Copper-catalyzed enantioselective inverse electron-demand hetero-diels-alder reactions of diazadienes with enol ethers: Efficient synthesis of chiral pyridazines

Article abstract of DOI:10.1002/adsc.201300723

A highly efficient and enantioselective inverse electron-demand hetero-Diels-Alder reaction of in situ generated 1,2-diaza-1,3-dienes with enol ethers has been developed with the use of a chiral copper/bisoxazoline complex as the catalyst. The reaction ex

Full text of DOI:10.1002/adsc.201300723

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DOI: 10.1002/adsc.201300723
Copper-Catalyzed Enantioselective Inverse Electron-Demand
Hetero-Diels–Alder Reactions of Diazadienes with Enol Ethers:
Efficient Synthesis of Chiral Pyridazines
Shuang Gao,a Jia-Rong Chen,a,b,* Xiao-Qiang Hu,a Hong-Gang Cheng,a
Liang-Qiu Lu,a and Wen-Jing Xiaoa,*
a
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal
University, 152 Luoyu Road, Wuhan, Hubei 430079, Peopleꢀs Republic of China and Collaborative Center of Chemical
Science and Engineering (Tianjin), Peopleꢀs Republic of China
Fax : (+86)-27-6786-2041; phone: (+86)-27-67862041; e-mail: chenjiarong@mail.ccnu.edu.cn or wxiao@mail.ccnu.edu.cn
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, 693 Xiongchu
b
Road, Wuhan, Hubei 430079, Peopleꢀs Republic China
Received: August 11, 2013; Revised: September 28, 2013; Published online: November 27, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300723.
Abstract: A highly efficient and enantioselective in-
verse electron-demand hetero-Diels–Alder reaction
of in situ generated 1,2-diaza-1,3-dienes with enol
ethers has been developed with the use of a chiral
copper/bisoxazoline complex as the catalyst. The re-
action exhibits high enantioselectivity (up to 90%
ee) and provides a robust and powerful method to
synthesize a variety of structurally diverse and
densely substituted chiral pyridazine derivatives in
good to excellent yields.
reactions.[6] Accordingly, employment of new and ver-
satile multiazadienes for the construction of multini-
trogen-containing heterocycles is a desirable but chal-
lenging task for synthetic organic chemists.
Recently, 1,2-diaza-1,3-dienes have been estab-
lished as a useful type of synthetic building blocks
and found broad applications in cyclization and cyclo-
addition reactions for the preparation of various ni-
trogen-containing heterocyclic compounds,[7] as well
as their enantioselective variants. In this regard, Bolm
and co-workers developed an elegant chiral copper
complex-catalyzed enantioselective formal [4+1]an-
nulation of in situ generated 1,2-diaza-1,3-dienes af-
fording a wide range of highly functionalized dihydro-
Keywords: chiral lewis acid catalysis; 1,2-diaza-1,3-
dienes; inverse electron-demand hetero-Diels–
Alder (IEDDA) reaction; pyridazine derivatives
pyrazoles
with
excellent
enatiomeric
purity
(Scheme 1a).[8] Our group recently disclosed
a [4+3]cycloaddition reaction between in situ gener-
ated 1,2-diaza-1,3-dienes and C,N-cyclic azomethine
The inverse electron-demand hetero-Diels–Alder imines (Scheme 1b),[9] which provided an efficient
(IEDDA) reaction[1] has been recognized as one of access to seven-membered polysubstituted tetrazepine
the most powerful methods to efficiently construct derivatives with excellent chemoselectivity. Remarka-
various biologically important and synthetically valua- bly, the ready availability of the highly reactive 1,2-
ble heterocycles.[2,3] In this context, various azadienes diaza-1,3-dienes from the a-halo hydrazone precusors
have been widely utilized in the inverse electron- further highlighted the synthetic potential of these re-
demand aza-Diels–Alder reaction for the enantiose- agents. Inspired by these works, we postulated that
lective synthesis of nitrogen-containing heterocycles. such a type of reagent could also participate in the
For example, aldehyde-derived 2-azadienes[4] and N- asymmetric IEDDA reaction for the synthesis of bio-
protected 1-aza-1,3-butadienes[5] have been successful- active and enantioenriched chiral pyridazine deriva-
ly applied in the asymmetric IEDDA reaction tives[10] by asymmetric catalysis (Scheme 1c).[11,12] Im-
through Lewis acid catalysis and organocatalysis strat- portantly, this method should lead to the formation of
egies, affording the corresponding tetrahydroquino- a challenging N,O-acetal stereocenter in the prod-
line and piperidine derivatives, respectively. Despite ucts.[13]
these impressive achievements, the use of multiaza-
Inspired by Bolmꢀs work[8] and Kobayashiꢀs insight-
dienes as the electron-deficient component has been ful investigation into the chemistry of N-acylhydra-
largely unexplored in catalytic asymmetric IEDDA zones,[14] we initially applied their catalytic systems to
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Scheme 1. Construction of biologically relevant and synthetically valuable nitrogen heterocycles with in situ generated 1,2-
diaza-1,3-dienes.
Table 1. Optimization of reaction conditions for the asymmetric IEDDA reaction of a-halo-N-acylhydrazones with ethyl
vinyl ether 2a.[a]
Entry
1
Lewis acid
Cu(OTf)2
Ligand
Yield [%][b]
er[c]
1
2
3
4
5
6
7
8
9
10
11
12
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1b
1b
1b
1a
G
L1
L2
L6
L3
L4
L5
L6
L7
L4
L4
L7
L7
L4
L4
L4
L4
61
86
18
53
86
75
99
87
94
88
99
95
96
91
97
93
56:44
50:50
50:50
50:50
82:18
55:45
62:38
28:72
84:16
75:25
47:53
49:51
88:12
91:9
Zr(O-t-Bu)4
Ni(ClO4)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
(OTf)2
ACHTUNGTRENNUNG
[d]
ACHTUNGTRENNUNG
Zn
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
T
13[e,f]
14[e,g]
15[e,g,h]
16[e,g,h]
92:8
94:6
A
[a]
Reaction conditions: 1 (0.15 mmol), 2a (4.5 mmol), Lewis acid (10 mol%), Ligand (10 mol%), Na2CO3 (0.3 mmol),
CH2Cl2 (1.5 mL), 2–5 days.
Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
[b]
[c]
[d]
[e]
[f]
Hexahydrate of Ni
ACHTUNGTREN(UNNG ClO4)2 was used.
Cu(OTf)2 (20 mol%), ligand (20 mol%) were used.
ACHTUNGTRENNUNG
The reaction was run at 08C.
[g]
[h]
The reaction was run at À208C.
DCE as the solvent. DCE=1,2-dichloroethane.
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Copper-Catalyzed Enantioselective Inverse Electron-Demand Hetero-Diels–Alder Reactions
ence on the reaction efficiency with the combination
of Cu(OTf)2 and L4 and L7 being the most promising
AHCTUNGTRENNUNG
candidates (Table 1, entries 5 and 8). With these cata-
lysts, the protecting groups for the N atom were next
investigated. It was found that better results were ob-
tained in the cases of arylacyl groups than in others,
such as Boc and Ts (Table 1, entries 5, 8–10 vs. 11 and
12). Finally, with hydrazones 1a and 1b as the model
substrates, investigation of other reaction parameters
such as catalyst loading, temperature, and reaction
media gave the optimal reaction conditions: 20 mol%
of Cu
(OTf)2/L4 as the catalyst in DCE at À208C
(Table 1, entry 15, 3ba: 97% yield and 92:8 er;
entry 16, 3aa: 93% yield and 94:6 er).[15]
Figure 1. Selected chiral ligands examined in this study; p-
Tol =4-MeC6H4.
With the optimal conditions established, a wide
range of a-halogenated N-benzylhydrazones were ex-
the model reaction of N-benzylhydrazone 1a with amined to probe the generality of the catalytic asym-
ethyl vinyl ether 2a (Table 1, entries 1 and 2). It was metric IEDDA reaction. As highlighted in Table 2, in
found that the reaction did indeed proceed smoothly addition to 1a and 1b, the hydrazones 1f–1n with both
to provide the desired product 3aa in good yield electron-donating and electron-withdrawing substitu-
albeit with only moderate enantioselectivity in the ents on the benzene ring were well tolerated, and the
presence of 10 mol% of CuACHTUNRGTNEUNG(OTf)2/(R)-BINAP com- corresponding products were generated with excellent
plex (Table 1, entry 1). Encouraged by this result, we yields (89–99%) and high enantioselectivities
further examined different combinations of various (Table 2, entries 3–12, up to 95:5 er). Notably, the a-
Lewis acids and chiral ligands (Figure 1). Representa- Cl- and a-Br-substituted hydrazones, such as 1l and
tive results of entries 3 to 8 in Table 1 showed that 1m, gave rise to similar results (Table 2, entries 9 and
both Lewis acids and ligands have significant influ- 10). Significantly, ester- (1p) and thienyl-substituted
Table 2. Scope of a-halo-N-acylhydrazones for the catalytic asymmetric IEDDA reaction.[a]
Entry
X, R1, R2
3
Yield [%][b]
er[c]
1
2
3
4
5
6
7
8
Cl, Ph, Ph (1a)
Cl, 2-MeOC6H4, Ph (1b)
Cl, Ph, 4-FC6H4 (1f)
Cl, Ph, 4-ClC6H4 (1g)
Br, 2-MeOC6H4, 4-MeOC6H4 (1h)
Br, 2-MeOC6H4, 4-MeC6H4 (1i)
Cl, 2-MeOC6H4, 4-FC6H4 (1j)
Br, 2-MeOC6H4, 4-BrC6H4(1k)
Cl, 2-MeOC6H4, 4-ClC6H4 (1l)
Br, 2-MeOC6H4, 4-ClC6H4 (1m)
Br, 2-MeOC6H4, 3-ClC6H4 (1n)
Br, 2-MeOC6H4, 3-NO2C6H4 (1o)
Br, 2-MeOC6H4, CO2Et (1p)
Br, 2-MeOC6H4, thienyl (1q)
3aa
3ba
3fa
3ga
3ha
3ia
3ja
3ka
3la
93
97
92
89
93
98
96
97
99
98
97
90
77
66
94:6
92:8
94:6
95:5
90:10
88:12
92:8
90:10
93:7
9
10
11
12[d]
13[d]
14[e]
3la
91:9
3na
3oa
3pa
3qa
89:11
76:24
70:30
67:33
[a]
Reaction conditions: 1 (0.15 mmol), 2a (4.5 mmol), Cu
(1.5 mL), À208C, 2–5 days.
ACHTUGNERTN(NUNG OTf)2 (20 mol%), L4 (20 mol%), Na2CO3 (0.3 mmol), DCE
[b]
[c]
[d]
[e]
Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
The reaction was run at À208C for 4 days, then at 08C for 1 day.
The reaction was run at 08C.
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Figure 2. X-ray crystallographic structure of (R)-3fa (ther-
mal ellipsoids are set at 30% probability, Flack parameter=
0.09 (19).
Scheme 2. Scope of dienophiles for the catalytic asymmetric
IEDDA reaction.
(1q) hydrazones can be successfully employed in this
transformation, providing the corresponding products
3pa and 3qa with moderate yields and enantiomeric
excesses (Table 2, entries 13 and 14).
Figure 3. Proposed transition state.
Furthermore, the scope of the dienophiles was also mechanism of the reaction, a plausible sterochemical
evaluated by subjecting a variety of substituted ethers model was proposed for the reaction based on the re-
to the optimal conditions (Scheme 2). For example, n- lated works of Bolm,[8] Kobayashi,[14d] and Friestad
propyl vinyl ether 2b and n-butyl vinyl ether 2c (Figure 3).[17] In the postulated transition state, the O
proved to be suitable for the reaction, giving the cor- atom of the acyl group and the N atom of the in situ
responding products with good chemical yields and generated 1,2-diaza-1,3-dienes might coordinate with
enantioselectivities (3ab, 3bb–3bc: 92–96% yield, the Lewis acidic copper, and thereafter the cycloaddi-
91:9–93:7 er). Cyclic 2,3-dihydrofuran 2d can also tion could proceed with a favoured endo-selectivity.
react well with hydrazone 1b to afford product 3bd in
In conclusion, we have described the first example
excellent yield with good regio- and stereoselectivity of asymmetric IEDDA reaction of in situ-generated
(97% yield, >99:1 dr and 84:16 er).[6b,f] The construc- 1,2-diaza-1,3-dienes and enol ethers by the use of
tion of a quaternary carbon stereocenter is always a catalytic amount of chiral copper triflate/bisoxazo-
a challenging task in asymmetric synthesis due to the line complex. The reaction allows a rapid access to
steric hindrance. Note that this methodology could be the biologically important and synthetically valuable
successfully applied to the synthesis of pyridazine 3be pyridazine derivatives with good efficiency and enan-
bearing a quaternary N,O-acetal carbon stereocenter tioselectivity. Further studies to probe the application
although only moderate enantioselectivity was deliv- of 1,2-diaza-1,3-dienes to other catalytic asymmetric
ered at the current stage. In addition to enol ethers, cycloadditions and the evaluation of biological activi-
the dienophiles can also be extended to substituted ty of the products are under way in our laboratory.
styrenes and enamines. For example, the reactions
with 4-methoxystyrene and N-vinylcarbazole proceed-
ed very well under the standard conditions, affording
Experimental Section
the corresponding pyridazines 3af and 3ag in good
yields with moderate to good enantioselectivities.
The absolute configuration of product 3fa (98:2 er
after a single crystallization) was unambiguously es-
tablished to be R based on the X-ray crystallographic
analysis of its structure (Figure 2).[16] Although further
General Procedure for the Catalytic Asymmetric
IEDDA Reaction
Cu
ACHTUNGRTEN(NUNG OTf)2 (0.03 mmol, 10.9 mg) and L4 (0.03 mmol, 8.9 mg)
were dissolved in DCE (1.5 mL, 0.1M), and the mixture was
extensive studies are required to elucidate the precise stirred at room temperature for 30 min. Then, a-halo-N-ben-
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Copper-Catalyzed Enantioselective Inverse Electron-Demand Hetero-Diels–Alder Reactions
zylhydrazone 1 (0.15 mmol) and Na2CO3 (0.3 mmol, 32 mg)
were added to the mixture. After stirring at À208C for an-
other 30 min, dienophile 2 (4.5 mmol) was introduced into
the above mixture, and then stirring was continued at
À208C for 2–5 days. After the reaction was completed as
detected by TLC, saturated NH4Cl (10 mL) was added to
quench the reaction. The mixture was extracted with di-
chloromethane twice (30 mL). Then, the organic layers were
combined and dried over anhydrous Na2SO4. The residue
after filtration and evaporation was purified through flash
column chromatography on silica gel (petroleum ether/ethyl
acetate=6:1 to 1:1) to afford the desired product 3. The
enantioselectivity and diastereoselectivity of the products
were determined by chiral HPLC analysis.
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Acknowledgements
We are grateful to the National Science Foundation of China
(No. 21002036, 21272087, 21202053 and 21232003), the Na-
tional Basic Research Program of China (2011CB808600),
and Wuhan Institute of Technology (No. RGCT201202) for
support of this research.
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[15] Please see the Supporting Information for more details.
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