted into
indoline 3a under the standard reaction conditions. The
In conclusion, we have developed an efficient asymmetric
catalytic synthesis of 2,2,3-trisubstiuted indolines via intra-
molecular trapping of ammonium ylides with enones. The use
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DOI: 10.1021/acs.orglett.8b01744
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01744.
Experimental procedures, full spectroscopic data for all
new compounds, and copies of 1H, 13C NMR, and
HPLC spectra (PDF)
Accession Codes
CCDC 1844834 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: liuxh@scu.edu.cn.
*E-mail: xmfeng@scu.edu.cn.
ORCID
(10) (a) Liu, X. H.; Lin, L. L.; Feng, X. M. Acc. Chem. Res. 2011, 44,
574−587. (b) Liu, X. H.; Lin, L. L.; Feng, X. M. Org. Chem. Front.
2014, 1, 298−302. (c) Liu, X. H.; Zheng, H. F.; Xia, Y.; Lin, L. L.;
Feng, X. M. Acc. Chem. Res. 2017, 50, 2621−2631. (d) Liu, X. H.;
Dong, S. X.; Lin, L. L.; Feng, X. M. Chin. J. Chem. 2018, 36, 791−797.
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X. M. Chem. Commun. 2017, 53, 8763−8766. (b) Xiao, X.; Mei, H. J.;
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Xiaohua Liu: 0000-0001-9555-0555
Xiaoming Feng: 0000-0003-4507-0478
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(21625205 and 21332003) for financial support.
■
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