Asymmetric [3+2]-Cycloaddition of Morita-Baylis-Hillman Carbonates with Maleimides Catalyzed by Chiral Ferrocenylphosphines

Article abstract of DOI:10.1080/00397911.2014.943346

New chiral ferrocenylphosphines LB1-LB9 were designed and prepared through simple synthetic approaches. These air-stable ferrocenylphosphines were applied to promote asymmetric [3+2]-cycloaddition of Morita-Baylis-Hillman carbonates with maleimides, among which LB7 was shown to have good catalytic activity to afford the corresponding multifunctional cyclopentenes in up to 59% yield and up to 53% ee under mild reaction conditions. A plausible reaction mechanism was proposed.

Full text of DOI:10.1080/00397911.2014.943346

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Asymmetric [3+2]-Cycloaddition of
Morita–Baylis–Hillman Carbonates
with Maleimides Catalyzed by Chiral
Ferrocenylphosphines
a
a
a
a
Aiping Tu , Haiwen Hu , Tieqi Du & Weihui Zhong
a
Key Laboratory for Green Pharmaceutical Technologies and Related
Equipment of Ministry of Education, College of Pharmaceutical
Sciences , Zhejiang University of Technology , Hangzhou , China
Accepted author version posted online: 16 Jul 2014.Published
online: 26 Sep 2014.
To cite this article: Aiping Tu , Haiwen Hu , Tieqi Du & Weihui Zhong (2014) Asymmetric
[3+2]-Cycloaddition of Morita–Baylis–Hillman Carbonates with Maleimides Catalyzed by Chiral
Ferrocenylphosphines, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 44:23, 3392-3399, DOI: 10.1080/00397911.2014.943346
To link to this article: http://dx.doi.org/10.1080/00397911.2014.943346
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1
Synthetic Communications , 44: 3392–3399, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.943346
ASYMMETRIC [3þ2]-CYCLOADDITION OF MORITA–
BAYLIS–HILLMAN CARBONATES WITH MALEIMIDES
CATALYZED BY CHIRAL FERROCENYLPHOSPHINES
Aiping Tu, Haiwen Hu, Tieqi Du, and Weihui Zhong
Key Laboratory for Green Pharmaceutical Technologies and Related
Equipment of Ministry of Education, College of Pharmaceutical Sciences,
Zhejiang University of Technology, Hangzhou, China
GRAPHICAL ABSTRACT
Abstract New chiral ferrocenylphosphines LB1–LB9 were designed and prepared through
simple synthetic approaches. These air-stable ferrocenylphosphines were applied to promote
asymmetric [3þ2]-cycloaddition of Morita–Baylis–Hillman carbonates with maleimides,
among which LB7 was shown to have good catalytic activity to afford the corresponding
multifunctional cyclopentenes in up to 59% yield and up to 53% ee under mild reaction
conditions. A plausible reaction mechanism was proposed.
Keywords Asymmetric [3þ2]-cycloaddition reaction; catalyst; chiral ferrocenylphos-
phines; cyclopentene
INTRODUCTION
Cyclopentene scaffolds are capital units in many bioactive molecules and
[1]
natural products, and asymmetric [3þ2]-cycloaddition is one of the important
methods to construct various of functionalized cyclopentenes, which has attracted
the broad attention of researchers. Recently the 1,3-dipolar synthons derived from
Morita–Baylis–Hillman (MBH) carbonates were found to be ideal intermediates
to afford multifunctional cyclopentenes via asymmetric [3þ2]-cycloaddition pro-
[
2]
moted by chiral phosphines derived from a-amino acid, dipeptide, and so on.
As we know, ferrocene was widely used to construct effective chiral ligands in
[3]
metal catalysis. Surprisingly, relatively few enantioselective variants of chiral
Received May 5, 2014.
Address correspondence to Weihui Zhong, Key Laboratory for Green Pharmaceutical
Technologies and Related Equipment of Ministry of Education, College of Pharmaceutical Sciences,
Zhejiang University of Technology, Hangzhou 310014, P. R. China. E-mail: weihuizhong@zjut.edu.cn
3
392

[3þ2]-CYCLOADDITION BY CHIRAL FERROCENYLPHOSPHINE
3393
ferrocenylphosphine-catalyzed asymmetric [3þ2]-cycloaddition have been reported
[
except for 2-phospha[3]-ferrocenophanes with planar chirality. However, the prep-
4]
ꢁ
aration of 2-phospha[3]-ferrocenophanes involved low temperatures (ꢀ70 C) and
[4a,4b]
use of the dangerous NaBH CN.
3
Thus, it is highly desirable to develop efficient
and mild approaches for the synthesis of chiral ferrocenylphosphines.
Herein, we report the asymmetric [3þ2]-cycloaddition of MBH carbonates
with maleimides catalyzed by chiral ferrocenylphosphines, which were designed
and prepared in simple synthetic ways under mild conditions and showed good air
[
5]
stability.
RESULTS AND DISCUSSION
Initially, we designed a series of new chiral ferrocenylphosphines LB1–LB9
Fig. 1), which were readily prepared by the condensation of the derivatives of fer-
(
rocene with the corresponding isothiocyanate, (S)-methyl pyrrolidine-2-carboxylate
[6]
or other reactants, respectively.
Then these catalysts LB1–LB9 were evaluated by promoting the [3þ2]-
cycloaddition of MBH carbonate 1a with N-phenyl maleimide 2a in toluene at room
temperature (Table 1). A racemic product 3a was obtained in 31% yield when chiral
bifunctional ferrocenylphosphines LB1 was used as catalyst (Table 1, entry 1). LB2,
containing an amide group, also generated the racemic mixture 3a with 83% yield
within 12 h (Table 1, entry 2). Taking into account the steric hindrance between
amide and diphenylphosphine, we designed another type of chiral bifunctional
Figure 1. Chiral ferrocenylphosphines LB1–LB9.

3394
A. TU ET AL.
Table 1. Asymmetric [3þ2]-cycloaddition of MBH carbonate 1a with N-Ph maleimide 2a catalyzed by
a
LB1–LB9
b
c
Entry
Catalyst
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
LB 1
LB 2
LB 3
LB 4
LB 5
LB 6
LB 7
LB 8
LB 9
47
12
20
52
46
70
48
48
48
31
83
59
19
31
19
37
25
42
Racemic
Racemic
Racemic
3
Racemic
2
48
18
23
a
Unless otherwise specified, the reactions were performed with 1a (0.5 mmol) and 2a (1 mmol) and
catalyst (0.05 mmol) in toluene (1.0 mL) at room temperature.
b
Isolated yield.
The ee value was determined by HPLC analysis using a chiral column.
c
ferrocenylphosphine, such as LB3 derived from m-diphenylphosphine benzoic acid.
To our disappointment, LB3 still afforded the racemic product 3a in 59% yield
(Table 1, entry 3). Compared with LB1–LB3, LB4 without Brønsted acid afforded
product 3a in 19% yield with 3% ee (Table 1, entry 4). Similarly, chiral phosphine
LB5 or LB6 produced the functionalized cyclopentenes with 31% yield and 0% ee
or in 19% yield and 2% ee, respectively (Table 1, entries 5 and 6). Perhaps the dipho-
sphine groups of LB5 and LB6 did not show a good synergetic effect and they only
increased the steric hindrance of catalysts.
Notably, LB7 containing (S)-methyl pyrrolidine-2-carboxylate shown good
catalytic activity to afford 3a in 37% yield and 48% ee (Table 1, entry 7), which
suggested that the introduction of the heterocycle group was necessary to increase
the ring strain of intermediate III (Scheme 1). To confirm our conjecture, chiral fer-
rocenylphosphines LB8 containing pyrrole section and LB9 containing morpholine
section were prepared and afforded the corresponding products in 25% yield with
1
8% ee and 42% yield with 23% ee, respectively (Table 1, entries 8 and 9). Chiral fer-
rocenylphosphine LB7 was the best catalyst for the [3þ2] cycloaddition of MBH car-
bonate 1a with N-phenyl maleimide 2a.
On the other hand, the reaction temperature has a significant effect on the
enantioselectivity and conversion rate. Initially the reaction temperature was set to
ꢁ
ꢁ
ꢀ
60 C, but no reaction was found (Table 2, entry 1). At 25 C, 52% ee values of
ꢁ
the product 3a were detected; if the temperature continued to rise to 60 C, the ee

[3þ2]-CYCLOADDITION BY CHIRAL FERROCENYLPHOSPHINE
3395
Scheme 1. Plausible reaction mechanism.
Table 2. Screening of different reaction temperatures and solvents for asymmetric [3þ2]-cycloaddition
a
catalyzed by LB7
ꢁ
b
c
Entry
Temp. ( C)
Solvent
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
ꢀ60
ꢀ25
0
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
168
108
84
—
28
29
37
36
50
35
24
21
33
—
—
52
49
48
46
36
50
49
42
45
—
25
40
60
25
25
25
25
25
48
36
29
57
Et
THF
2
O
72
47
CHCl
DCM
3
1
1
0
1
76
120
CH
3
CN
a
Unless otherwise specified, the reactions were performed with 1a (0.5 mmol), 2a (1 mmol), and LB7
0.05 mmol) in toluene (1.0 mL) at room temperature.
(
b
Isolated yield.
The ee value was determined by HPLC analysis using a chiral column.
c

3396
A. TU ET AL.
ꢁ
values were reduced to 36% (Table 2, entries 2–6). The optimal temperature is 25 C.
Furthermore, various solvents were screened using LB7 as catalyst and 1a and 2a as
ꢁ
substrates at 25 C. The reaction generated smoothly in various solvents to afford 3a
in moderate yields and ee values (Table 2, entries 4 and 7–10), among which toluene
was proved to be the optimal solvent. Because of the poor solubility of substrates in
acetonitrile, the reaction did not occur even if the reaction time was prolonged to 5
days (Table 2, entry 11).
With the optimal reaction conditions identified, we next turned our attention to
examine the scope of chiral ferrocenylphosphine LB7–catalyzed asymmetric [3þ2]-
cycloaddition using a variety of MBH carbonates with maleimides (Table 3). The
position and electronic effect of the substituents on the aromatic ring of MBH car-
bonates not only have effects on the yield and enantioselectivity but also affect the
a
Table 3. Asymmetric [3þ2]-cycloaddition of MBH carbonates with maleimides catalyzed by LB7
1
R
2
R
b
c
Entry
Ar
X
Product
Yield (%)
ee (%)
1
2
3
4
5
6
4-NO
Ph
4-MeC
4-MeOC
4-FC
2
C
6
H
4
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Bn
4-ClC
10
30
30
30
30
30
20
30
30
30
30
30
30
10
10
10
10
10
10
30
3a
3b
3c
3d
3e
3f
37
33
27
16
41
16
46
57
30
59
37
18
34
29
26
38
29
22
32
—
48
43
40
37
41
42
48
48
41
47
34
24
53
6
H
4
6
H
4
6
H
4
2-ClC
3-ClC
3-ClC
4-ClC
6
6
6
6
H
4
4
4
4
d
7
H
H
H
3g
3g
3h
3i
8
9
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
3-BrC
4-BrC
6
H
H
4
4
6
3j
Cinnamyl
2-Furyl
3k
3l
e
4-NO
4-NO
4-NO
4-NO
4-NO
4-NO
n-Pr
2
C
2
C
2
C
2
C
2
C
2
C
6
6
6
6
6
6
H
H
H
H
H
H
4
3m
3n
3o
3p
3q
3r
3s
—
4
4
4
4
4
26
52
45
44
42
—
6
H
4
6
4-MeOC
Ph
Ph
H
4
n-Bu
Me
Ph
a
Unless otherwise specified, the reactions were performed with 1a (0.5 mmol), 2a (1 mmol), and LB7
0.05 mmol) in toluene (1.0 mL) at room temperature.
(
b
Isolated yield.
The ee value was determined by HPLC analysis using a chiral column.
c
d
ꢁ
The reaction temperature is 40 C.
The ee value was not detected.
e

[3þ2]-CYCLOADDITION BY CHIRAL FERROCENYLPHOSPHINE
3397
loading of catalyst. For instance, the asymmetric [3þ2]-cycloaddition between the
MBH carbonate derived from p-nitronbenzaldehyde and maleimides could smoothly
proceed with 10 mol% LB7. However, other MBH carbonates could not generate the
cycloproducts under the similar conditions even when prolonging the reaction time.
ꢁ
In most of cases, 20 mol% of LB7 was essential to promote this addition at 40 C
(Table 3, entry 7), but if the load of LB7 was increased to 30%, the reaction could
be carried out at room temperature (Table 3, entry 8). Compared with electron-
donating groups, electron-withdrawing groups on the aromatic ring of MBH carbo-
nates have a significance effect on the yield and enantioselectivity (Table 3, entries 1,
2
, and 3). The MBH carbonate with an ortho-chloro group on the aromatic ring
could produce 3g in 16% yield and 42% ee due to steric effect (Table 3, entry 6).
While those with meta-substitute on the aromatic ring afforded the corresponding
products in a greater yields and ee values than ortho- or para-substitute (Table 3,
entries 6 and 8–11). The MBH carbonate derived from 2-furyl aldehyde gave the
greatest ee value (53% ee, Table 3, entry 13). In the case of MBH carbonate 2s
derived from n-butyraldehyde, no prodcut was detected under the similar conditions
(Table 3, entry 20).
Furthermore, the substituents effect of the N-aryl maleimides on yield and
enantioselectivity was also evaluated. Generally, electron-withdrawing groups
increased the reactivity of maleimides and the reaction enantioselectivity (Table 3,
entries 16 and 17). If varying substituents from phenyl to methyl or benzyl, the cor-
responding products 3m and 3n were isolated in a lower yields and ee values (Table 3,
entries 14 and 15).
[2c,2e]
On the basis of predecessors’ works
and our experimental results, a plaus-
ible reaction mechanism was proposed (Scheme 1). First of all, the attack of LB7 to
the MBH carbonate 1 generates the phosphonium salt I, accompanied by the release
of CO and deprotonation of I by in situ–generated tert-butoxide anion to produce
2
the ylide II, which then undergoes c-addition to maleimide 2 to generate intermedi-
ate III. Intermediate III affords the final cycloproduct 3 through intramolecular
cyclization and the elimination of catalyst.
CONCLUSION
In summary, chiral ferrocenylphosphines-catalyzed asymmetric [3þ2]-
cycloaddition of MBH carbonates with maleimides was developed. LB7 was found
to be air stable enough and have moderate catalytic activities to give the functiona-
lized cyclopentenes in up to 59% yield and up to 53% ee value. Applying the chiral
ferrocenylphosphines to other asymmetric reactions is under way in our laboratory.
EXPERIMENTAL
All starting materials were commercially available and were used without
further purification. Melting points were determined on a B u¨ chi B-540 capillary
melting-point apparatus and are uncorrected. Optical rotations were determined
1
13
by using a Autopol V polarimeter. H NMR and C NMR spectra were recorded
on a Varian 400-MHz spectrometer at 400 and 100 MHz for solution in CDCl with
3
tetramethylsilane (TMS, d ¼ 0) as an internal standard. The chemical shift (d) were

3398
A. TU ET AL.
reported in parts per million (ppm) and coupling constants J were expressed in hertz
(
3
Hz). P NMR were measured in CDCl with 85% H PO as an internal standard on
1
3 3 4
a Varian 400-MHz spectrometer at 161.9 MHz. Mass spectra were obtained with a
Trace DSQ mass spectrometer in electrospray ionization (ESI) mode.
High-resolution mass spectra were acquired with an Agilent 6210 TOF mass spec-
trometer. The enantiomeric excesses of target molecules were obtained with
chiral-phase high-performance liquid chromatographic (HPLC) analysis, using Agi-
lent 1100 HPLC (chromatographic column: Daicel Chiralcel AD-H, OD-H, or AS-H
column and eluting with n-hexane=i-PrOH) with DAD UV detector.
Typical Procedure for N-[(S)-1’-(R)-2’-Diphenylphosphinoferrocenyl]
ethyl-(R)-methyl Pyrrolidine-2-carboxylate (LB7)
A solution of (S)-1-(R) diphenylphosphisoferrocenyl ethyl acetate (0.23 g,
0
(
.5 mmol) and (S)-methyl pyrrolidine-2-carboxylate (0.26 g, 2.0 mmol) in methanol
5 mL) was refluxed for 5 h. The solvent was removed under induced pressure, and
the residue was purified by column chromatography to afford LB7 (0.69 g, 66%)
ꢁ
20
ꢁ
1
as a yellow solid; mp: 105.2–106.4 C; ½aꢂ ¼ þ277:0 (c 1.0, CHCl ); H NMR
D
3
(400 MHz, CDCl ) d 7.60–7.55 (m, 2H), 7.38–7.35 (m, 3H), 7.21–7.18 (m, 5H),
3
4
1
3
1
(
8
.42 (s, 2H), 4.30 (s, 1H), 3.92 (s, 1H), 3.91 (s, 5H), 3.62 (s, 3H), 2.76–2.70 (m,
H), 2.47–2.37 (m, 1H), 2.27–2.21 (m, 1H), 1.65–1.58 (m, 2H), 1.49 (d, J ¼ 6.8 Hz,
1
3
H), 1.45–1.40 (m, 2H); C NMR (100 MHz, CDCl ) d 176.2, 140.5, 138.2, 135.3,
3
35.1, 132.3, 132.1, 128.8, 127.8, 127.7, 127.5, 127.4, 127.2, 97.5, 75.0, 71.3, 69.6
3
5C), 69.3, 68.7, 61.9, 51.5, 51.3, 45.1, 29.0, 22.7, 15.7; P NMR (162 MHz, CDCl ,
1
3
þ
5% H PO ): d ꢀ22.77; MS (ESI) m=z (%): 525.0 (100) [M þ 1]. HRMS (ESI) calcd.
3
4
for C H FeNNaO P 548.1418 [M þ Na]; found: 548.1420.
3
0
32
2
Typical Procedure for (3aR,4S,6aS)-Methyl 4-(4-Nitrophenyl)-
1
5
,3-dioxo-2-phenyl-1,2,3,3a,4,6a-hexahydrocyclopenta[c]pyrrole-
-carboxylate (3a)
A solution of methyl 2-(((tert-butoxycarbonyl)oxy)-(4-nitrophenyl)methyl)-
acrylate 1a (0.17 g, 0.5 mmol), N-phenyl maleimide 2a (0.17 g, 1 mmol), and chiral
ferrocenylphosphine LB7 (26.25 mg, 0.05 mmol) in toluene (1.0 mL) was stirred at
room temperature for 48 h. The solvent was removed under reduced pressure, and
the residue was purified by column chromatography to give 3a (0.07 g, 37%) as a
ꢁ
20
D
ꢁ
1
white solid. Mp 62.3–63.4 C; ½aꢂ ¼ þ97:4 (c 1.0, CHCl ); H NMR (400 MHz,
3
CDCl ) d 8.21 (d, J ¼ 8.8 Hz, 2H), 7.52–7.43 (m, 2H), 7.43–7.39 (m, 3H),
3
7
1
1
1
.28–7.25 (m, 2H), 7.05 (dd, J ¼ 2.4, 1.6 Hz, 1H), 4.77–4.75 (m, 1H), 4.37–4.33 (m,
1
3
H), 3.68 (s, 3H), 3.51 (dd, J ¼ 7.6, 2.4 Hz, 1H); C NMR (100 MHz, CDCl ) d
3
75.9, 173.3, 162.7, 148.6, 147.2, 140.3, 138.7, 131.2, 129.1 (2C), 128.8, 127.9 (2C),
26.4 (2C), 124.2 (2C), 53.2, 53.1, 52.3, 52.2; MS (ESI) m=z (%): 391.1 (100)
ꢀ
[
M ꢀ 1]. HRMS (ESI) calcd. for C H N O 391.0930 [M-H]; found: 391.0917.
2
1
15
2
6
Enantiomeric excess was determined by HPLC with a Chiral OD-RH column
k ¼ 214 nm; eluent: acetonitrile=water ¼ 60=40; flow rate: 0.5 mL=min; t
[
1
¼
major
6.365 min, t
¼ 13.167 min; ee ¼ 48%].
minor

[3þ2]-CYCLOADDITION BY CHIRAL FERROCENYLPHOSPHINE
3399
FUNDING
We thank the National Natural Science Foundation of China (21276238) for
financial support.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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