Ag2CO3/CA-AA-amidphos multifunctional catalysis in the enantioselective 1,3-dipolar cycloaddition of azomethine ylides

Article abstract of DOI:10.1021/acs.orglett.5b03430

The new Ag₂CO₃/CA-AA-amidphos complexes have been demonstrated as highly efficient multifunctional catalysts in the asymmetric 1,3-dipolar cycloaddition of azomethine ylides. Under optimal conditions, highly functionalized endo-4 pyrrolidines were obtained with excellent yields (up to 99% yield) and enantioselectivities (up to 96% ee).

Full text of DOI:10.1021/acs.orglett.5b03430

Letter
pubs.acs.org/OrgLett
Ag2CO3/CA-AA-AmidPhos Multifunctional Catalysis in the
Enantioselective 1,3-Dipolar Cycloaddition of Azomethine Ylides
Haifei Wang,* Qifu Deng, Zhipeng Zhou, Shunqin Hu, Zhiguo Liu, and Li-Yi Zhou
School of Packaging and Materials Engineering, Hunan University of Technology, Zhuzhou 412007, Hunan Province, P.R. China
S
* Supporting Information
ABSTRACT: The new Ag2CO3/CA-AA-amidphos complexes have been
demonstrated as highly efficient multifunctional catalysts in the asymmetric
1,3-dipolar cycloaddition of azomethine ylides. Under optimal conditions,
highly functionalized endo-4 pyrrolidines were obtained with excellent yields
(up to 99% yield) and enantioselectivities (up to 96% ee).
atalytic asymmetric 1,3-dipolar cycloaddition of azome-
Recently, amidophosphane ligands have aroused great
Cthine ylides to electron-deficient alkenes has inspired attention in the asymmetric reaction,13 such as Pd-catalyzed
asymmetric allylations,13a,c hydrogenations,13f Michael-type
much research interest, as the reaction is one of the most
conjugate additions,13d and imine additions.13b,e However, to
powerful methods and diversity-oriented synthesis is desired for
the best of our knowledge, coordination between a multifunc-
tional amidophosphane and a metal ion has not been reported
in the 1,3-dipolar cycladdition of azomethine ylides. Here, we
report that cinchona alkaloid-amino acid derived amidophos-
phanes (CA-AA-amidphos) serve as the desired multifunctional
precatalysts in combination with transition-metal ions to
cooperatively catalyze 1,3-dipolar cycloaddition of azomethine
ylides with high diastereo- and enantioselectivities (Figure 1).
the construction of highly substituted pyrrolidine rings, which
are structural motifs widely present in many natural alkaloids
and pharmaceutically useful agents.1 Since the pioneering work
of Grigg employing stiochiometric amounts of chiral metal
complexes,2 effective chiral catalytic systems based on a
combination of chiral metal Lewis acids and tertiary amine
bases have thus far been reported to catalyze the process with
high endo/exo-diastereo- and enantioselectivities.3−5 In partic-
ular, the catalytic systems, including Zhang’s Ag(I)/xylyl-FAP/
i-Pr2NEt,6 Schreiber’s Ag(I)/QUINAP/i-Pr2NEt,7 Carretereo’s
Cu(I)/Fe-sulphos/Et3N,8 Wang’s Cu(I)/TF-Biphamphos/
Et3N,9 and Jørgensen’s Zn(II)/t-BuBox/Et3N,5a afford high
endo-selectivities and asymmetric inductions in the intermo-
lecular 1,3-dipolar cycloaddition of glycine derivatives with
electron-deficient alkenes, while Komatsu’s Cu(II)/BINAP/
Et3N,10 Zhang’s Cu(I)/P,N-ligands/Et3N,11 and Deng’s Cu-
5d
(II)/N,O-ligands/K2CO3 have been reported as efficient
Figure 1. Pseudoenantiomeric cinchona alkaloid-amino acid derived
potential amidophosphine precatalysts.
catalytic systems with some exo-selective cycloadditions.
Despite these impressive advances, there are still some
problems that have yet to be solved for the reaction: (1) an
excess of base such as tertiary amine or inorganic base is
involved in most cases; (2) these bidentate ligands are mainly
focused on bisphosphanes, nitrogenated phosphanes, sulfur-
containing phosphanes, and bisimines, which easily form
relatively rigid and stable complexes with metal cations; and
(3) applicable substrates of various azomethine ylides,
especially derived from aminoesters other than glycinate, are
still limited. Furthermore, successful examples of obtaining the
opposite absolute configuration with such high enantioselectiv-
ities are very rare.12 It is thus highly desirable to develop an
innovative, multifunctional catalytic system which is easily
prepared with a tertiary amine and a different phosphane, while
it can also tolerate variation of the dipole components.
We first investigated the 1,3-dipolar cycloaddition of the
iminoester 3a and diethyl maleate 2a using a combination of
different silver(I) salts and cinchonidine-derived amidophos-
phane 1a, which was used in the isocyanoacetate aldol reaction
by Dixon’s group (Table 1, entries 1−5).14 From a screen of
silver(I) salts, we found that the Ag2CO3/1a system gave
relatively high reactivity and enantioseletivity (Table 1, entry
5). For obtaining higher enantioselectivity in the cycloaddition,
we envisioned it may be effective by incorporating primary
amino acid moieties into the amidophosphane 1a, and thus, a
Received: December 1, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03430
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Screening Studies of Asymmetric 1,3-Dipolar
Cycloaddition of the Iminoester 3a with Diethyl Maleate 2a
Table 2. Enantioselective Ag2CO3-Catalyzed 1,3-Dipolar
a
a
Cycloaddition of Various Iminoesters 3 with the Maleates 2
b
c
entry
2
3, R2
cat.
4, yield (%)
ee (%)
1
2
3
4
5
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2a
2a
2a
2a
3b, 4-MePh
3c, 4-MeOPh
3d, 4-FPh
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1h
1h
1h
1h
4b, 95
4c, 92
4d, 93
4e, 99
4f, 92
4g, 99
4h, 90
4i, 99
4j, 95
4k, 92
4l, 81
4m, 47
4n, 94
4b, 91
4c, 97
4d, 99
4e, 96
94
96
94
3e, 4-ClPh
3f, 4-BrPh
92
91
d
6
3g, 4-CNPh
3h, 4-tBuPh
3i, 1-naphthyl
3j, 2-naphthyl
3k, 2-thienyl
3l, 2-furyl
91
c
d
7
8
9
90
entry
precat
Ag(I) salts
time (h)
yield (%)
ee (%)
b
91
1
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
AgOAc
AgOTf
AgClO4
Ag2O
18
18
24
2
41
34
<10
82
91
89
90
96
95
96
92
94
79
−17
−4
b
91
2
d
b
10
88
3
11
89
4
−12
−24
26
e
12
3m, Cyclohexyl
3a, Ph
86
5
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
2
13
14
15
16
17
93
6
2
3b, 4-MePh
3c, 4-MeOPh
3d, 4-FPh
−91
−95
−94
−93
7
2
74
8
2
81
9
2
90
3e, 4-ClPh
10
11
12
2
96
a
All reactions run using 2a (0.2 mmol), 3a (0.3 mmol), and toluene
1g
1h
1f
2
86
b
c
d
(1.4 mL). Isolated yield. Determined by HPLC. 2.8 mL of toluene
was used. Run in 24 h.
2
−92
88
e
e
13
16
a
All reactions run using 2a (0.2 mmol), 3a (0.3 mmol), and toluene
b
c
(1.4 mL). 4 mol % of Ag(I) salt was used. Isolated yield.
in high yields (90−99%) and excellent enantiselectivties (90−
96% ee) in the presence of precatalyst 1f (Table 2, entries 1−
9). α-Iminoesters 3k,l containing heteroatoms in their aromatic
rings also worked well without significant loss in selectivity
(Table 2, entries 10 and 11). Noticeably, when R2 was aliphatic
cyclohexyl, the endo-4m was successfully obtained with slightly
low enantioselectivity (86% ee) and moderate yield (47%) in
24 h (Table 2, entry 12). Dimethyl maleate as a popular
dipolarophile in the reported literature was also used to react
with α-iminoester 3a in 94% yield and 93% enantioselectivity
(Table 2, entry13).15 Gratifyingly, when precatalyst 1h was
used, reversal of the absolute configuration was successfully
realized with excellent levels of yield and enantioselectivity in
the reactions of iminoester 3b−e and diethyl maleate regardless
of electron-rich, electron-neutral, or electron-deficient groups
on the phenyl ring of iminoester 3 (Table 2, entries 14−17).
Encouraged by the results for less sterically hindered
azomethine ylides from glycinate, 1,3-dipolar cycloaddition of
the iminoesters 3 derived from α-substituted amino esters and
the maleates 2 was also investigated, as shown in Table 3.15 The
tested substrates generate pyrrolidines with a quaternary center
at the 2-position; several limited successful protocols have been
reported in achieving moderate to high levels of enantiose-
lectivity.16 As depicted in Table 3, the reaction of iminoesters 3
derived from alanine with the maleates 2 using Ag2CO3/1f
catalytic system led to pyrrolidines 4o−u with perfect endo-
selectivity and excellent enantioselectivity ranging from 88% to
96% regardless of the steric hindrance and electronic properties
of the phenyl ring of iminoesters 3 (Table 3, entries 1−7). We
also examined the iminoesters derived from phenylglycine
(Table 3, entry 8) and tryptophan (Table 3, entry 9). Good
enantioselectivity (83−91% ee) was observed in both cases,
although the process was sluggish. These results strongly
d
e
Determined by HPLC. temp = 0 °C.
number of cinchonidine-amino acid derived amidophosphines
1b−g were prepared. Interestingly, by investigating the effect of
Ag2CO3/1b−g systems on the cycloaddition, good to excellent
reversals of enantioselectivity were observed (Table 1, entries
6−11). Glycine-containing precatalyst 1b was not very effective
(Table 1, entry 6); however, high enantioselectivity (74% ee)
was achieved with the L-alanine-containing precatalyst 1c
(Table 1, entry 7). L-Phenylalanine-containing precatalyst 1d
and L-phenylglycine-containing precatalyst 1e were better than
the precatalyst 1c with an L-alanine moiety (Table 1, entries 8
and 9).We were pleased to find that precatalyst 1f, with a L-
valine moiety incorporated, afforded the desired adduct in 96%
yield and 96% ee (Table 1, entry 10). Further tuning of the
steric hindrance at the amino acid side chain did not result in
better precatalyst 1g (Table 1, entry 11). When the
pseudoenantiomeric 1h derived from cinchonine and D-valine
was used, the opposite configuration was also observed with
excellent enantioselectivity of −92% ee (Table 1, entry 12).
However, lowering the reaction temperature from room
temperature to 0 °C had detrimental effects on the yield
(77% yield) and the enantioselectivity (88% ee) (Table 1, entry
13). Thus, the optimal conditions for the asymmetric
cycloaddition of azomethine ylides is Ag2CO3/1f or 1h/toluene
at room temperature.
1,3-Cycloaddition of various iminoesters 3 and the maleates
2 in the presence of precatalyst 1f or 1h was investigated under
the optimized experimental conditions. As shown in Table 2, α-
iminoesters 3b−j from aromatic aldehydes with different steric
hindrance and electronic properties reacted with diethyl
maleate 2a to afford the corresponding endo-4b−j exclusively
B
DOI: 10.1021/acs.orglett.5b03430
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
precatalyst 1f with a slightly decreased enantioselectivity to 84%
ee, whereas the latter required prolonged reaction time to 4 h
for obtaining 100% conversion without a loss of enantiose-
lectivity (Table 4, entries 3 and 4). In terms of the above
experiments, we postulate the tertiary amine in precatalyst 1f
should have the same significant rate acceleration effect as Et3N.
To further understand the role of N−H bonds on the two
amides, ligands 1k−m which were synthesized by replacing the
different hydrogen atoms of the amides in 1j to benzyl, methyl,
and benzyl and methyl, respectively, were investigated.
Interestingly, when any one of the ligands 1k−m was used to
catalyze the process with Et3N, lower reactivity and
enantioselectivity were observed (Table 4, entries 5−7).
Furthermore, ligand 1l led to the reversal of the absolute
configuration, while ligand 1m afforded only 75% conversion
even if the reaction time was prolonged to 12 h (Table 4,
entries 6 and 7). These results indicate the importance of the
amide N−H bonds in the rate-determining step and stereo-
control.
These data suggest a cooperative mechanism where the
silver(I) ion, the tertiary amine, the two N−H bonds on the
amide group, and amidophosphane in 1f are all essential for the
observed reactivity. Although further mechanistic studies are
required, we postulate that the precatalyst 1f has three distinct
roles: (1) the tertiary amine as a Brønsted base can accelerate
the reaction by the deprotonation of the azomethine ylide; (2)
the two amide groups as H-bond donors should have the
potential interaction with the substrates; and (3) the
amidophosphane group serves as an effective ligand in
combination with silver ions to afford high levels of
enantiocontrol.
In conclusion, we have developed the Ag2CO3/CA-AA-
amidphos complexes serving as a class of novel and highly
efficient multifunctional catalysts for the asymmetric 1,3-dipolar
cycloaddition reaction without the addition of an extra excess of
base. High to very high levels of reactivity, selectivity, and
structural scope were uniformly observed for various
azomethine ylides. In addition, reversal of the absolute
configuration of the endo-adducts is also realized with excellent
levels of yield and enantioselectivity, which benefits from both
pseudoenantiomers of the new catalyst systems easily prepared
from commercially available reagents. Furthermore, the
possible contributions of the functional groups in the CA-AA-
amidphos precatalysts to the 1,3-dipolar cycloaddition have also
been investigated. Further investigations in this field of catalysis
are ongoing and will be reported in due course.
Table 3. Asymmetric 1,3-Dipolar Cycloaddition of α-
Substituent Iminoesters 3 Catalyzed by Ag2CO3/1f
a
b
c
entry
2
3, R2, R3
3o, Ph/Me
time (h) 4, yield (%) ee (%)
1
2
3
4
5
6
7
8
2a
2b
2a
2a
2a
2a
2a
2a
2a
48
48
48
48
48
48
48
72
72
4o, 83
4p, 95
4q, 90
4r, 87
4s, 99
4t, 96
4u, 99
4v, 67
4w, 61
93
3o, Ph/Me
95
92
96
95
94
88
91
86
3q, 4-MePh/Me
3r, 4-MeOPh/Me
3s, 4-FPh/Me
3t, 4-ClPh/Me
3u, 2-naphthyl/Me
3v, Ph/Bn
d
9
3w, Ph/E
a
All reactions run using 2a (0.2 mmol), 3a (0.3 mmol), and toluene
b
c
d
(1.4 mL). Isolated yield. Determined by HPLC. E = 3-
indolylmethyl.
suggest that use of a suitable catalytic system can lead to a high
diastereo- and enantioselectivity in reactions with a variety of
substrates.17
To ascertain the possible contributions of the functional
groups in the precatalyst 1f to the observed reactivity,
imidophosphane 1i and L-valine-derived amidophosphanes
1j−m without the scaffolds of cinchona alkaloids were
synthesized to catalyze the cycloaddition with or without
Et3N. As shown in Table 4, the amidophosphane group on
phenyl ring was also irreplaceable; once the group was changed
into iminophosphane, poor results were obtained, and
especially the enantioselectivity was decreased to 2% ee
(Table 4, entries 1 and 2). When the ligand 1j was used with
or without Et3N, the former had the same reaction rate as
Table 4. Studies of the Reactivity of Precatalyst 1f, 1i, and L-
Valine-Derived Amidophosphines 1j−m in the
a
Cycloaddition
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03430.
b
c
entry
precat
base
time (h)
conv (%)
ee (%)
Experimental procedures and detailed characterization
data of all new compounds (PDF)
1
2
3
4
5
6
7
1f
2
5
100
100
100
100
100
100
75
95
2
1i
1j
Et3N
2
84
1j
4
84
AUTHOR INFORMATION
■
1k
1l
Et3N
Et3N
Et3N
5
16
Corresponding Author
6
−11
12
1m
12
*E-mail: whf2107@hotmail.com.
Notes
a
All reactions run using 2a (0.2 mmol), 3a (0.3 mmol), and toluene
b
c
1
(1.4 mL). Determined by H NMR. Determined by HPLC.
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.5b03430
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (No. 21202042) and Hunan Provincial Natural Science
Foundation of China (No. 13JJ4090) for financial support.
(9) Wang, C.-J.; Liang, G.; Xue, Z.-Y.; Gao, F. J. Am. Chem. Soc. 2008,
130, 17250−17251.
(10) Oderaotoshi, Y.; Cheng, W.; Fujitomi, S.; Kasano, Y.; Minakata,
S.; Komatsu, M. Org. Lett. 2003, 5, 5043−5046.
(11) Gao, W.; Zhang, X.; Raghunath, M. Org. Lett. 2005, 7, 4241−
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