N,N′-Dioxide/Zinc Bis(trifluoromethylsulfonyl)imide Complex Catalyzed Enantioselective Diels-Alder Reaction of Cyclopentadiene with Alkynones

Article abstract of DOI:10.1002/adsc.201500069

An N,N′-dioxide/zinc bis(trifluoromethylsulfonyl)imide complex has been developed as an efficient catalyst for the highly enantioselective Diels-Alder reaction of cyclopentadiene with alkynones. Various 2-acyl substituted norbornadiene derivatives were obtained in moderate to high yields (up to 99%) with good enantiomeric excesses (up to 95%).

Full text of DOI:10.1002/adsc.201500069

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DOI: 10.1002/adsc.201500069
N,N’-Dioxide/Zinc Bis(trifluoromethylsulfonyl)imide Complex
Catalyzed Enantioselective Diels–Alder Reaction of Cyclopenta-
diene with Alkynones
Tengfei Kang,a Zhen Wang,a Lili Lin,a Yuting Liao,a Yuhang Zhou,a Xiaohua Liu,a
and Xiaoming Fenga,*
a
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, Peopleꢀs Republic of China
Fax: : (+86)-28-8541-8249; e-mail: xmfeng@scu.edu.cn
Received: January 26, 2015; Revised: April 8, 2015; Published online: May 18, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500069.
Abstract: An N,N’-dioxide/zinc bis(trifluoromethyl-
sulfonyl)imide complex has been developed as an
efficient catalyst for the highly enantioselective
Diels–Alder reaction of cyclopentadiene with alky-
nones. Various 2-acyl substituted norbornadiene de-
rivatives were obtained in moderate to high yields
(up to 99%) with good enantiomeric excesses (up
to 95%).
substituted norbornadienes, which are attractive struc-
tures and might be useful as chiral diene ligands.
Recently, remarkable attention has been paid
toward the design of such chiral diene ligands since
they have proven to be privileged ligands for several
rhodium-catalyzed carbon-carbon bond forming reac-
tions.[10] We have demonstrated previously that chiral
N,N’-dioxides ligands developed by our group can
complex with various metal salts to promote numer-
ous asymmetric transformations.[11] Herein, we report
on a N,N’-dioxide/Zn(NTf2)2 complex that can effi-
ciently catalyze the DA reaction of 1,3-cyclopenta-
diene with alkynones.[12,13]
Keywords: alkynones; asymmetric catalysis; Diels–
Alder reaction; N,N’-dioxide; norbornadiene
Initially, we screened the DA reaction between al-
kynone 1a and cyclopentadiene catalyzed by 5 mol%
As one of the most important and effective transfor- of chiral N,N’-dioxide L–PiEt2Me (Figure 1) com-
mations in organic chemistry, the Diels–Alder (DA) plexed with various different metal salts at 308C in
reaction, has been widely applied in total synthesis.[1] CH2Cl2. The complexes of lanthanide Sc(OTf)3 and
As a result, tremendous efforts in the past decades Y(OTf)3 gave very low ee values (Table 1, entries 1–
have been devoted to developing efficient catalysts 2). The complex of Zn(OTf)2 produced the desired
for the asymmetric DA cycloaddition of 1,3-dienes product 3a in moderate yield and improved ee (57%
with dienophiles to obtain optically pure six-mem- yield, 41% ee, Table 1, entry 3). Encouraged by this
bered cyclic products.[2,3] Compared to the well-devel- result, another zinc salt, Zn(NTf2)2, was tested; the
oped DA reactions employing sp2-hybridized dieno- yield was sharply improved to 99% without signifi-
philes,[2,4] cycloadditions using sp-hybridized dieno- cant improvement in the enantioselectivity (Table 1,
philes are less prevalent owing to the lower reactivity entry 4). Next, the structure of the ligand was exam-
À
of the C C triple bond versus the corresponding
alkene in the transformation. Since the pioneering
work of Yamamoto[5a] and Evans,[6] only boron-based
catalysts[5] and copper complexes[7] have demonstrated
success as highly efficient chiral catalysts for the enan-
tioselective DA reaction between alkyne dieno-
philes[8,9] and 1,3-dienes. Hence, there remains
a desire to develop new and more efficient chiral cat-
alysts for the asymmetric DA cycloaddition of acety-
lenic dienophiles. For example, the DA reaction of al-
kynones with cyclopentadiene can construct 2-acyl Figure 1. Ligands used in this study.
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Table 1. Optimization of the reaction conditions.
Entry[a]
Ligand
Metal
Time [h]
T [oC]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
L–PiEt2Me
L–PiEt2Me
L–PiEt2Me
L–PiEt2Me
L–PiMe2
L–PiPr2
Sc(OTf)3
Y(OTf)3
21
21
15
15
9
30
30
30
30
30
30
30
30
30
81
57
57
99
88
62
77
80
77
96
92
93
63
20
11
41
43
32
52
53
69
75
80
87
90
91
Zn(OTf)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
Zn(NTf2)2
9
L–PrPr2
3.5
3.5
12
21
48
69
70
L–RaPr2
L–RaPr2
L–RaPr2
L–RaPr2
L–RaPr2
L–RaPr2
9[d]
10[e,f]
11[e,f]
12[e,f]
13[e,f]
0
À30
À40
À50
[a]
Unless otherwise noted, all reactions were performed with L-metal (5 mol%, 1:1), 1a (0.10 mmol), 2 (0.50 mmol, 40 mL)
in CH2Cl2 (1.0 mL).
Isolated yield.
Determined by HPLC analysis.
50 mg 4 Š MS was added.
The reaction was conducted with 0.30 mmol scale and 100 mg 4 Š MS was added.
0.60 mL CH2Cl2 was used.
[b]
[c]
[d]
[e]
[f]
ined. A large steric hindrance of the amide moiety in À508C, only 63% yield was obtained even after 70 h
the ligand played a positive role on the enantioselec- (Table 1, entry 13). Thus, the optimal condition for
tivity of the reaction. When decreasing the hindrance the DA reaction was established as 5 mol% catalyst
of ortho-substituents on the aniline ring of the N,N’- loading and 100 mg 4 Š MS as additive in CH2Cl2 at
dioxide to 2,6-dimethyl groups, the ee was decreased À408C.
to 32% (Table 1, entry 5). Alternatively, increasing
With the optimized reaction conditions in hand
the bulk to 2,6-diisopropyl groups increased the ee to (Table 1, entry 12), a series of alkynones were exam-
52% (Table 1, entry 6). Variation of the chiral back- ined. Initially, variation of the ketone substituent R1
bone revealed that the l-ramipril-derived L–RaPr2 was investigated (Table 2, entries 1–15). Alkynones
was superior to the S-pipecolic acid-derived L–PiPr2 with either electron-withdrawing or electron-donating
and l-proline-derived L–PrPr2 (Table 1, entry 8 vs en- substituted phenyl R1 transformed to the correspond-
tries 6 and 7), giving a promising 80% yield and 69% ing products in moderate to excellent yields and high
ee (Table 1, entry 8). To our delight, it was observed ee. The enantioselectivity did not appear to be influ-
that the addition of 4 Š molecular sieves (MS) to the ence significantly by electronic effects, but steric ef-
catalytic system increased the ee of 3a to 75% fects do appear to lower the observed ee; the 2-meth-
(Table 1, entry 9). With an aim to further improve the ylphenyl substituted alkynone gave an ee of only 71%
enantioselectivity, the reaction temperature was ex- (Table 2, entry 11). The observed yields, on the other
amined next. As expected, lower reaction tempera- hand, were influenced strongly by both the electronic
tures caused lower reactivity but higher enantioselec- nature and the position of the substituents on the aro-
tivity. When the temperature was reduced from 308C matic ring. The yields of most cycloadducts bearing
to 08C to À308C (Table 1, entries 9–11), the ee con- electron-withdrawing R1 groups were higher than
tinually improved (87% at À308C).
those with electron-donating groups, even under
Further reduction of the reaction temperature to lower reaction temperatures. The 3,4-dichlorophenyl-
À408C resulted in an ee of 90%; the high yield could substituted 1o was most remarkable: complete con-
be maintained (93%) by prolonging the reaction time version to the corresponding 3o with 93% ee was ob-
to 69 h (Table 1, entry 12). Below this reaction tem- served after 26 h (Table 2, entry 15). Alkynones bear-
perature, minimal improvements in enantioselectivity ing 2-naphthyl or heteroaryl R1 moieties also proved
were offset by dramatic reductions in reactivity; at to be suitable substrates, affording yields ranging
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N,N’-Dioxide/Zinc Bis(trifluoromethylsulfonyl)imide Complex
Table 2. Substrate scope of the DA reaction.
[a]
Entry
R1
R2
Time [h]
T [8C]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
69
36
82
36
72
72
72
72
82
82
89
48
86
À40
À40
À30
À40
À50
À50
À50
À30
À30
À30
À20
À40
À30
93(3a)
90 (3b)
89 (3c)
87 (3d)
72 (3e)
95 (3 f)
90 (3g)
87 (3h)
70 (3i)
92 (3j)
75 (3k)
75 (3l)
65(3m)
90(1R,4S)
91(1R,4S)
93(1R,4S)
92(1R,4S)
94(1R,4S)
95(1R,4S)
95(1R,4S)
94(1R,4S)
88(1R,4S)
90(1R,4S)
71(1R,4S)
91(1R,4S)
91(1R,4S)
3-BrC6H4
4-BrC6H4
3-FC6H4
4-FC6H4
4-CF3C6H4
3-CF3C6H4
3-PhO-4-FC6H3
4-MeC6H4
3-MeC6H4
2-MeC6H4
3-MeOC6H4
4-MeOC6H4
9
10
11
12
13
14
H
86
À30
57 (3n)
90(1R,4S)
15
16
17
18
3,4-Cl2C6H3
2-naphthyl
2-thienyl
3-furyl
n-pentyl
Ph
Ph
Ph
4-ClC6H4
H
H
H
H
26
82
82
48
89
98
42
42
6
À40
À30
À30
À40
À20
30
30
30
0
99 (3o)
88 (3p)
87 (3q)
72 (3r)
79 (3s)
30 (3t)
23 (3u)
40 (3v)
85(3w)
93(1R,4S)
90(1R,4S)
90(1R,4S)
94(1R,4S)
72(1R,4S)
90(1R,4S)
89(1R,4S)
57(1R,4S)
93(1R,4S)
19[e]
20[e,f]
21[d,e]
22[d,e,h]
23[g]
H
n-pentyl
Me
TMS
I
[a]
Unless otherwise noted, all reactions were performed with L-metal (5 mol%, 1:1), 1a (0.30 mmol), 2 (1.50 mmol, 120 mL)
and 100 mg 4 Š MS in CH2Cl2 (0.6 mL).
Isolated yield.
Determined by HPLC analysis.
7.5 mol% catalyst was used.
200 mL cyclopentadiene was used.
20 mol% catalyst was used.
The reaction was performed with 0.1 mmol scale.
The absolute configuration was determined to be (1R, 4S) by comparing the specific rotation with the literature data.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
from 72% to 88% in 90–94% ee (Table 2, entries 16– entry 23). Moreover, the cycloadduct 3w can be trans-
18). Moreover, the n-pentyl substituted alkynone 1s formed easily to either the 3-nonsubstituted deriva-
converted to the expected cycloadduct 3s in 79% tive or 3-methyl derivative following reported proce-
yield and 72% ee (Table 2, entry 19).
dures.[5a] The absolute configuration of the product 3v
Next, the alkynyl substituent R2 was varied. Alky- was determined to be (1R, 4S) by comparing the spe-
nones 1t and 1u proved to be less reactive, requiring cific rotation with the literature data.[5c] The configu-
an increase in reaction temperature to 308C and pro- ration of the other products were determined also to
longed reaction times to 98 h to afford the corre- be (1R, 4S) by a comparison of their CD spectra with
sponding cycloadduct 3t and 3u in 30% yield (90% that of 3v.
ee) and 23% yield (89% ee), respectively (Table 2,
To illustrate the practical synthetic value of the re-
entries 20–21). When TMS-substituted alkynone 3v action, a gram-scale synthesis of the chiral cycload-
was used, the desired product was obtained in 40% duct 3g was performed. As shown in Scheme 1, treat-
yield with 57% ee (Table 2, entry 22). Notably, sub- ment of 0.99 g of 1g (5.0 mmol) with 2.0 mL of 2
strate 1w bearing an iodo group reacted quite well (25.0 mmol) under the optimal reaction conditions
with cyclopentadiene to give the desired product 3w (Table 2, entry 7) afforded 0.9 g (70% yield) of the
in 85% yield with 93% ee at 08C in only 6 h (Table 2, desired product 3g in 95% ee (Scheme 1(a)). Further-
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Tengfei Kang et al.
for financial support. We thank Prof. Jason Chruma for help
with language polishing.
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