Unusual ester-directed regiochemical control in endo-selective asymmetric 1,3-dipolar cycloadditions of azomethine ylides with β-sulfonyl acrylates

Article abstract of DOI:10.1002/chem.201102430

Ester-controlled regioselectivity! A highly efficient, asymmetric 1,3-dipolar cycloaddition of azomethine ylides with unsymmetrically 1,2-diactivated sulfonyl acrylates has been developed, accompanied by an unusual regioselectivity that is controlled by t

Full text of DOI:10.1002/chem.201102430

DOI: 10.1002/chem.201102430
Unusual Ester-Directed Regiochemical Control in endo-Selective
Asymmetric 1,3-Dipolar Cycloadditions of Azomethine Ylides with
b-Sulfonyl crylates
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Min-Chao Tong,[a] Jun Li,[a] Hai-Yan Tao,[a] Yu-Xue Li,*[b] and Chun-Jiang Wang*[a, b]
Five-membered nitrogen heterocycles, especially highly
substituted pyrrolidines, are widely featured in pharmaceuti-
cals, natural alkaloids, or organocatalysts and also represent
useful building blocks in organic synthesis.[1] During the last
decade, asymmetric 1,3-dipolar cycloadditions of azome-
thine ylides and electron-deficient alkenes have been report-
ed to generate stereochemically complex pyrrolidines with
moderate to high enantio- and diastereoselectivities.[2] De-
Scheme 1. Catalytic asymmetric 1,3-dipolar cycloaddition of unsymmetri-
spite excellent results achieved for this transformation, most
dipolarophiles applied in these reactions are mono-activated
or symmetrically double-activated, electron-deficient al-
kenes.[3–8] However, unsymmetrically 1,2-disubstituted al-
kenes with two different electron-withdrawing groups have
been seldom studied in the catalytic 1,3-dipolar cycloaddi-
tion, probably owing to the synthetic challenges associated
with the regio-, enantio- and diastereoselectivity. Recently,
an excellent result was achieved by Carretero and co-work-
ers when employing unsymmetrically 1,2-diactivated (Z)-sul-
fonyl acrylate as the dipolarophile and CuI/(R)-Segphos
complex as the catalyst. Here, the high regioselectivity was
mainly controlled by the sulfonyl group, accompanied by
exo-preferred diastereoselectivity[9a] (Scheme 1 left).
Recently, we reported a new family of chiral TF-Bipham-
Phos ligands that exhibited high diastereoselectivity and
good to excellent enantioselectivity in the transition-metal-
catalyzed asymmetric 1,3-dipolar cycloaddition of azome-
thine ylides with various mono-activated and symmetrically
double-activated, electron-deficient alkenes.[10] Encouraged
by these achievements and intense curiosity about the per-
formance of unsymmetrically substituted 1,2-diactivated di-
polarophiles, herein, we describe that AgI/TF-BiphamPhos
complexes serve as efficient catalysts for the highly endo-se-
lective 1,3-dipolar cycloaddtion of azomethine ylides and
cally 1,2-diactivated sulfonylacrylate with imino esters.
(Z)-sulfonyl acrylate with unusual regioselectivity, which is
exclusively controlled by the ester group rather than the sul-
fonyl group (Scheme 1 right). Azomethine ylide versatility is
one of the key features of the present method: excellent re-
activity, selectivity, and structural scope were uniformly ob-
served for various azomethine ylides, especially for those de-
rived from a-substituted amino acids, with which a unique
quaternary stereogenic center was generated efficiently.
Initially, we began our investigation by testing the reac-
tion of (Z)-sulfonyl acrylate 2 and imino ester 3a with
AgOAc/rac-(Æ)-TF-BiphamPhos (1a) as the catalyst and
Et3N as the base. The reaction was finished in less than
30 min at room temperature and delivered a single isomer
4a in 78% yield with high diastereoselectivity (>98:2,
Scheme 2).[11] Surprisingly, the regioselectivity in this case
Scheme 2. AgOAc/(Æ)-TF-BiphamPhos-catalyzed 1,3-dipolar cycloaddi-
ton of imino ester 3a with unsymmetrically 1,2-diactivated sulfonylacry-
late 2.
[a] M.-C. Tong, J. Li, H.-Y. Tao, Prof. Dr. C.-J. Wang
College of Chemistry and Molecular Sciences
Wuhan University, 430072 (China)
Fax : (+86)27-68754067
was controlled by the ester group other than the sulfonyl
group. These conclusions were further confirmed by X-ray
diffraction analysis of the enantiomerically pure compound
4j (see below).
Having established the unusual ester-controlled regiose-
lectivity and endo selectivity exerted by the AgOAc/rac-(Æ)-
TF-BiphamPhos complex in this cycloaddition reaction, we
then conducted the asymmetric reaction to evaluate the
E-mail: cjwang@whu.edu.cn
[b] Prof. Dr. Y.-X. Li, Prof. Dr. C.-J. Wang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Fenglin Road, Shanghai 230032 (China)
E-mail: liyuxue@sioc.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102430.
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COMMUNICATION
Table 1. Screening studies on the catalytic asymmetric 1,3-dipolar cyclo-
Table 2. Substrate scope of the AgOAc/(R)-1e-catalyzed asymmetric 1,3-
dipolar cycloaddition of sulfonylacrylate 2 with various imino esters 3.[a]
addition of sulfonylacrylate 2a with imino ester 3a.[a]
Entry
R
4
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
p-ClPh (3a)
p-BrPh (3b)
p-FPh (3c)
p-CNPh (3d)
o-ClPh (3e)
Ph (3 f)
p-MePh (3g)
o-MePh (3h)
m-MePh (3i)
p-MeOPh (3j)
2-naphthyl (3k)
2-furyl (3l)
2-thienyl (3m)
isopropyl (3n)
cinnamyl (3o)
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4k
4l
4m
–
82
84
82
85
83
85
85
86
89
73
83
87
84
–
99
99
99
99
96
99
99
97
96
99
99
96
99
–
Entry Ligand [M]
Solvent Time
[min]
Yield[b]
[%]
ee[c]
[%]
1
2
3
4
5
6
7
8
9
(S)-1a AgOAc
(S)-1a [CuACHTUNGTRENNUNG(MeCN)4BF4] CH2Cl2 30
CH2Cl2 30
76
76
72
73
70
80
78
82
80
87
83
88
63
80
96
97
99
98
(S)-1b AgOAc
(S)-1c AgOAc
(S)-1d AgOAc
(R)-1e AgOAc
(R)-1e AgOAc
(R)-1e AgOAc
(S)-1e AgOAc
CH2Cl2 30
CH2Cl2 30
CH2Cl2 30
CH2Cl2 30
toluene 30
EtOAc 30
EtOAc 30
–
–
–
[a]All reactions were carried out with 2 (0.23 mmol) and 3 (0.30 mmol)
in EtOAc (1 mL). [b]Isolated yield. [c]ee and>98:2 d.r. were determined
by chiral HPLC analysis. The minor diastereomer was not detected in the
1
crude H NMR spectrum.
[a]All reactions were carried out with 2 (0.23 mmol) and 3a (0.30 mmol)
in solvent (1 mL) as indicated. [b]Isolated yield. [c]ee and >98:2 d.r.
were determined by HPLC analysis. The minor diastereomer was not de-
ed, and the results are summarized in Table 2. A wide array
of glycinate-derived imino esters 2 stemming from aromatic
aldehydes bearing electron-deficient (Table 2, entries 1–5),
electron-neutral (Table 2, entry 6), and electron-rich sub-
stituents (Table 2, entries 7–10) on the aryl ring all per-
formed well, providing the corresponding endo products in
high yields (62–85%), exclusive regioselectivity, excellent
diastereoselectivities (>98:2 d.r.) and enantioselectivities
(96–99% ee). The substitution pattern of the arene only had
little effect on the reactivity and selectivity of the reaction;
ortho-substituted imino esters 3e and 3h underwent this
transformation smoothly, leading to the corresponding endo
adducts 4e and 4h with 96 and 97% ee, respectively
(Table 2, entries 5 and 8). Naphthyl aldehyde-derived imino
ester 3k could also be successfully applied in this reaction
and a high yield (83%) and excellent enantioselectivity
(99%) were obtained (Table 2, entry 11). The current cata-
lytic system was efficient for the heteroaromatic 2-furyl and
2-thienyl imino esters 3l and 3m, which delivered the de-
sired adducts with 96 and 99% ee, respectively (Table 2, en-
tries 12 and 13). However, no cycloaddition occurred when
alkyl- or alkenyl-substituted imino esters were tested under
the optimal reaction conditions (Table 2, entries 14 and 15).
Compared with the literature results,[9a] one of the key
features of this ester-controlled and endo-selective 1,3-dipo-
lar cycloaddition is that high yields and excellent selectivi-
ties were uniformly observed for the imino esters derived
from a-substituted amino acids, such as (Æ)-alanine, (Æ)-leu-
cine, (Æ)-2-aminobutyric acid, and (Æ)-phenylalanine, af-
fording the highly substituted pyrrolidines bearing a unique
nitrogen-substituted quaternary stereogenic center.[12]
1
tected in the crude H NMR spectrum.
enantioselectivity with chiral TF-BiphamPhos ligand. Al-
though both silver(I) and copper(I) salts combined with 1a
efficiently afforded the desired endo-4a with high regio- and
diastereoselectivity, AgOAc/1a complex gave a better result
in terms of the enantioselectivity (Table 1, entries 1 and 2).
Subsequently, other TF-BiphamPhos ligands were examined
to further improve the enantioselectivity, and the represen-
tative results are summarized in Table 1. The catalytic activi-
ty of ligands 1a and 1b was found to be superior to that of
ligands 1c and 1d (Table 1, entries 1 and 3–5). These results
demonstrated the importance of steric and electronic effects
of the ligands on the enantioselectivity: when the phenyl
group on the phosphorus atom of ligand 1a was replaced by
a more sterically hindered, but electron-withdrawing 3,5-
bis(trifluoromethyl)phenyl group, the enantioselectivity de-
creased significantly; ligand 1d containing cyclohexyl groups
also displayed a detrimental effect. Ligand 1e, bearing two
bromine atoms at the 3,3’-positions of the TF-BiphamPhos
backbone, emerged as the most effective chiral ligand and
provided endo-4a in high yield and excellent enantioselec-
tivity of 96% (Table 1, entry 6). A subsequent survey of sol-
vent effects indicated that EtOAc was the solvent of choice
in terms of the reactivity and enantioselectivity (Table 1, en-
tries 6–8).
Under the optimized reaction conditions, the scope and
limitations of this 1,3-dipolar cycloaddition were investigat-
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Y.-X. Li, C.-J. Wang et al.
Scheme 3. Synthetic transformation of the cycloadducts 4 f and 4n.
To further confirm the unusual regiochemial control of
this cycloaddition and determine the absolute configuration
of the adduct, a single-crystal X-ray diffraction study of 4j
was performed and its absolute configuration was unequivo-
cally assigned as (2R,3R,4S,5R)[13] (Figure 1).
The cycloaddition products containing a sulfonyl group
can be readily converted into synthetically useful and other-
wise inaccessible compounds, as exemplified in Scheme 3.
Desulfonylation of 4 f, followed by tautomerization, led to
the cyclic imine 5 without loss of diastereo- and enantiomer-
ic excess. Treatment of the cycloadduct 4n, derived from an
a-substituted imino ester, under the same reaction condi-
tions afforded 2,5-dihydro-1H-pyrrole derivative 6, probably,
because the similar elimination and tautomerization route
was efficiently suppressed by the quaternary carbon stereo-
genic center adjacent to the N atom.
To rationalize the mechanism and the stereoselectivity of
this cycloaddition reaction, density functional theory (DFT)
studies have been performed.[14] The full reaction pathway
was explored with the substrates 2, 3 f (R1 =Ph), and a sim-
plified ligand of (R)-1e, in which methyl groups were used
instead of phenyl groups on the phosphorus. The configura-
tions of the calculated models are in correspondance to the
adduct 4 in the experiment. As shown in Figure 2, this cyclo-
addition reaction proceeds through a stepwise mechanism.
In these five structures, the distance between the N atom of
the NH2 group and the Ag center ranges from 3.288 to
3.846 ꢁ, indicating that there are no strong interactions be-
Figure 1. X-ray crystallographic structure of (2R,3R,4S,5R)-4j.
Figure 2. The calculated reaction pathway with simplified models. The relative free energies DGsol are in kcalmolÀ1, the distances are in ꢁ. Calculated at
M06/(6-31G* and 6-31+G**) level in CH2Cl2 solvent.
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Asymmetric 1,3-Dipolar Cycloaddition
COMMUNICATION
tween them. Each structure has double hydrogen bonds be-
tween the two electron-withdrawing groups in the (Z)-sulfo-
in good agreement with the experimental observations, and
the most favorable transition state TS1-a will lead to prod-
uct 4 f. Other transition states are at least 4.5 kcalmolÀ1
higher in energy, indicating excellent enantioselectivity (cal-
culated ee=99.8%). The transition states in Figure 3 have
some common features: the carbon atom Cc, linked with the
nyl acrylate and the NH2 group of the chiral ligand. Ap
ACHTUNGTNERpNUNG ar-
A
ACHTUNGTRENNUNG
high endo selectivity, since they direct the two substituents
of the acrylate towards the Ag center at the same side with
the N atom in the azomethine ylide. The nucleophilic attack
(TS1) of Ca on the azomethine ylide to the Cc of the acrylate
À
sulfonyl group, is more reactive and forms the first C C
bond with Ca or Cb of the azomethine ylide. The transition
À
À
leads to a zwitterionic intermediate Int, in which the Ca Cc
states forming the Cc Ca bond (TS1-a and TS1-a’) are more
À
bond is nearly fully formed (1.600 ꢁ), the accumulated
NBO (natural bond orbitals) charges delocalized on Cd, and
the ester group is À0.712. In TS1, and especially in Int, the
double hydrogen bonds become stronger, indicating that
they can stabilize the transition state and the intermediate,
that is, activate the acrylate. The ring-closing step TS2 leads
to the final product P.
stable than those forming the Cc Cb bond (TS1-b and TS1-
b’).
The unusual ester-directing regioselectivity can be ration-
alized as follows: the resonance structures in Scheme 4a
As shown in Figure 2, the stereoselectivity should be con-
trolled by the first step (TS1) of the reaction. Four transition
states of the first step, corresponding to the four possible
stereoisomers, were constructed and fully optimized
(Figure 3). The complete models of substrates 2, 3 f, and the
ligand (R)-1e were used in the calculation. The results are
Scheme 4. Small models A and B to rationalize the regioselectivity.
show that Ca has more negative charge and consequently a
higher reactivity towards nucleophilic attack. In the reactant
R, shown in Figure 1, the calculated NBO charges on Ca and
Cb are À0.236 and À0.013, respectively. On the other hand,
for the (Z)-sulfonyl acrylate in a stepwise mechanism, Cc
should more readily undergo nucleophilic attack, owing to
the accumulated charge being delocalized on Cd and the
ester group (A in Scheme 4b). However, an isolated nega-
tive charge will accumulate on Cc, when Cd undergoes nucle-
ophilic attack (B in Scheme 4b). The small models A and B
with R=H in Scheme 4b were calculated (see Figure S2 in
the Supporting Information);[14] A is more stable than B by
9.5 kcalmolÀ1. The negative charge on Cc in B is À0.940,
which is much larger than that on Cd (À0.623) in A. There-
fore, in a stepwise reaction, when the large negative charge
on Cc in B cannot be stabilized by the catalyst or other
groups in the system, and there are no significant steric ef-
fects exerting on the sulfonyl and ester groups, the best reac-
tion mode is that Ca of the azomethine ylide attacks Cc of
the acrylate.
The origin of the enantioselectivity can be easily under-
stood from Figures 3 and 4. In TS1-a, the phenyl group of
the azomethine ylide takes the empty right side, whereas in
the less stable TS-a’ this phenyl group is at the crowded left
side and encounters a steric clash with the TF-Bipham back-
bone of the ligand. Therefore, ligand 1e with a larger Br
atom as the R1 group can produce higher ee values.
Figure 3. The four optimized transition states corresponding to the four
possible stereoisomers of TS1. The relative free energies DGsol are in kcal
molÀ1, the distances are in ꢁ. Calculated at M06/(6-31G* and 6-31+
G**) level in CH2Cl2 solvent.
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Y.-X. Li, C.-J. Wang et al.
(20090141110042), and the Fundamental Research Funds for the Central
Universities.
Keywords: asymmetric catalysis · cycloaddition · density
functional calculations · diactivated alkenes · regioselectivity
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Figure 4. The top view of TS1-a and TS1-a’.
In conclusion, we have reported the first catalytic, asym-
metric 1,3-dipolar cycloaddition of imino esters and unsym-
metrically 1,2-diactivated (Z)-sulfonyl acrylate with unusual
regioselectivity that is controlled by the ester group rather
than the sulfonyl group. The highly efficient AgI/TF-Bi-
phamPhos catalytic system exhibited excellent diastereo-
and enantioselectivity, and a broad substrate scope. Subse-
quent transformation of the cycloadducts led to expedient
preparation of synthetically useful cyclic imine and dihydro-
pyrrole derivatives. Theoretical calculations revealed a step-
wise mechanism for this highly selective 1,3-dipolar cycload-
dition, and the hydrogen-bonding interactions between the
two different electron-withdrawing groups in (Z)-sulfonyl
acrylate and the NH2 group of the chiral ligand TF-Bipham-
Phos played a significant role in the high endo selectivity.
The unusual ester-directed regioselectivity and excellent
enantioselectivity were also rationalized.
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Experimental Section
General procedure for the asymmetric 1,3-dipolar cycloaddition of azo-
methine ylides with (Z)-methyl 3-(phenylsulfonyl)-acrylate catalyzed by
AgOAc/TF-BiphamPhos complex 1e: Under an argon atmosphere, (R)-
TF-BiphamPhos (1e, 5.73 mg, 0.007 mmol) and AgOAc (1.2 mg,
0.007 mmol) were dissolved in ethyl acetate (1 mL) and stirred at 08C for
approximately 1 h. Then, the imine substrate (0.30 mmol) was added, fol-
lowed by Et3N (0.04 mmol) and (Z)-methyl 3-(phenylsulfonyl)-acrylate
(2, 52 mg, 0.23 mmol). When the starting material was consumed (moni-
tored by TLC), the mixture was filtered through Celite and the filtrate
was concentrated to dryness. The crude product was analyzed by
1H NMR spectroscopy to determine the endo to exo ratio. Then, the resi-
due was purified by column chromatography to give the corresponding
cycloaddition product as a white solid, which was directly analyzed by
chiral HPLC to determine the enantiomeric excess.
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Acknowledgements
This work is supported by the National Natural Science Foundation of
China (20972117, 20872168), the Program for New Century Excellent
Talents in university (NCET-10-0649), the Program for Changjiang
Scholars and Innovative Research Team in University of the Ministry
of Education (IRT1030), 973 program (2011CB808600), SRFDP
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Asymmetric 1,3-Dipolar Cycloaddition
COMMUNICATION
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[11] The AgI or CuI/rac-(Æ)-TF-BiphamPhos (1a)-catalyzed reaction of
(E)-sulfonyl acrylate 2 with glycine imino ester 3a delivered a mix-
ture of isomers that could not be separated by chromatography.
[12] Quaternary Stereocenters: Challenges and Solution for Organic Syn-
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[13] Crystal data for (2R,3R,4S,5R)-4j: C21H23NO7S; Mr =433.46; T=
293 K; monoclinic; space group P21; a=12.9796(8), b=5.7493(4),
c=14.8287(10) ꢁ; V=1032.33(12) ꢁ3; Z=2; 3329 unique reflec-
tions; final R1 =0.0282 and wR2 =0.0728 for 3428 observed [I>
2s(I)] reflections. Flack c=0.03(6) CCDC 822412 contains the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] See the Supporting Information for details.
Received: August 5, 2011
Published online: October 5, 2011
Chem. Eur. J. 2011, 17, 12922 – 12927
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12927