Asymmetric [3 + 2] Cycloaddition Employing N, N′-Cyclic Azomethine Imines Catalyzed by Chiral-at-Metal Rhodium Complex

Article abstract of DOI:10.1021/acs.orglett.8b01264

An efficient asymmetric 1,3-dipolar cycloaddition of α,β-unsaturated 2-acyl imidazoles with N,N′-cyclic azomethine imines catalyzed by a chiral-at-metal rhodium complex is reported. The corresponding N,N′-bicyclic pyrazolidine derivatives with three conti

Full text of DOI:10.1021/acs.orglett.8b01264

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric [3 + 2] Cycloaddition Employing N,N′-Cyclic Azomethine
Imines Catalyzed by Chiral-at-Metal Rhodium Complex
Jun Gong,†,‡ Qian Wan,† and Qiang Kang*,†
†Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou 350002, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: An efficient asymmetric 1,3-dipolar cycloaddition of α,β-unsaturated 2-acyl imidazoles with N,N′-cyclic
azomethine imines catalyzed by a chiral-at-metal rhodium complex is reported. The corresponding N,N′-bicyclic pyrazolidine
derivatives with three contiguous tertiary stereocenters were obtained in good yields (up to 99%) with excellent stereoselectivities
(>20:1 dr and >99% ee). Remarkably, as little as 0.5 mol % of a chiral Rh(III) complex can promote a gram-scale reaction with
excellent yield and enantioselectivity.
Scheme 1. Asymmetric 1,3-Dipolar Cycloadditions
Catalyzed by Metal Complexes
1,3-Dipolar cycloaddition (1,3-DC) has been a powerful
method for construction of a variety of five-membered
heterocycles from relatively simple precursors.1 1,3-Dipoles,
such as azomethine ylides,2 nitrones,3 nitrile imines,4 and nitrile
oxides,5 have been employed for the cycloaddition reactions.
Since the first example of a catalytic asymmetric 1,3-DC by use
of N,N′-cyclic azomethine imines6,7 as suitable substrates in
2003 as reported by Fu’s group, a variety of organo8 and
organometallic catalysts9 have been developed for enantiose-
lective construction of N,N′-bicyclic pyrazolidine derivatives
with biological activities.10 Aside from well-developed chiral
ligand/Cu catalytic systems,9a−k other asymmetric organo-
metallic catalysts have been explored.9l−n In 2007, Suga and co-
workers reported an asymmetric 1,3-DC reaction between
fused azomethine imines and 3-acryloyl-2-oxazaolidinones
catalyzed by a chiral BINIM−Ni(II) complex (Scheme 1a).9l
Recently, Feng’s group developed a highly efficient chiral N,N′-
dioxide−Mg(OTf)2 complex catalyzed asymmetric 1,3-DC
reaction between methyleneindolinones and N,N′-cyclic
azomethine imines for synthesis of pyrazolidine products with
contiguous quaternary−tertiary stereocenters (Scheme 1b).9n
Despite these impressive achievements, the development of
efficient asymmetric organometallic catalysts, which could solve
long-standing problems such as high catalyst loading (usually
5−10 mol %) and long reaction time, is still in great demand.
Chiral-at-metal rhodium complexes developed by the
Meggers group have proven to be powerful chiral Lewis acid
catalysts in which the metal center serves not only as a Lewis
acid but also as the exclusive source of chirality.11 A variety of
asymmetric transformations have been successfully achieved
with high catalytic efficiency and enantioselectivity.12 As a
continuation of our interest in chiral-at-metal rhodium
catalysis,13 herein we report a highly efficient asymmetric 1,3-
DC reaction of α,β-unsaturated 2-acyl imidazoles with N,N′-
cyclic azomethine imines catalyzed by chiral-at-metal Rh(III)
complexes, affording the N,N′-bicyclic pyrazolidine derivatives
with three contiguous tertiary stereocenters (Scheme 1c).
We started our studies on this topic by investigating the
reaction of α,β-unsaturated 2-acyl imidazole 1a with N,N′-cyclic
azomethine imine 2a in the presence of chiral-at-metal Rh(III)
complex Λ-Rh113g (2 mol %), which was synthesized by our
Received: April 20, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01264
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
group. To our delight, the reaction proceeded smoothly in 1,2-
dichlorethane (DCE) to afford the desired product 3a in 99%
yield with >20:1 dr and 82% ee (Table 1, entry 1). Encouraged
Scheme 2. Substrate Scope of α,β-Unsaturated 2-Acyl
Imidazoles.
a
a
Table 1. Optimization of the Reaction Conditions
b
c
d
entry catalyst
solvent
time (h) yield (%)
dr
ee (%)
1
2
3
4
5
6
7
8
Λ-Rh1
Λ-Rh2
Λ-Rh3
Λ-Rh3
Λ-Rh3
Λ-Rh3
Λ-Rh3
none
DCE
16
16
16
16
16
36
36
24
99
99
99
99
99
98
98
0
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
nd
82
90
99
99
99
97
99
nd
DCE
DCE
THF
toluene
MeCN
CH2Cl2
DCE
e
a
Reaction conditions: 1a (0.20 mmol), 2a (0.24 mmol), catalyst (2.0
mol %) in solvent (0.4 mL) at 50 °C under argon atmosphere.
b
c
d
Isolated yield. Determined by crude 1H NMR. Chiral HPLC
e
analysis. At 30 °C. nd = not determined.
by this promising result, we examined other chiral Rh(III)
complexes. When Λ-Rh213f was used, the enantioselectivity was
improved to 90% (entry 2). Gratifyingly, Λ-Rh3 developed by
Meggers’ group12a was the best one in terms of enantiose-
lectivity, giving the desired product in 99% yield with 99% ee
and >20:1 dr (entry 3). Various solvents such as THF, toluene,
MeCN, and CH2Cl2 also could provide comparable results
(entries 4−7). A control experiment in the absence of a catalyst
failed to provide any product, thereby demonstrating that this
reaction crucially depends on the chiral-at-metal Rh(III)
complexes (entry 8).
With the optimized reaction conditions in hand (Table 1,
entry 3), we next investigated the scope of α,β-unsaturated 2-
acyl imidazoles in this reaction (Scheme 2). The introduction
of electron-donating and electron-withdrawing groups on the
phenyl ring of α,β-unsaturated 2-acyl imidazoles had little
influence on both the yields and stereoselectivities. The desired
products 3b−j were obtained in high yields (94−99%) with
excellent stereoselectivities (98−99% ee and >20:1 dr).
Naphthyl-based 1k and heteroaromatic 1l,m were all well
converted in good yields with excellent stereoselectivities (98−
99% ee and >20:1 dr, 3k−m). Methyl- or cyclopropyl-
substituted α,β-unsaturated 2-acyl imidazoles were also
tolerated well, affording products 3n and 3o in good yields
with excellent ee (>20:1 dr, 99−99.6% ee). Moreover, replacing
the N-isopropylimidazole with N-methylimidazole or N-
phenylimidazole had no significant influence on the reactivity
and selectivity (3p−q). Styryl-substituted α,β-unsaturated 2-
acyl imidazole 1r was also suitable substrate for this reaction,
although moderate yield was obtained (68% yield, 98% ee, 3r).
Further investigation of the substrate scope of N,N′-cyclic
azomethine imines was carried out. As illustrated in Scheme 3,
azomethine imine with an electron-donating methyl or
methoxy group on the phenyl ring could react smoothly with
1a to afford the corresponding products (4a,b). Azomethine
imines equipped with halogen atoms were all converted in good
a
Reaction conditions: 1a−r (0.20 mmol), 2a (0.24 mmol), and Λ-Rh3
(2.0 mol %) in DCE (0.4 mL) at 50 °C under argon atmosphere. All
isolated yields were based on substrate 1. The ee values were
determined by HPLC analysis using chiral stationary phase. The dr
b
1
values were detected by crude H NMR. 3.0 mol % of Λ-Rh3 was
used.
yields with excellent ee (96−99% yields, 98−99.4% ee, 4c−f).
Azomethine imines bearing a strong electron-withdrawing
substituent (−CF3 or −NO2) on the phenyl ring could react
with 1a with excellent stereoselectivities, albeit with a slightly
lower yield (87−88% yield, 4g,h). Heteroaromatic and 1-
naphthyl-substituted azomethine imines were tolerable under
the optimal reaction conditions, delivering the desired products
4i−k in 76−99% yields with satisfactory stereoselectivities
(94−98% ee, 10:1 to >20:1 dr). To our delight, replacing the
aromatic group with an aliphatic cyclohexyl group on an
azomethine imine could also provide the corresponding
product 4l in 74% yield with 93% ee and >20:1 dr, although
4 mol % of Λ-Rh3 was needed.
To demonstrate the synthetic utility of the current protocol,
a gram-scale reaction of 1a (1.06 g, 4.4 mmol) and azomethine
imine 2a (919.8 mg, 5.28 mmol) was conducted in the
presence of 1.0 mol % Λ-Rh3 (Scheme 4a). Gratifyingly, the
reaction proceeded smoothly to afford 3a in 98% yield (1.79 g),
with 99% ee and >20:1 dr. Remarkably, when as low as 0.5 mol
% of Λ-Rh3 was employed, the product 3a still could be
obtained in 98% yield with excellent enantioselectivity (96%
ee). The gram-scale reaction exhibits extraordinary reactivity
and stereoselectivity of a chiral Rh(III) complex on the title
reactions. Moreover, the product 3a could be converted into
optically active alcohol 5 and aldehyde 6 by reduction and
B
DOI: 10.1021/acs.orglett.8b01264
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of the tert-butyl groups. The highly selective approach of the
N,N′-cyclic azomethine imine from the Re-face of the
coordinated substrate leads to the desired product. The
absolute configuration of 3e was determined to be (1S,2R,3S)
by X-ray crystallographic analysis, and other products were
assigned by analogy (Figure 1, for details, see the Supporting
Information).
Scheme 3. Substrate Scope of N,N′-Cyclic Azomethine
a
imines
In conclusion, we have developed a highly efficient
asymmetric 1,3-dipolar cycloaddition of α,β-unsaturated 2-
acylimidazoles with N,N′-cyclic azomethine imines catalyzed by
chiral-at-metal rhodium complexes, affording the N,N′-bicyclic
pyrazolidine derivatives with three contiguous tertiary stereo-
centers in up to 99% yields, >20:1 dr, and >99% ee. The
reaction features simple operation, low catalyst loading, wide
substrate scope, and mild reaction conditions. Remarkably, this
protocol exhibits extraordinary advantages in terms of reactivity
and enantioselectivity, given the fact that as little as 0.5 mol %
of Λ-Rh3 can realize the title reaction on gram scale, yielding
the desired product with excellent stereoselectivity.
ASSOCIATED CONTENT
■
S
* Supporting Information
a
Reaction conditions: 1a (0.20 mmol), 2b−m (0.24 mmol) and Λ-
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01264.
Rh3 (2.0 mol %) in DCE (0.4 mL) at 50 °C under argon atmosphere.
All isolated yields were based on substrate 1. The ee values were
determined by HPLC analysis using chiral stationary phase. The dr
values were detected by crude H NMR. 4.0 mol % of Λ-Rh3 was
used.
b
1
1
Experimental procedures, characterization data, and H
and 13C NMR spectra and HPLC chromatograms for
obtained compounds (PDF)
Scheme 4. Gram-Scale Reaction and Synthetic
Transformations
Accession Codes
CCDC 1835990 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 Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kangq@fjirsm.ac.cn.
ORCID
Qiang Kang: 0000-0002-9939-0875
Notes
further removal of imidazole moiety without any loss in
enantiomeric excess (Scheme 4b).
The authors declare no competing financial interest.
On the basis of the previous investigations,12a a model of
stereoselective control is proposed (Figure 1). The α,β-
unsaturated 2-acyl imidazole can be activated by the chiral
Rh(III) complex through bidentate N,O-coordination. The Si-
face of the coordinated substrate is effectively shielded by one
ACKNOWLEDGMENTS
■
We thank Prof. Daqiang Yuan (Fujian Institute of Research on
the Structure of Matter, the Chinese Academy of Sciences) for
his kind help with X-ray analysis. This work was supported by
the Strategic Priority Research Program of the Chinese
Academy of Sciences, Grant No. XDB20000000, and the 100
Talents Program of the Chinese Academy of Sciences.
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DOI: 10.1021/acs.orglett.8b01264
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