Cu(I)-Catalyzed Highly Enantioselective [3 + 3] Cycloaddition between Two Different 1,3-Dipoles, Phthalazinium Dicyanomethanides and Iminoester-Derived Azomethine Ylides

Article abstract of DOI:10.1021/ol503169d

(Chemical Equation Presented). The Cu(I)-catalyzed highly enantioselective [3 + 3] cycloaddition between two different 1,3-dipoles, phthalazinium dicyanomethanides and iminoester-derived azomethine ylides, has been achieved under mild reaction conditions, providing novel chiral heterocyclic compounds, 2,3,4,11b-tetrahydro-1H-pyrazino[2,1-a]phthalazine derivatives, in high yields with excellent diastereo- and enantioselectivies (up to 99% yield, 99% ee, >20:1 dr).

Full text of DOI:10.1021/ol503169d

Letter
pubs.acs.org/OrgLett
Cu(I)-Catalyzed Highly Enantioselective [3 + 3] Cycloaddition
between Two Different 1,3-Dipoles, Phthalazinium
Dicyanomethanides and Iminoester-Derived Azomethine Ylides
Chunhao Yuan, Honglei Liu, Zhenzhen Gao, Leijie Zhou, Yalin Feng, Yumei Xiao, and Hongchao Guo*
Department of Applied Chemistry, China Agricultural University, Beijing 100193, China
S
* Supporting Information
ABSTRACT: The Cu(I)-catalyzed highly enantioselective [3 + 3] cycloaddition between two different 1,3-dipoles,
phthalazinium dicyanomethanides and iminoester-derived azomethine ylides, has been achieved under mild reaction conditions,
providing novel chiral heterocyclic compounds, 2,3,4,11b-tetrahydro-1H-pyrazino[2,1-a]phthalazine derivatives, in high yields
with excellent diastereo- and enantioselectivies (up to 99% yield, 99% ee, >20:1 dr).
1,3-Dipolar cycloaddition reactions represent an important class
of synthetic methods for the convergent synthesis of a wide range
of heterocycles.1 Among various cycloadditions, [3 + 3]
cycloaddition reactions have attracted much attention during
the past decade2,3 and have emerged as an important alternative
tool to the [4 + 2] cycloaddition for the synthesis of six-
membered heterocycles and total synthesis of natural prod-
ucts.2,3 However, successful examples of metal-catalyzed [3 + 3]
cycloadditions, especially enantioselective [3 + 3] cycloadditions,
are still limited.4,5
Generally, [3 + 3] cycloaddition occurs via stepwise reaction of
a stable 1,3-dipole with reactive dipolar species.2,4,5 To achieve a
[3 + 3] cycloaddition, making a compatible combination of stable
dipoles with reactive dipolar species is key. Through reactions of
the phosphonium ylide formed in situ from a phosphine and
allenoate or electron-deficient alkene with the stable dipole,
azomethine imines, we developed several phosphine-catalyzed
cycloadditions, not only [3 + 3] but also [3 + 2], [4 + 3], and [3 +
2 + 3] cycloadditions, leading to a broad range of dinitrogen-
fused heterocycles.6 In 2013, we reported the first copper-
catalyzed highly diastereo- and enantioselective [3 + 3]
cycloaddition of azomethine ylides formed in situ with nother
stable dipole azomethine imines, providing a concise and
expedient excess of a variety of optically active hexahydro-8H-
pyrazolo[1,2-a][1,2,4]triazin-8-one derivatives with potential
biological activity.5h As a continuation of our research on
cycloaddition reactions, we are anticipating to find new
cycloaddition reactions by using a combination of a stable 1,3-
dipole with reactive dipolar species.
are available for cycloaddition chemistry, thus providing a diverse
dipole source for the development of [3 + 3] cycloaddition.1
Among these 1,3-dipoles, phthalazinium dicyanomethanide is a
particularly attractive one as a result of its stability, easy
preparation, and favorable reactivity. Its thermal [3 + 2]
cycloaddition reactions with alkenes, alkynes, phosphaalkynes,
or thiones have intensively been studied.7 Besides these thermal
cycloadditions, in 2011, Carrillo and Vicario reported an
imidazolidinone-catalyzed enantioselective [3 + 2] cycloaddition
of phthalazinium dicyanomethanide with α,β-unsaturated
aldehydes, affording highly substituted chiral pyrrolo-
phthalazines.8 As a classic Sustmann type-II 1,3-dipole,9
phthalazinium dicyanomethanide may conduct a cycloaddition
reaction via dipole HOMO-control or dipole LUMO-control
mode depending on the dipolarophile that it encounters.7c−e On
the basis of this behavior, we reasoned that [3 + 3] cycloaddition
of phthalazinium dicyanomethanide with other 1,3-dipoles might
occur when both dipoles are encountered. Iminoester-derived
azomethine ylide, which was generated in situ and has functioned
as a versatile metalated 1,3-dipole in various 1,3-dipolar
cycloadditions,10 would be an ideal candidate to pair with
phthalazinium dicyanomethanide for an interesting [3 + 3]
cycloaddition. Herein, we report a novel Cu(I)-catalyzed
enantioselective [3 + 3] cycloaddition of two different 1,3-
dipoles, phthalazinium dicyanomethanide and iminoester-
derived azomethine ylide, to give chiral heterocycles under
mild reaction conditions (Scheme 1).
Our initial investigations focused on the reactions of
phthalazinium dicyanomethanide 1a with α-iminoester 2a
Numerous heteroatom-containing 1,3-dipoles such as nitrone,
azomethine imine, azomethine ylide, carbonyl ylide, nitrile imine,
nitrile oxide, azide, diazoalkane, diazoacetate, and carbonyl oxide,
Received: October 30, 2014
Published: December 18, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
and 61% ee (entry 1). The P,S-ligand L2 could also promote the
reaction to give the product in 89% yield and 60% ee (entry 2).
The use of ferrocenyl P,N ligand L3 resulted in the product 3aa
in 67% yield with increased 80% ee and >20:1 dr. The
homologous ligand L4 provided the product in similar 65%
yield, but in low 47% ee. Gratifyingly, iPr-Phosferrox L5
displayed high activity and excellent enantio- and diastereose-
lectivity, providing the product 3aa in 82% yield, 95% ee and
>20:1 dr (entry 5). Using tBu-Phosferrox L6 as chiral ligand, the
product 3aa was obtained in 87% yield and >20:1 dr, albeit in
lower 84% ee (entry 6). With the use of Cu(CH3CN)4BF4/L5 as
the catalyst, two haloalkane solvents, CHCl3 and 1,2-dichloro-
ethane (DCE), were tried. Both solvents were compatible for
catalysis. However, in chloroform, the reaction gave the desired
product in lower yield and moderate ee (75% yield and 44% ee)
(entry 7); in the contrast, DCE gave excellent results (86% yield,
96% ee, >20:1 dr) (entry 8). Lowering the reaction temperature
to −10 and −20 °C respectively, gave a little increased yield of
product (entries 9−10). When the volume of solvent was
reduced from 5 to 3 mL, the yield and ee were slightly increased
to 91% and 99%, respectively (entry 11 vs 10). Reducing the
catalyst loading to 5 mol % still afforded an 83% yield of product
in 97% ee (entry 12), albeit requiring 32 h of reaction time. Such
a significantly longer reaction time is probably attributed to the
heterogeneous nature of the reaction. On the basis of the above
observations, subsequent reactions were performed at −20 °C
using Cu(CH3CN)4BF4/L5 (10 mol %) and DBU (20 mol %) in
3 mL of DCE.
Scheme 1. Copper-Catalyzed [3 + 3] Cycloaddition Reactions
of Phthalazinium Dicyanomethanides with Iminoester-
Derived Azomethine Ylides
(Table 1). The dipole 1a is a stable ylide and was easily prepared
in almost quantitative yield by treating phthalazine with
a
Table 1. Screening of the Reaction Conditions
With the optimized conditions identified, we investigated the
[3 + 3] cycloaddition of a variety of α-iminoesters 2 with
phthalazinium dicyanomethanide 1a. As shown in Table 2, a wide
range of aryl substituted α-iminoesters reacted smoothly with
phthalazinium dicyanomethanide to afford the desired cyclo-
addition products in high yields (87−99%) with excllent
enantioselectivities (90−99% ee) and good to excellent
diastereoselectivity (14:1 to >20:1 dr) (entries 1−22). Both
electron-donating and -withdrawing substituents on the benzene
ring of the α-iminoesters were well tolerated in the cycloaddition
reactions, and the substitution pattern had no significant
influence on the activity and stereoselectivity (entries 1−20).
In particular, those α-iminoesters with electron-donating
substituents on the benzene ring yielded the cycloaddition
products in high yields with uniformly excellent stereoselectivites
(97−99% ee, >20:1 dr) (entries 12−20). Both naphthyl and
thienyl-substituted iminoesters are also compatible substrates for
the reaction, providing the corresponding product in high yields
and excellent enantioselectivities (entries 21−22). Moreover,
cinnamyl-substituted iminoesters underwent the cycloaddition
reaction to give the cycloadduct in 90% yield with 96% ee and
>20:1 dr (entry 23). It is worth noting that alkyl-substituted
substrates also displayed satisfactory activity and excellent
enantioselectivity (entry 24). Compared with aryl-substituted
iminoesters, the alkyl-substituted substrate is relatively inert and
required more basic inorganic base to promote the reaction. The
absolute and relative configurations as depicted were determined
by X-ray diffractional analysis of the cycloaddition product 3ah.12
We next studied the reaction of several phthalazinium
dicyanomethanide 1 with α-iminoesters 2a.11 As indicated in
Table 3, all phthalazinium dicyanomethanides afforded the
cycloaddition products in high yields (80−93%) and with
excellent enantioselectivities (90−99% ee), regardless of the
electronic properties and substitution pattern of substituents on
the phthalazine ring (entries 1−4). However, it is noteworthy
b
c
entry
L
solvent
t (°C)
t/h
yield (%)
ee (%)
d
1
L1
L2
L3
L4
L5
L6
L5
L5
L5
L5
L5
L5
DCM
DCM
DCM
DCM
DCM
DCM
CHCl3
DCE
0
0
12
5
85
89
67
65
82
87
75
86
90
89
91
83
61
2
60
d
3
0
12
12
12
12
24
5
80
d
4
0
−47
5
0
95
84
44
96
95
97
99
97
6
0
7
0
8
0
9
DCE
−10
−20
−20
−20
6
10
11
DCE
8
e
DCE
8
f
12
DCE
32
a
Unless otherwise stated, reactions of 1a (0.1 mmol) and 2a (0.2
mmol) were carried out in the presence of Cu(CH3CN)4BF4 (0.01
mmol), Ligand (0.01 mmol), and DBU (0.02 mmol) in 5 mL of the
b
c
solvent. Isolated yield. Unless otherwise stated, dr is >20:1,
determined by 1H NMR analysis. Ee was determined by chiral
d
e
HPLC analysis. Entry 1, dr 15:1; entry 3, dr 10:1; entry 4, dr 8:1. For
entry 17, 3 mL of DCE were used. 5 mol % catalyst was used.
f
tetracyanoethyl-eneoxide in THF at rt.11 We first examined the
reaction in dichloromethane (DCM) at 0 °C in the presence of
10 mol % of Cu(CH3CN)4BF4, 10 mol % of chiral ligand, and 20
mol % of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Since the
solubility of phthalazinium dicyanomethanide is poor in the
solvent, the reaction is heterogeneous. As the reaction proceeds,
the substrate slowly dissolves and is exhausted. Using 2,2′-
bis(diphenylphosphanyl)-1,1′-binaphthyl (BINAP) (L1) as a
chiral ligand, the desired product 3aa was obtained in 85% yield
27
dx.doi.org/10.1021/ol503169d | Org. Lett. 2015, 17, 26−29

Organic Letters
Letter
a
We also performed the present asymmetric reaction on the
gram scale. As shown in Scheme 2, in the presence of 10 mol % of
Table 2. Scope of Iminoester-Derived Azomethine Ylides
Scheme 2. Gram-Scale Synthesis and Further
Transformations of the Cycloadduct
b
c
entry
2
R
t/h
3
yield (%)
ee (%)
1
2a
2b
2c
2d
2e
2f
C6H5
8
5
3aa
3ab
3ac
3ad
3ae
3af
91
84
95
86
87
84
93
94
85
99
95
86
87
99
95
90
90
95
92
93
95
80
90
61
99
94
94
2
2-FC6H4
3
3-FC6H4
8
d
4
4-FC6H4
12
7
95
5
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-BrC6H4
3-BrC6H4
4-BrC6H4
4-PhC6H4
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-EtC6H4
2-OMeC6H4
4-OMeC6H4
4-SMeC6H4
2,4-diMeC6H3
3,4-diMeC6H3
2-naphthyl
2-thienyl
92
90
97
90
92
97
95
98
97
97
98
97
99
98
98
98
99
6
9
7
2g
2h
2i
7
3ag
3ah
3ai
8
12
48
24
7
9
10
11
12
13
14
15
16
17
18
19
20
21
2j
3aj
2k
2l
3ak
3al
24
8
2m
2n
2o
2p
2q
2r
3am
3an
3ao
3ap
3aq
3ar
3as
3at
7
12
24
8
8
2s
2t
7
Cu(I) catalyst, asymmetric [3 + 3] cycloadddition of
phthalazinium dicyanomethanide 1a (1.75 g) with α-iminoester
2a delivered the desired chiral 2,3,4,11b-tetrahydro-1H-
pyrazino[2,1-a]phthalazine 3aa in 95% yield (3.30 g) with 94%
ee, demonstrating the synthetic potential of the present
asymmetric reaction. Further treatment of the product 3aa
with H2/Pd/C led to reduction of a cyano group to an amine,
affording the corresponding derivative 4 in 48% yield with 98%
ee and >20:1 dr, and an unidentified ring-opening product. With
the use of LiALH4, the ester group of 3aa was reduced to give the
corresponding derivative 5 in 63% yield with 97% ee and >20:1
dr.
20
12
12
12
48
2u
2v
2w
2x
3au
3av
3aw
3ax
e
d
22
95
23
cinnamyl
96
f
d
24
i-Bu
92
a
Unless otherwise stated, reactions of 1a (0.1 mmol), 2 (0.2 mmol),
Cu(CH3CN)4BF4 (0.01 mmol), L5 (0.01 mmol), and DBU (0.02
b
mmol) were carried out in DCE (3 mL) at −20 °C. Isolated yields.
c
Unless otherwise stated, dr is >20:1, determined by 1H NMR analysis.
d
Ee was determined by chiral HPLC analysis. Entry 4, dr: 14:1; entry
22, dr: 18:1; entry 24, dr: 5:1. The reaction was performed at −30 °C.
e
f
2 equiv of Cs2CO3 were used.
In summary, a highly enantioselective [3 + 3] cycloaddition of
phthalazinium dicyanomethanides and iminoester-derived azo-
methine ylides has been developed with the use of a phosphine−
oxazoline−Cu(I) complex as the chiral catalyst, leading to chiral
2,3,4,11b-tetrahydro-1H-pyrazino[2,1-a]phthalazine derivatives
in high yields with excellent dr and ee values. A variety of
iminoester-derived azomethine ylides and phthalazinium
dicyanomethanides are compatible with the mild reaction
conditions, making this reaction a highly valuable method for
the diversity-directed synthesis of biologically important chiral
heterocyclic compounds.
a
Table 3. Scope of Phthalazinium Dicyanomethanides
b
c
d
entry
1
3
yield (%)
dr
ee (%)
1
2
3
4
1b (R = 5-OMe)
1c (R = 8-OMe)
1d (R = 6,7-2Me)
1e (R = 5-Cl)
3ba
3ca
3da
3ea
91
93
90
87
9:1
15:1
90
91
97
99
>20:1
>20:1
a
Reactions of 1 (0.1 mmol), 2a (0.2 mmol), Cu(CH3CN)4BF4 (0.01
mmol), L5 (0.01 mmol), and DBU (0.02 mmol) were carried out in
DCE (3 mL) at −20 °C. For entries 1 and 2, the reactions of 1b and
1c were performed in one pot by using the mixture of 1b and 1c as the
substrates, and then the resulting products were separated by flash
column. The yields were calculated according to the ratio of two
substrates obtained from HPLC analysis. For entry 4, a similar
ASSOCIATED CONTENT
* Supporting Information
■
S
b
c
procedure had been used. Isolated yields. Determined by 1H NMR
Experimental procedures, spectral data, and crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
d
analysis. Determined by chiral HPLC analysis.
that the substitution pattern of substituents on phthalazine ring
seems to have a certain influence on the diastereoselectivity. For
example, the cycloaddition reaction of 5-OMe substituted
phthalazinium dicyanomethanide (1b) led to the corresponding
product 3ba with 9:1 dr, while an 8-OMe substituted substrate
(1c) yielded the heterocyclic product 3ca with 15:1 dr (entry 1 vs
2).
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: hchguo@cau.edu.cn.
Notes
The authors declare no competing financial interest.
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
This work is supported by the NSFC (21172253 and 21372256).
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