Phosphine-catalyzed [3 + 2] cycloaddition of 4,4-Dicyano-2-methylenebut-3- enoates with benzyl buta-2,3-dienoate and penta-3,4-dien-2-one

Article abstract of DOI:10.1021/cs300751a

4,4-Dicyano-2-methylenebut-3-enoates are employed in the phosphine-catalyzed [3 + 2] cycloaddition with allenoates for the first time, affording regiospecific [3 + 2]-annulation products in moderate to good yields. The multifunctional chiral thiourea-phos

Full text of DOI:10.1021/cs300751a

Research Article
pubs.acs.org/acscatalysis
Phosphine-Catalyzed [3 + 2] Cycloaddition of 4,4-Dicyano-2-
methylenebut-3-enoates with Benzyl Buta-2,3-dienoate and Penta-
3,4-dien-2-one
Xiao-Nan Zhang† and Min Shi*,†,‡
†Laboratory for Advanced Materials & Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China
‡Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. China
S
* Supporting Information
ABSTRACT: 4,4-Dicyano-2-methylenebut-3-enoates are employed in the
phosphine-catalyzed [3 + 2] cycloaddition with allenoates for the first time,
affording regiospecific [3 + 2]-annulation products in moderate to good yields.
The multifunctional chiral thiourea-phosphines having an axially chiral
binaphthyl scaffold are effective catalysts for the asymmetric variant of this
reaction, giving the α-regioisomers in good yields and moderate
enantioselectivities.
KEYWORDS: allenoate, 4,4-dicyano-2-methylenebut-3-enoates, multifunctional chiral phosphines, [3 + 2] cycloaddition
Scheme 1. Phosphine-Catalyzed [3 + 2] Cycloaddition of
Allenoates with α-Substituted Acrylates
INTRODUCTION
■
Functionalized five-membered carbocycles as structural motifs
are often found in natural products and medicinally important
agents.1,2 Among the known synthetic methods, phosphine-
catalyzed [3 + 2] cycloaddition, developed by Lu in 1995,3−8 is
considered to be a powerful synthetic approach. Impressive
progress in the phosphine-catalyzed [3 + 2] cycloaddition of
allenoates with electron-deficient species such as olefins and
imines has been made, thereby providing new pathways to
functionalized five-membered carbo- and heterocycles.9−36
Following Lu and Zhang’s pioneering efforts,3,9 further
application of this annulations strategy to the total syntheses
of natural products and the development of their asymmetric
variants have also been important achievements.7,37−44 Despite
the above significant achievements, compared to the wide-
spread applications of phosphine-catalyzed [3 + 2] annulations,
the design and development of novel phosphine-mediated
processes with regard to allenoates are still under-explored.
Five-membered carbocycles with a quaternary stereogenic
center are interesting substructures often found in many natural
products and bioactive molecules,45−47 and we recently became
interested in exploring organocatalytic methods to access highly
functionalized cyclopentenes bearing one all-carbon quaternary
stereogenic center.48 It is known that phosphine-catalyzed [3 +
2] annulations between α-substituted acrylates and allenes may
be utilized to construct such five-membered ring systems
(Scheme 1). However, when the phosphine-catalyzed [3 + 2]
cycloaddition of allenoates with electron-deficient olefins is
concerned, α-substituted acrylates remain as elusive sub-
strates.9,49−51 Thus, the design and development of new
phosphine-catalyzed [3 + 2] cycloaddition of allenoates with α-
substituted acrylates is highly desirable. To the best of our
knowledge, the direct application of α,α-dicyanoolefin-sub-
stituted acrylates in the phosphine-catalyzed [3 + 2] cyclo-
addition has not been reported before. Thus, it is highly
desirable to develop a synthetic variant in which these acrylates
can be used directly in such a cyclization.
Received: November 21, 2012
Revised: January 6, 2013
Published: February 11, 2013
© 2013 American Chemical Society
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Research Article
Herein we wish to report a novel [3 + 2] cycloaddition
between α,α-dicyanoolefin-substituted acrylates with allenoates
mediated by phosphine, providing a facile protocol to construct
highly functionalized cyclopentenes bearing one all-carbon
quaternary stereogenic center in good yields (Scheme 1).
Moreover, the 4,4-dicyano-2-methylenebut-3-enoates as new α-
substituted acrylates can be conveniently prepared from alkyl
propiolates and α,α-dicyanoolefins.52
Table 2. Substrate Scope for [3 + 2] Cycloaddition of 4,4-
Dicyano-2-methylenebut-3-enoates with Allenoates.
a
We began our investigation by using the [3 + 2]
cycloaddition between 4,4-dicyano-2-methylenebut-3-enoate
1a and benzyl allenoate 2a as a model reaction (Table 1).
time
b
entry
R1
R2
(d)
yield (%)
c
1
1b, o-MeOC6H4
1c, m-MeOC6H4
1d, p-MeC6H4
1e, p-FC6H4
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2a, OBn
2b, OEt
2c, OtBu
2d, Me
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
2
2
2
2
2
3b, 47 (3b′, 39)
3c, 84
3d, 80
3e, 84
3f, 80
2
a
3
Table 1. Screening of Solvents and Catalysts.
4
5
1f, p-ClC6H4
6
1g, m-BrC6H4
1h, p-BrC6H4
1i, Ph
3g, 82
3h, 76
3i, 80
7
8
b
9
1j, p-CF3C6H4
1k, furan-2-yl
3j, 42
entry
solvent
catalyst
time (d)
yield (%)
10
11
12
13
14
15
16
17
18
19
20
3k, 72
3l, 85
1
toluene
CH2Cl2
Et2O
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
2
2
2
2
2
2
2
2
2
2
2
2
3
4
42
1l, naphtha-2-yl
1m, 3,4,5-triMeOC6H2
1a, p-MeOC6H4
1a, p-MeOC6H4
1d, p-MeC6H4
1e, p-FC6H4
2
trace
trace
trace
trace
trace
trace
8
3m, 84
3n, 82
3o, 51
3p, 81
3q, 86
3r, 80
3
4
CH3CN
THF
5
6
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
2d, Me
7
PPh2Me2
1f, p-ClC6H4
2d, Me
8
PPh2Me
1g, m-BrC6H4
1k, furan-2-yl
2d, Me
3s, 78
9
dppb
trace
trace
trace
87
2d, Me
3t, 72
10
11
12
13
DABCO
1m, 3,4,5-triMeOC6H2
2d, Me
3u, 80
(4-CH3OC6H4)3P
(4-FC6H4)3P
(4-CF3C6H4)3P
(4-FC6H4)3P
a
Reactions were performed with 1 (0.10 mmol) and 2a (0.20 mmol)
in the presence of 20 mol % of (4-FC6H4)3P in toluene (2 mL) at
room temperature. Isolated yields. Regioisomer of 3b and 3b’.
75
b
c
c
14
84
a
Reactions were performed with 1a (0.10 mmol) and 2a (0.20 mmol)
in the presence of 20 mol % of catalyst in solvent (2 mL) at room
reactions proceeded smoothly to give the corresponding
annulation products 3b−3j in 42−86% yields, respectively
(Table 2, entries 1−9). Only in the case of 4,4-dicyano-3-
(ortho-methoxyphenyl)-2-methylenebut-3-enoate 1b were two
regioisomers 3b and 3b′ formed in good total yields, perhaps
because of the steric influence (Table 2, entry 1). When R1 is a
heteroaromatic group (R1 = furan-2-yl) or a sterically more
bulky 2-naphthalene moiety or a more substituted aromatic
group (R1 = 3,4,5-(MeO)3C6H2), the reactions also proceeded
efficiently to afford the corresponding products 3k−3m in 72−
85% yields (Table 2, entries 10−12). The reactions also worked
well upon changing the ester groups in allenoates, providing the
corresponding products 3n and 3o in 82% and 51% yields,
respectively (Table 2, entries 13 and 14). Using penta-3,4-dien-
2-one 2d as substrate, whether R1 is an electron-rich or
-deficient aromatic ring or a heteroaromatic group (R1 = furan-
2-yl), the reactions also proceeded efficiently, affording the
corresponding cycloadducts 3p−3u in 72−86% yields,
respectively (Table 2, entries 15−20). The relative config-
uration of 3p has been assigned by X-ray diffraction. The
ORTEP drawing is shown in Figure 1, and the CIF data are
presented in the Supporting Information.
b
c
temperature. Isolated yields. 10 mol % of catalyst was used.
We initially utilized 20 mol % of PPh3 as catalyst and 1a (1.0
equiv) and 2a (2.0 equiv) as substrates to investigate the
influence of solvents on this cycloaddition reaction. The results
of these experiments are summarized in Table 1. It was found
that toluene is the best solvent in this reaction, albeit giving the
corresponding annulation product 3a in only 42% yield within
2 days (Table 1, entries 1−5). The examination of other
phosphines such as PBu3, PPhMe2, PPh2Me, or dppb and 1,4-
diazabicyclic-[2,2,2]-octane (DABCO) revealed that PPh3 was
the best catalyst for this reaction (Table 1, entries 6−10).
Changing the nucleophilicity of triarylphosphine, we consec-
utively examined the catalysts (4-CH3OC6H4)3P, (4-FC6H4)3P,
and (4-CF3C6H4)3P and identified that (4-FC6H4)3P was the
most efficient catalyst, producing 3a in 87% yield within 48 h
(Table 1, entries 11−13). Reducing the employed amounts of
(4-FC6H4)3P to 10 mol % also gave 3a in 84% yield upon
prolonging the reaction time to 4 days (Table 1, entry 14)
Having determined the optimal reaction conditions, we
turned our attention to the substrate scope and limitations of
this phosphine-catalyzed [3 + 2] cycloaddition of 4,4-dicyano-
2-methylenebut-3-enoates with allenoates, and the results are
summarized in Table 2. All reactions proceeded smoothly to
give the corresponding products 3 in moderate to excellent
yields under the optimal reaction conditions (Table 2). Using
benzyl allenoate 2a as substrate, we found that regardless of
whether R1 is an electron-rich or -deficient aromatic ring, the
On the basis of our previous work on chiral phosphines as
nucleophilic catalysts in asymmetric catalysis,48,50,53−59 we next
attempted to identify the best chiral phosphine catalyst and the
optimal reaction conditions for the asymmetric version of this
novel [3 + 2] cycloaddition between 4,4-dicyano-2-methyl-
enebut-3-enoates and allenoates (Table 3 and the Supporting
Information, Table SI-1). We initiated the study by
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corresponding product 3d in 78% yield, albeit in only 36% ee
(Table 3). Subsequently, we examined several multifunctional
chiral thiourea-phosphines CP2-CP4 having an axially chiral
binaphthyl scaffold, and identified that CP4 was the more
efficient catalyst, producing 3d in 88% yield along with 53% ee
(Table 3). Gratifyingly, an improvement was realized using
CP5 and CP6 through a slight structural modification of CP4,
which could produce 3d in better results. As a result, we found
that 3d could be obtained in 78% yield along with 63% ee using
CP6 as the catalyst (Table 3). Subsequently we also examined
several multifunctional chiral urea-phosphine catalysts CP7−
CP9 having an axially chiral binaphthyl scaffold, revealing that
they were not as efficient as that of CP6, affording 3d in 82−
84% yields with 51−60% ee values, respectively (Table 3).
Moreover, various multifunctional thiourea-phosphines CP10−
CP18, derived from amino acids, were also synthesized and
subsequently examined in this asymmetric reaction under
identical conditions, and the results indicated that they were
also not as efficient as that of CP6 (Table 3). Furthermore, the
cycloaddition reaction even did not take place if using chiral
phosphine CP19 as the catalyst. On the basis of the above
results, the best reaction conditions are shown in Scheme 2,
Figure 1. ORTEP drawing of 3p.
Table 3. Screening of Chiral Phosphine Catalysts for [3 + 2]
Cycloaddition of 4,4-Dicyano-2-methylenebut-3-enoates
a
with Allenoates.
Scheme 2. Optimization of the Reaction Conditions for the
Asymmetric [3 + 2] Cycloaddition Reaction.a
that is, using 20 mol % CP6 as the catalyst and carrying out the
reaction in toluene at room temperature for 2 days, giving 3d in
78% yield along with 63% ee (for other chiral phosphine
catalysts and optimization of the reaction conditions, please see
the Supporting Information, Table SI-1).
Under the optimal reaction conditions, we also investigated
the influence of the substrates 1 on this cycloaddition reaction.
The results of these experiments revealed that using a sterically
more bulky 2-naphthalene moiety-substituted 1l or a more
substituted aromatic group-substituted 1m as substrate did not
have significant influence on the ee value of the products,
affording the corresponding products 3l in 70% yield along with
a 60% ee value and 3m in 74% yield along with a 56% ee value,
respectively (Scheme 3).
On the basis of the above experimental results and previous
work on the phosphine-catalyzed [3 + 2] cycloaddition with
allenoates,3,23,24,29,60 a plausible reaction mechanism has been
presented in Scheme 4. The reaction might be initiated with the
formation of the phosphonium enolate intermediate I via the
nucleophilic attack of the phosphine catalyst at the allene
moiety. Then the subsequent nucleophilic attack of phospho-
nium enolate intermediate I on 4,4-dicyano-2-methylenebut-3-
enoate 1 with its anionic allyl carbon atom results in
intermediate II, which might keep an equilibrium with
intermediate II′.48 Intramolecular Michael addition of inter-
a
Reactions were performed with 1d (0.10 mmol) and 2a (0.20 mmol)
in the presence of 20 mol % of CP in toluene (2 mL) at room
temperature. Isolated yields. Determined by HPLC analysis.
b
c
investigating the reaction between 4,4-dicyano-2-methylene-3-
p-tolylbut-3-enoate 1d and benzyl allenoate 2a in the presence
of chiral phosphine CP1 (20 mol %) derived from axially chiral
bi(2-naphthol) in toluene at room temperature for 2 days. To
our delight, the desired annulation indeed took place, giving the
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of this new methodology to synthesize interesting bioactive
compounds.
Scheme 3. Influence of Substrates 1 on This Asymmetric [3
+ 2] Cycloaddition Reaction
EXPERIMENTAL SECTION
■
General Procedure for Tris(4-fluorophenyl)phosphine
Catalyzed [3 + 2] Cycloaddition of 4,4-Dicyano-2-
methylenebut-3-enoates with Benzyl Buta-2,3-dien-
oate, Ethyl Buta-2,3-dienoate, tert-Butyl Buta-2,3-dien-
oate, and Penta-3,4-dien-2-one. General Procedure. To a
solution of tris(4-fluorophenyl)phosphine (0.02 mmol) in
toluene (1.0 mL) was added the corresponding benzyl buta-
2,3-dienoate, ethyl buta-2,3-dienoate, tert-butyl buta-2,3-dien-
oate, or penta-3,4-dien-2-one (0.2 mmol). The reaction mixture
was stirred at room temperature. To this resulting reaction
mixture (1.0 mL) a solution of 4,4-dicyano-2-methylenebut-3-
enoate (1) (0.1 mmol) was slowly added. The reaction mixture
was stirred at room temperature until the reaction was
complete (monitoring by TLC). Then the solvent was removed
under reduced pressure and the residue was purified by a flash
column chromatography to afford the desired cyclic products 3.
1-(2,2-Dicyano-1-(4-methoxyphenyl)vinyl)cyclopent-3-
ene-1,3-dicarboxylate (3a). A colorless oil, 39 mg, 87% yield;
IR (KBr): ν 2954, 2841, 2230, 1741, 1712, 1604, 1508, 1456,
Scheme 4. Plausible Reaction Mechanism
1
1244, 1206, 1177, 1100, 1028, 836, 736, 698 cm−1; H NMR
(400 MHz, CDCl3, TMS): δ 2.87−2.98 (m, 2H, CH2), 3.46−
3.59 (m, 2H, CH2), 3.84 (s, 3H, OCH3), 3.85 (s, 3H, OCH3),
5.16 (s, 2H, CH2), 6.65−6.66 (m, 1H, =CH), 6.97 (d, 2H, J =
8.8 Hz, ArH), 7.16 (d, 2H, J = 8.8 Hz, ArH), 7.33−7.39 (m,
5H, ArH); 13C NMR (100 MHz, CDCl3, TMS): δ 43.1, 44.8,
53.9, 55.4, 59.3, 66.4, 88.9, 111.8, 112.5, 114.5, 127.2, 128.2,
128.3, 128.5, 128.6, 133.4, 135.5, 139.6, 161.5, 163.2, 172.1,
181.7; MS (ESI) m/z (%): 465.0 (100) [M+Na+]; HRMS
(ESI) Calcd. For C26H22N2O5Na+1 (M+Na+) requires
465.1426, Found: 465.1440.
General Procedure for Chiral Phosphines Catalyzed [3
+ 2] Cycloaddition of 4,4-Dicyano-2-methylenebut-3-
enoates with Benzyl Buta-2,3-dienoate, Ethyl Buta-2,3-
dienoate, tert-Butyl Buta-2,3-dienoate, and Penta-3,4-
dien-2-one. General Procedure. To a solution of chiral
phosphines (0.02 mmol) in toluene (1.0 mL) was added the
corresponding benzyl buta-2,3-dienoate, ethyl buta-2,3-dien-
oate, tert-butyl buta-2,3-dienoate, or penta-3,4-dien-2-one (0.2
mmol). The reaction mixture was stirred at room temperature.
To this resulting reaction mixture (1.0 mL) a solution of 4,4-
dicyano-2-methylenebut-3-enoate (1) (0.1 mmol) was slowly
added. The reaction mixture was stirred at room temperature
until the reaction was complete (monitoring by TLC). Then
the solvent was removed under reduced pressure and the
residue was purified by a flash column chromatography to
afford the desired cyclic products 3.
3-Ethyl 1-Methyl 1-(2,2-Dicyano-1-p-tolylvinyl)cyclopent-
3-ene-1,3-dicarboxylate (3d). A colorless solid, 34 mg, 80%
yield; mp 53−54 °C; IR (KBr): ν 3032, 2954, 2231, 1743,
1713, 1644, 1455, 1433, 1347, 1242, 1205, 1178, 1100, 747,
735, 698 cm−1; 1H NMR (400 MHz, CDCl3, TMS): δ 2.40 (s,
3H, CH3), 2.85−2.96 (m, 2H, CH2), 3.44−3.56 (m, 2H, CH2),
3.86 (s, 3H, OCH3), 5.15 (s, 2H, CH2), 6.64−6.65 (m, 1H,
=CH), 7.07 (d, 2H, J = 8.0 Hz, ArH), 7.27 (d, 2H, J = 8.0 Hz,
ArH), 7.31−7.40 (m, 5H, ArH); 13C NMR (100 MHz, CDCl3,
TMS): δ 21.4, 43.1, 44.7, 53.8, 59.2, 66.3, 89.5, 111.6, 112.1,
126.4, 128.2, 128.3, 128.5, 129.8, 132.4, 133.5, 135.5, 139.6,
141.4, 163.2, 172.0, 182.2; MS (ESI) m/z (%): 449.1 (100) [M
+Na+]; HRMS (ESI) Calcd. For C26H22N2O4Na1+ (M+Na+)
mediate II produces intermediate III, which isomerizes to
intermediate IV through a proton transfer. Intermediate IV
gives the desired highly functionalized cyclopentene 3 and
regenerates the phosphine catalyst by the elimination of
catalyst.
In conclusion, we have utilized 4,4-dicyano-2-methylenebut-
3-enoates in phosphine-catalyzed [3 + 2] cycloaddition with
allenoates for the first time, affording an efficient synthetic
approach to highly functionalized cyclopentenes bearing one
all-carbon quaternary stereogenic center in excellent regiose-
lectivity and moderate to good yields. The asymmetric
annulation reaction was promoted effectively by multifunctional
chiral thiourea-phosphines having an axially chiral binaphthyl
scaffold, giving the α-regioisomers in good yields and moderate
enantioselectivities. This is also the first example in which α,α-
dicyanoolefin-substituted acrylates are used directly in
phosphine-catalyzed asymmetric [3 + 2] cycloaddition with
allenoates. Further efforts are in progress toward the application
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectroscopic and analytic data of the
compounds 3, CIF data of 3p, as well as the optimization of the
reaction conditions for the asymmetric [3 + 2] cycloaddition
reaction are included. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
(33) Xu, S.; Zhou, L.; Ma, R.; Song, H.; He, Z. Chem.Eur. J. 2009,
15, 8698−8702.
Corresponding Author
*E-mail: mshi@mail.sioc.ac.cn.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We thank the Shanghai Municipal Committee of Science and
Technology (11JC1402600), the National Basic Research
Program of China (973)-2010CB833302, the Fundamental
Research Funds for the Central Universities, and the National
Natural Science Foundation of China for financial support
(21102166, 21072206, 20472096, 20872162, 20672127,
21121062 and 20732008).
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