Asymmetric formal [3+2] cycloaddition reaction of isocyanoesters to 2-oxobutenoate esters by a multifunctional chiral silver catalyst

Article abstract of DOI:10.1002/chem.201100636

Silver-standard selectivity! The first highly enantioselective formal [3+2] cycloaddition reaction of isocyanoesters with 2-oxobutenoate esters catalyzed by a chiral silver complex proceeded readily under the catalysis of a multifunctional chiral silver complex in high yields and enantiometric excess (ee) values of up to 98 % (see scheme). Copyright

Full text of DOI:10.1002/chem.201100636

DOI: 10.1002/chem.201100636
Asymmetric Formal [3+2] Cycloaddition Reaction of Isocyanoesters to ACHTUNGTRENNUNG
2-Oxobutenoate Esters by a MultiACTHNUTRGENUGNfunctional Chiral Silver Catalyst
Jin Song, Chang Guo, Peng-Hao Chen, Jie Yu, Shi-Wei Luo, and Liu-Zhu Gong*[a]
Chiral pyrrolidine constitutes a core structural motif prev-
alent in numerous natural alkaloids and pharmaceutically
active substances and has proven widely applicable in total
synthesis as a type of building block. In the last decade,
great efforts have been devoted to the construction of opti-
cally pure pyrrolidine derivatives, leading to the appearance
of many elegant catalytic methods.[1,2] Optically active dihy-
dropyrroles possess a carbon–carbon double bond and
thereby can be adapted for diverse synthetic purposes by ex-
ploiting a wide range of reactions that are compatible with
olefins.[3] However, approaches to access the molecules of
this type in an enantioselective fashion are rather limited,
but include the asymmetric Heck reaction of dihydropyr-
roles with halogenated (or pseudo-halogenated) com-
pounds,[4] partial hydrogenation of trisubstituted pyrroles,[5]
and a sequential process of N-butoxycarbonyl (Boc)-protect-
ed imines with propargylated malononitriles (recently dis-
covered by Jørgensen).[6] Specifically, formal cycloaddition
reactions of metalated isocyanides to electronically poor ole-
fins offers the most straightforward method to produce
highly functionalized 2,3-dihydropyrroles in comparison
with other alternatives.[7] However, the enantioselective var-
iants have not been available until our recent report of a
highly enantioselective organocatalytic formal cycloaddition
reaction of a-substituted isocyanoesters with nitroolefins by
cinchona alkaloid derivatives.[8] Unfortunately, the expan-
sion of this protocol to an unsaturated carbonyl compound
2a provided a moderate yield with low levels of stereoselec-
tivity.[9] To address this challenge, and inspired by the asym-
metric ferrocenyl–gold(I)/silver(I)-catalyzed aldol reactions
of isocyanoacetate and tosylmethyl isocyanides with alde-
hydes,[10,11] we turned our attention to a Lewis-acid-cata-
lyzed asymmetric version of the cyclization between isocya-
noesters and 2-oxobutenoate esters. Although a similar reac-
tion catalyzed by silver acetate has been reported,[12] no
enantioselective version is available. Herein, we report the
first highly enantioselective formal [3+2] cycloaddition re-
action of isocyanoesters to 2-oxobutenoate esters catalyzed
by a chiral silver complex, which performed as a multifunc-
tional catalyst to control the stereochemistry.
Our initial investigation focused on screening chiral li-
gands (Scheme 1) for the silver-acetate-catalyzed cycloaddi-
Scheme 1. Chiral ligands evaluated for the titled reaction.
tion reaction of methyl a-phenylisocyano acetate (1a) with
methyl 2-oxo-4-phenylbutenoate (2a) conducted in dichloro-
methane at 08C.[13] Although high yields were obtained, dis-
appointingly all the chiral ligands, including (S)-BINAP 5,[14]
Hoveyda ligand 6,[15] (R)-phosphoramidite ligand 7,[16] and
(S)-MOP 8–10,[17] offered racemic products. Inspired by the
work of Hayashi and co-workers,[11a] which indicated that
the selective generation of a tri-coordinated silver(I) catalyst
would bring about higher stereoselectivity through slow ad-
dition of isocyanoacetate to a solution of the silver(I) cata-
lyst and substrates, we then examined our ligands again
through slow addition of isocyanoacetate over two hours
(see the Supporting Information). To our great delight, Hov-
eyda ligand 6 (the analogues of which have provided high
stereoselectivity with AgI and CuI for other type of reac-
tions)[15] gave an improved result (26% enantiometric excess
(ee) for the major diastereomer, Table 1, entry 2). No signifi-
cant enhancement in the ee value was achieved by using
other ligands except those with a hydroxyl group, as shown
[a] J. Song, C. Guo, P.-H. Chen, J. Yu, S.-W. Luo, Prof. L.-Z. Gong
Hefei National Laboratory for Physical Sciences
at the Microscale and Department of Chemistry
University of Science and Technology of China
Hefei, 230026 (P.R. China)
Fax : (+86)551-3606266
E-mail: gonglz@ustc.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100636.
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COMMUNICATION
Table 1. Evaluation of ligands (L*) and optimization of reaction condi-
tions.[a]
bond with one of the acetate oxygens. As such, the silver
complex might work as a multifunctional catalyst in the cyc-
lization reaction by acting as a silver(I) Lewis acid, an ace-
tate base, and proton as a Brønsted acid capable of activat-
ing substrates by a hydrogen-bonding interaction.[18,20] Varia-
tion of the silver source revealed that the basicity of the
counter ion in the silver catalyst precursors not only plays
an important role in the catalytic activity, but also is crucial
to the enantioselectivity (Table 1, entries 5–9). Thus,
AgOAc, AgTFA, Ag2CO3, and PhCO2Ag all exhibited ex-
cellent catalytic activity in combination with 9a. In contrast,
AgOTf, which proved to be an excellent silver catalyst pre-
cursor in asymmetric aldol reaction of tosylmethyl isocya-
nide with aldehydes,[11b] was unable to activate the nucleo-
phile 1a and hence was catalytically inactive for the reac-
tion, presumably due to the much lower basicity of the tri-
flate than other counter ions. In terms of the stereoselectiv-
ity, AgOAc emerged as the catalyst precursor of choice. The
ligand 9a could offer 90% ee upon conducting the reaction
at À108C, whereas its structural analogues 9b and 10
showed slightly lower enantioselectivities (Table 1, en-
tries 10–12). A survey of solvents identified chloroform as
the most suitable media; the highest level of stereoselectiv-
ity was observed in these conditions (92% ee for the major
diastereomer, Table 1, entries 13–18). The absolute configu-
ration of 3aa was assigned by X-ray crystal structure analy-
sis (see the Supporting Information).
Entry
AgI
L*
Solvent
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
CF3CO2Ag
Ag2CO3
AgOTf
PhCO2Ag
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
5
6
7
8
9a
9a
9a
9a
9a
9a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
91
80
96
94
95
82
87
n.r.[f]
88
95
86
90
2:1
2:1
1:1
2:1
2:1
1:1
1:1
–
3:1
2:1
3:1
3:1
3:1
3:1
4:1
3:1
3:1
1:1
6 (0)
26 (10)
3 (2)
5 (0)
82 (72)
27 (41)
42 (51)
–
9
87 (84)
90 (70)
88 (80)
88 (81)
88 (78)
92 (83)
84 (56)
80 (56)
84 (68)
46 (39)
10[e]
11[e]
12[e]
13[e]
14[e]
15[e]
16[e]
17[e]
18[e]
9b CH2Cl2
10
CH2Cl2
ACHTUNGTERN(NUNG ClCH2)2 95
CHCl3
CCl4
toluene
THF
CH3CN
9a
9a
9a
9a
9a
9a
90
55
99
90
95
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), AgOAc
(10 mol%), and ligand (12 mol%). Isocyanoacetate 1a was added by a
syringe pump over a period of 2 h to a solution of 2a and catalyst at 08C.
[b] Yields of isolated products. [c] Determined by 1H NMR spectroscopy.
[d] Determined by HPLC analysis; ee values shown in parentheses for
the minor diastereomer. [e] at À108C. [f] n.r.=no reaction.
Having established the optimal conditions, the generality
of this reaction between a-phenylisocyano acetate (1a) and
2-oxobutenoate esters was next explored (Table 2). Either
electronically rich or electronically poor phenyl-substituted
substrates underwent smooth cyclization reactions to give
in 9 and 10. Most interestingly, the chiral ligand 9a offered
remarkably higher levels of enantioselectivity than 8, clearly
demonstrating that the presence of a hydroxyl group is cru-
cial to the induction of high enantioselectivity (Table 1, en-
tries 4 and 5). In addition, these results suggest that the hy-
drogen-bonding interaction between the hydroxyl of the
ligand 9a and the substrate is important to control the ste-
reoselectivity. The similar hydrogen-bonding interaction be-
tween 9a and substrates was also found in organocatalyzed
Morita–Baylis–Hillman reactions.[18] Fortunately, a crystal of
the complex formed from silver acetate and (S)-9a was ob-
tained and its X-ray structure acquired (Figure 1). To the
best of our knowledge, this is the first silver crystal of such
type with an additional hydroxyl Brønsted acid functionali-
ty.[19] In this complex, only the phosphorus coordinates to
the silver, whereas the hydroxyl proton forms a hydrogen
Table 2. Scope for 2-oxobutenoate esters.[a]
Entry
3
R
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
4-ClC6H4
4-BrC6H4
4-FC6H4
4-MeC6H4
4-CNC6H4
3-ClC6H4
3-MeOC6H4
3,4-Cl2C6H3
91
81
73
95
84
91
85
98
3:1
4:1
3:1
3:1
4:1
3:1
3:1
4:1
94 (75)
90 (74)
92 (90)
91 (84)
91 (71)
90 (75)
94 (78)
93 (81)
9
3aj
94
3:1
91 (83)
10
11
12
3ak
3al
3am
2-furyl
2-naphthyl
c-C6H11
90
96
86
2:1
2:1
5:1
93 (91)
90 (78)
93 (80)
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), AgOAc
(10 mol%), and ligand (12 mol%) in CHCl3 at À108C. Isocyanoacetate
1a was added by a syringe pump over a period of 2 h to a solution of 2a
and catalyst. [b] Yields of isolated products. [c] Determined by 1H NMR
spectroscopy. [d] Determined by HPLC analysis; ee values are shown in
parentheses for the minor diastereomer.
Figure 1. Crystal structure of the silver acetate complex with (S)-9a.
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L.-Z. Gong et al.
the 4,5-dihydropyrroles, where-
as both reaction conversion and
stereoselectivity showed de-
pendence to some degree on
the electronic feature of sub-
stituents on the phenyl substitu-
ent. The 2-oxobutenoate, with a
relatively electronically rich
para-methylphenyl substituent,
provided
a comparably good
yield and stereochemical out-
come like those with electroni-
cally deficient groups (Table 2,
entry 4, entries 1–3, and 5).
Meta-substituted phenyl 2-oxo-
butenoates underwent the cycli-
zation reaction with high levels
Scheme 2. Stereoselective reductions of the dihydropyrrole into functionalized hydroxyacid esters. Bn=
benzyl; Boc=tert-butoxycarbonyl; DMAP=4-dimethylaminopyridine.
of enantioselectivity (Table 2, entries 6 and 7). Disubstituted
phenyl 2-oxobutenoates participated in the reaction to pro-
vide excellent yields and high levels of enantioselectivity for
the major diastereomers (Table 2, entries 8 and 9). A heter-
oaryl 2-oxo-butenoate is also a good substrate in a high
yield and excellent enantioselectivity (Table 2, entry 10).
The reaction occurred nicely using 2-oxobutenoate with a 2-
naphthyl substituent and also gave high enantioselectivity,
albeit with a comparably lower diastereomeric ratio (d.r.,
Table 2, entry 11). The cyclohexyl 2-oxobutenoate reacted
well with 1a to give a high yield and with a slightly higher
d.r. of 5:1 and 93% ee for the major diastereomer (Table 2,
entry 12).
A further study on the substrate scope was then focused
on the tolerance of the a-substituted isocyanoacetates
(Table 3). A variety of a-arylisocyano acetates could be tol-
erated in the reaction. Either electronically rich or electroni-
cally poor a-arylisocyano acetates participated in the cycli-
zation reaction in high yields and with excellent levels of
enantioselectivity. Particularly, both the 4-chloro- and 4-ace-
toxyphenyl isocyanoacetates afforded 98% ee (Table 3, en-
tries 2 and 4).
The 2,3-dihydropyrrole contains multiple functionalities
and thereby can be readily transformed into structurally di-
verse pyrrolidine derivatives through well-established meth-
ods. For example, 2,3-dihydropyrrole 3aa was first protected
with a benzyl (Bn) group by exposure to benzylbromide in
the presence of potassium tert-butoxide (Scheme 2). Then,
treatment with sodium borohydride followed to yield dihy-
dropyrrole 12 in high yield with a high diastereomeric ratio.
The hydrogenation of the Boc-protected product 13 was
able to generate pyrrolidine 14 with maintained ee values
under the catalysis of Pd/C. Both dihydropyrroles of type 12
and pyrrolidines of 14 (Scheme 2), which contain useful hy-
droxy ester functional groups, show potential value in the
design of new catalysts[21] and occur frequently in many bio-
logically active molecules.[1,22]
According to the crystal structure of the silver(I)/(S)-9a
complex (Figure 1) and previous reports on related transfor-
mations,[10,11a,23] we proposed a transition structure to explain
the observed stereochemistry (Scheme 3). a-Deprotonation
Table 3. Scope of a-substituted isocyanoacetates.[a]
Scheme 3. Proposed transition state to explain the stereochemistry ob-
served.
Entry
3
Ar
Yield [%][b]
d.r.[c]
ee [%][d]
of metallated isocyanide with the acetate anion generates an
enolate-like species, which is stabilized by a hydrogen bond
formed with the OH group on the catalyst. Simultaneously,
both carbonyl groups of the 2-oxobutenoate ester are able
to coordinate with the Ag center[24] and thus lower the
LUMO of the carbon–carbon double bond, which becomes
more electrophilic toward the enolate-like nucleophile from
its Re face, to facilitate an enantioselective Michael addition
and give the pyrrolidine products with high levels of stereo-
chemical control.
1
2
3
4
5
3bh
3ch
3dh
3eh
3 fb
3-ClC6H4
4-ClC6H4
4-MeOC6H4
4-AcOC6H4
4-FC6H4
78
93
78
81
86
3:1
4:1
3:1
3:1
3:1
93 (84)
98 (70)
96 (82)
98 (86)
93 (76)
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), AgOAc
(10 mol%), and ligand (12 mol%) in CHCl3 at À108C. Isocyanoacetate
1a was added by a syringe pump over a period of 2 h to a solution of 2a
and catalyst. [b] Yields of isolated products. [c] Determined by 1H NMR
spectroscopy. [d] Determined by HPLC analysis; ee values are shown in
parentheses for the minor diastereomer.
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Asymmetric Formal [3+2] Cycloaddition Reaction of Isocyanoesters
COMMUNICATION
[8] C. Guo, M.-X. Xue, M.-K. Zhu, L.-Z. Gong, Angew. Chem. 2008,
120, 3462; Angew. Chem. Int. Ed. 2008, 47, 3414.
[9] The similar reaction, catalyzed by cinchona alkaloid derivative
(Deng catalyst 4), proved to be unsuccessful. For an early applica-
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Chem. Soc. 2004, 126, 9906.
In conclusion, we have established the first chiral Lewis-
acid-catalyzed formal [3+2] cyclization reaction of isocya-
noesters with 2-oxobutenoate esters in high yields and excel-
lent levels of enantioselectivity by using a silver complex of
silver acetate and (S)-(2’-hydroxy-1,1’-binaphthyl-2-yl) di-
phenylphosphine. Most specifically, this is the first successful
application of a silver-OH-MOP complex in asymmetric cat-
alysis. In addition, we have also provided evidence that the
functionality of hydroxyl in this complex plays a crucial role
in the control of stereoselectivity. The crystal structure of
the complex suggested that it works as a multifunctional cat-
alyst in the asymmetric cyclization reaction. These findings
may reveal a strategy for designing new catalysts for this
class of cyclization reactions.
Experimental Section
General procedure: Under Ar, a solution of AgOAc (0.01 mmol) and
chiral ligand 9a (0.012 mmol) in CHCl3 (0.5 mL) were stirred at room
temperature for 1 h. Then, a solution of 2 (0.1 mmol) in CHCl3 (1.0 mL)
was added. After cooling to À108C, a solution of 1 (0.1 mmol) in CHCl3
(1.0 mL) was added by a syringe pump over a period of 2 h to a solution
of 2 and the catalyst. After the completion of reaction (monitored by
TLC), the mixture was purified by column chromatography to give the
product.
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Acknowledgements
We are grateful for financial support from Chinese Academy of Sciences,
MOST (973 project 2010CB833300), the Ministry of Education and
BASF (GLZ). We also thank Dr. Fei Liu in Australia for polishing the
English language in the manuscript.
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Keywords: asymmetric
isocyanoesters · multifunctional · silver
catalysis
·
cycloaddition
·
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Received: February 27, 2011
Revised: April 28, 2011
Published online: May 26, 2011
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Chem. Eur. J. 2011, 17, 7786 – 7790