Enantioselective synthesis of dihydropyrazoles by formal [4+1] cycloaddition of in situ-derived azoalkenes and sulfur ylides

Article abstract of DOI:10.1021/ja301196x

An unprecedented strategy to access highly enantioenriched dihydropyrazoles is described. It involves formal [4+1] cycloadditions of in situ-derived azoalkenes and sulfur ylides catalyzed by a chiral copper/Tol-BINAP complex. A variety of synthetically and biologically important dihydropyrazoles have been obtained with high enantioselectivities (up to 97:3 er) in good yields (83-97%).

Full text of DOI:10.1021/ja301196x

Communication
pubs.acs.org/JACS
Enantioselective Synthesis of Dihydropyrazoles by Formal [4+1]
Cycloaddition of in Situ-Derived Azoalkenes and Sulfur Ylides
Jia-Rong Chen,†,‡ Wan-Rong Dong,‡ Mathieu Candy,‡ Fang-Fang Pan,§ Manuel Jorres,‡
̈
and Carsten Bolm*,‡
†Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
152 Luoyu Road, Wuhan, Hubei 430079, China
‡Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany
§Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
S
* Supporting Information
attractive but underexploited strategy for the construction of
ABSTRACT: An unprecedented strategy to access highly
enantioenriched dihydropyrazoles is described. It involves
formal [4+1] cycloadditions of in situ-derived azoalkenes
and sulfur ylides catalyzed by a chiral copper/Tol-BINAP
complex. A variety of synthetically and biologically
important dihydropyrazoles have been obtained with
high enantioselectivities (up to 97:3 er) in good yields
(83−97%).
structurally diverse five-membered carbo- and heterocyclic
systems.11 In this context, ylides were identified as versatile 1,1′-
dipolar synthons that reacted with a variety of electron-deficient
conjugated components, affording multifunctionalized carbo-/
heterocyclic motifs.12 For example, Tang et al. reported formal
[4+1] annulations between cinchonidine-derived ammonium
salts and nitroolefins, leading to optically active isoxazoline N-
oxides with excellent diastereo- and enantioselectivities,12e and
Xiao and co-workers elegantly used an axial-to-central chirality
transfer strategy to stereoselectively react stable sulfur ylides
with ester-bearing unsaturated imines and nitroolefins.13 For
the current report it is also noteworthy that azoalkenes, which
can be readily formed by two-electron oxidation of α-halo-N-
sulfonyl hydrazones, are highly susceptible to conjugate
addition to give the corresponding α-functionalized hydra-
zones.14 In the light of all of those findings, we wondered about
the possibility of formal asymmetric [4+1] cycloadditions of in
situ-generated azoalkenes and sulfur ylides providing optically
active dihydropyrazoles under chiral Lewis acid catalysis
(Scheme 1). In this scenario, several challenges had to be
ihydropyrazoles are important five-membered aza-hetero-
Dcycles, which are present in a wide range of bioactive
compounds with anti-depressant, anti-cancer, anti-inflamma-
tory, anti-bacterial, and anti-viral activities.1,2 Moreover,
functionalized dihydropyrazoles are also of significance for
the preparation of natural products and applications in
asymmetric synthesis. As a result, these aza-heterocycles have
become attracting synthetic targets for the development of new
chemical reactions. In 2000, Kanemasa and Kanai reported the
first enantioselective 1,3-dipolar cycloaddition reactions3 of
diazoalkanes with acrylamides using a magnesium complex as
Lewis acid catalyst.4 Since this pioneering work, catalytic
asymmetric [3+2] cycloadditions of diazoalkanes,5 azomethine
imines,6 nitrile imine dipole precursors,7 and hydrazones8 have
been established as the most prominent strategies for the
synthesis of optically active dihydropyrazole derivatives.9
Scheme 1. Strategy for the Synthesis of Optically Active
Dihydropyrazoles
Recently, Muller and List reported an alternative method
̈
involving phosphoric acids-catalyzed asymmetric 6π electro-
cyclizations of α,β-unsaturated hydrazones to give dihydropyr-
azolines in high yields and enantioselectivities.10a Furthermore,
encountered: (1) the conditions for the generation of the
reactive azoalkene should not interfere with the following
asymmetric cycloaddition; (2) the Lewis acids should
preferentially coordinate with the azoalkene over the sulfur
ylide; (3) possible background reactions could be detrimental
to the enantioselectivity.15 The methodological difficulties were
mostly expressed by the lack of examples of catalytic
asymmetric cycloadditions of sulfur ylides leading to five-
membered carbo-/heterocyclic systems.16 Herein, we report a
̀
Briere and co-workers described a powerful domino aza-
Michael addition/cyclocondensation reaction for the enantio-
selective synthesis of 3,5-diaryldihydropyrazoles by phase-
transfer catalysis.10b While these works stand out as pioneering
efforts, wide applications of these methods are impeded by
drawbacks such as unsatisfactory yields, poor chemo- and/or
stereoselectivities, and limited substrate scope. Therefore, the
development of more general strategies for the construction of
enantioenriched dihydropyrazole derivatives with functional
diversity is still highly desirable.
The formal [4+1] cycloaddition of 1,3-conjugated systems
and two-electron, one-carbon synthons has been proven as an
Received: February 6, 2012
Published: April 13, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
a
successful introduction of such a strategy using a chiral copper
complex for catalytic asymmetric formal [4+1] cycloadditions,
providing dihydropyrazoles with high enantioselectivities in
good yields (Scheme 1).
Table 2. Scope of the Sulfur Ylides
Based on our previous work on sulfoximine chemistry,17,18
we initiated the study by investigating the reaction between N-
Boc hydrazone 1a and sulfur ylide 2a in the presence of the
chiral complex formed in situ from sulfoximine L118 and
copper(II) triflate. To our delight, the desired cycloaddition
occurred, giving the corresponding product 3aa in 80% yield,
albeit the er was only 58:42 (Table 1, entry 1).19 Subsequent
b
entry
1
R2 (substrate 2)
3
yield (%)
er
1
2
3
4
5
6
7
1d
1d
1d
1d
1d
1d
1d
1e
1e
1e
1e
1e
1e
1e
1e
1e
1e
1e
1e
Ph (2a)
3da
3db
3dc
3dd
3de
3df
3dg
3ea
3eb
3ec
3ed
3ee
3ef
89
91:9
91:9 (98:2)
c
4-BrC6H4 (2b)
4-ClC6H4 (2c)
3-NO2C6H4 (2d)
4-CNC6H4 (2e)
3-MeOC6H4 (2f)
2-naphthyl (2g)
Ph (2a)
89 (80)
85 (76)
92
c
90:10 (98:2)
92:8
Table 1. Asymmetric Formal [4+1] Cycloaddition of in Situ-
87
92:8
a
Derived Azoalkenes 1 with Sulfur Ylide 2a
95
90:10
90:10
97:3
81
d
8
84
9
4-BrC6H4 (2b)
4-ClC6H4 (2c)
3-NO2C6H4 (2d)
4-CNC6H4 (2e)
3-MeOC6H4 (2f)
2-naphthyl (2g)
4-FC6H4 (2h)
3,4-Cl2C6H3 (2i)
4-MeC6H4 (2j)
2-furyl (2k)
91
97:3
10
93
96:4
e
11
84
92:8
e
12
88
94:6
13
14
15
95
92:8
3eg
3eh
3ei
92
95:5
93
96:4
d
16
94
92:8
b
entry
1
metal salt
ligand
t (h)
yield (%)
er
d
17
3ej
92
95:5
d
1
2
3
4
5
6
1a
1a
1b
1c
1d
1d
1d
1d
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
L1
L2
L2
L2
L2
L3
L3
L3
1
1
80
80
85
72
90
80
89
83
58:42
66:34
55:45
55:45
73:27
77:23
91:9
18
3ek
3el
85
93:7
d
19
2-thienoyl (2l)
88
94:6
1
a
Reaction conditions: 1d or 1e (0.3 mmol, 1.0 equiv), 2 (0.45 mmol,
1.5 equiv), Cu(OTf)2 (10 mol %), Tol-BINAP (11 mol %), Na2CO3
(1.0 equiv), and THF (10 mL) under Ar at −20 °C. Determined by
HPLC using a chiral stationary phase. Values in parentheses are the
1
b
2
c
2
c
d
7
cd
,
17
36
results after single recrystallizations. With 2.0 equiv of the sulfur ylide.
e
8
92:8
Performed at −15 °C.
a
Reaction conditions: 1 (0.3 mmol, 1.0 equiv), 2a (0.45 mmol, 1.5
equiv), metal/ligand (10 mol %), Na2CO3 (0.5 equiv), THF (10 mL)
N-benzoyl hydrazone 1d, various electron-poor and -rich sulfur
ylides with different substitution patterns on the aromatic ring
reacted smoothly, giving the corresponding cyclized products in
high yields (85−95%, entries 2−6). The best er ratio was 92:8
(entry 4 and 5). All products were solids, and the er values
could be significantly increased (to 98:2) by a single
recrystallization (entries 2 and 3). This initial screening also
revealed that the presence of a 2-methoxybenzoyl group at N1
of the hydrazone (as in 1e) substantially improved the
enantioselectivity (entry 8 vs entry 1, er ratio of 97:3 vs
91:9). Accordingly, the investigation of the sulfur ylide scope
was continued using hydrazone 1e as substrate (entries 8−19).
Also in this case, both electron-withdrawing and -donating
groups at the para- and meta-positions of the phenyl ring were
well tolerated. Compared to the previous results with 1d, use of
1e as substrate generally led to better enantioselectivities (with
er values ranging from 92:8 to 97:3, entries 8−14). Moreover,
heterocycle-derived ylides 2k and 2l readily participated in this
transformation, giving rise to products 3ek and 3el with er
values of 93:7 and 94:6, respectively (entries 18 and 19).
The relationship between the absolute configurations of the
ligand and a product was unambiguously determined by X-ray
crystal structure analysis of product (S)-3db (Figure 1)
stemming from a copper catalysis with (R)-Tol-BINAP as
ligand.20
b
under Ar. Determined by HPLC using a chiral stationary phase.
c
Performed with 1.0 equiv of Na2CO3 and 11 mol % of the ligand at
d
−20 °C. As in footnote c, but performed at −30 °C instead of −20
°C.
ligand screening revealed that the use of BINAP increased the
er to 66:34 (entry 2). With the goal to induce a more effective
stereochemical control by promoting interactions between the
azoalkene intermediate and the catalyst, the R substituent at the
hydrazone acyl group was varied.8 As hypothesized, the nature
of this group had a significant impact on the enantioselectivity
(Table 1, entries 3−5). Notably, the er of the product increased
to 73:27 when benzoyl hydrazone 1d was employed (entry 5).
With this substrate, various other ligands were tested.
Gratifyingly, the use of Tol-BINAP (L3) afforded the product
with an er of 77:23 in 80% yield. Changing the metal salt as
well as applying other bisphosphines, P,N-ligands, and
sulfoximines gave inferior results (for details see Supporting
Information). An improvement was possible by optimizing the
reaction conditions, and finally [with 10 mol % of Cu(OTf)2,
11 mol % of ligand L3, and 1.0 equiv of Na2CO3 at −30 °C],
the product was obtained with an er of 92:8 in 83% yield
(Table 1, entry 8). For the subsequent investigations a
temperature of −20 °C was chosen, which allowed us to
shorten the reaction time from 36 to 17 h without significantly
affecting the er (91:9 for 3da, entry 7).
The catalytic asymmetric formal [4+1] cycloaddition was
then extended to other hydrazones. As shown in Table 3, an
array of α-chloro- and α-bromo N-benzoyl hydrazones reacted
well, and generally high yields and good enantioselectivities
Next, the substrate generality with respect to the sulfur ylides
was investigated. The results are summarized in Table 2. With
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Journal of the American Chemical Society
Communication
the er values of the products were only moderate (entries 12
and 13). The less bulky hydrazones 1s and 1t gave the
corresponding products with er values of 87:13 in up to 93%
yield (entries 14 and 15). Interestingly, in the case of ester-
substituted hydrazone 1u, the reaction worked very well,
providing product 3ua with 93:7 er in 93% yield (entry 16).
Moreover, the alkenyl-substituted hydrazone 1v proved to be
suitable, and the corresponding product 3va was isolated in
good yield (entry 17).
Figure 1. ORTEP diagram of the X-ray crystal structure of 3db. Platon
plot of 3db (100 K) with thermal ellipsoids at the 50% probability
level. Hydrogen atoms are omitted for clarity.
To demonstrate the synthetic potential of the method, the
reaction of hydrazone 1e and sulfur ylide 2a was carried out on
a gram scale (Scheme 2). To our delight, the catalyst loading
a
Table 3. Scope of the Hydrazones
Scheme 2. Gram Scale Experiment
b
entry
X, R1, R2
1
3
yield (%)
er
1
2
3
4
5
6
7
8
Cl, H, 4-ClC6H4
1f
3fa
94
97
95
84
92
88
96
86
83
88
97
90
93
84
93
93
92
91:9
could be reduced to 5 mol % of copper salt combined with 5.5
mol % of ligand, and the corresponding product (3ea) was
isolated in 92% yield having an er of 96:4.
Cl, H, 4-FC6H4
1g
1h
1i
3ga
3ha
3ia
90:10
90:10
93:7
Br, H, 4-MeC6H4
Cl, 2-MeO, 2,4-Cl2C6H3
Cl, 2-MeO, 4-FC6H4
Br, 2-MeO, 4-BrC6H4
Cl, 2-MeO, 4-ClC6H4
Br, 2-MeO, 4-MeOC6H4
Cl, 3-OMe, Ph
In summary, we have developed a copper-catalyzed
asymmetric formal [4+1] cycloaddition of in situ-generated
azoalkenes with sulfur ylides. It provides an efficient,
enantioselective access to a variety of optically active
dihydropyrazoles. To the best of our knowledge, the current
transformation represents the first example of its kind. Further
expansion of the reaction scope and mechanistic studies are
underway.
1j
3ja
94:6
1k
1l
3ka
3la
88:12
88:12
92:8
1m
1n
1o
1p
1q
1r
1s
1t
3ma
3na
3oa
3pa
3qa
3ra
3sa
3ta
c
9
92:8
c
10
Cl, 4-OMe, Ph
91:9
c
11
Cl, 4-Me, Ph
90:10
77:23
79:21
87:13
87:13
93:7
d
12
Cl, 2-OMe, Me
e
13
Cl, 2-OMe, t-Bu
ASSOCIATED CONTENT
* Supporting Information
■
f
14
Br, 2-OMe, i-Bu
S
f
15
Cl, 2-OMe, PhCH2CH2
Br, 2-OMe, CO2Et
Cl, 2-OMe, Ph-CHCH
Experimental procedures, characterization data, and crystallo-
graphic data (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
f
16
1u
1v
3ua
3va
g
17
71:29
a
Reaction conditions: 1 (0.3 mmol, 1.0 equiv), 2a (0.45 mmol, 1.5
equiv), Cu(OTf)2 (10 mol %), Tol-BINAP (11 mol %), Na2CO3 (1.0
AUTHOR INFORMATION
Corresponding Author
carsten.bolm@oc.rwth-aachen.de
■
b
equiv), and THF (10 mL) under Ar at −20 °C. Determined by
c
HPLC using a chiral stationary phase. Performed with 2.0 equiv of 2a
d
e
at −15 °C. Performed at 0 °C. Performed with 3.0 equiv of 2a at
f
g
Notes
−10 °C. Performed with 2.5 equiv of 2a at −40 °C. Performed with
2.5 equiv of 2a at −30 °C.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
were achieved. For example, in the case of hydrazones 1f−1h
bearing Cl, F, and Me in the para-position of aryl ring, the
corresponding products were obtained in 94−97% yield with er
values of up to 91:9 (entries 1−3). Also methoxybenzoyl
hydrazones 1i−1m with both electron-withdrawing and
-donating groups on the phenyl ring reacted efficiently with
sulfur ylide 2a affording the corresponding products in up to
96% yield and er values of up to 94:6 (entries 4−8). Because it
is known that the group at N1 can significantly affect the
biological activities of dihydropyrazoles,2 a few substrates with
different substitution patterns at the N1 benzoyl group were
applied. Gratifyingly, the reactions of hydrazones 1n−1p having
meta- or para-substituents at the phenyl ring proceeded
smoothly, providing the products in high yields and with
good enantioselectivities (entries 9−11). Notably, the reaction
could also be realized for aliphatic hydrazones. For example,
hydrazones 1q and 1r, derived from chloroacetone and 1-
chloropinacolone, respectively, could be employed, although
This study was supported by Fonds der Chemischen Industrie
and the National Natural Science Foundation of China (NO.
21002036). J.-R.C. and M.C. are grateful to the Alexander von
Humboldt Foundation for postdoctoral fellowships. W.-R.D.
and F.-F.P. are thankful to the China Scholarship Council for
doctoral stipends.
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