Asymmetric Synthesis of Spiropyrazolones by Rhodium-Catalyzed C(sp2)?H Functionalization/Annulation Reactions

Article abstract of DOI:10.1002/anie.201700021

Rhodium-catalyzed C(sp²)?H functionalization reactions of 4-aryl-5-pyrazolones followed by [3+2] annulation reactions with alkynes provide rapid access to highly enantioenriched five-membered-ring 4-spiro-5-pyrazolones. The use of a chiral SCpRh catalyst enabled the synthesis of a large range of spiropyrazolones with all-carbon quaternary stereogenic centers in up to 99 % yield and 98 % ee from readily available substrates.

Full text of DOI:10.1002/anie.201700021

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700021
German Edition: DOI: 10.1002/ange.201700021
Rhodium Catalysis
Asymmetric Synthesis of Spiropyrazolones by Rhodium-Catalyzed
2
À
C(sp ) H Functionalization/Annulation Reactions
Jun Zheng, Shao-Bo Wang, Chao Zheng, and Shu-Li You*
2
À
Abstract: Rhodium-catalyzed C(sp ) H functionalization
reactions of 4-aryl-5-pyrazolones followed by [3+2] annula-
tion reactions with alkynes provide rapid access to highly
enantioenriched five-membered-ring 4-spiro-5-pyrazolones.
The use of a chiral SCpRh catalyst enabled the synthesis of
a large range of spiropyrazolones with all-carbon quaternary
stereogenic centers in up to 99% yield and 98% ee from
readily available substrates.
ring have been published. In 2014, Lu and co-workers
developed an elegant phosphine-catalyzed [4+1] annulation
for the asymmetric synthesis of spiropyrazolones (Sche-
me 1a).[4] Later, Enders and co-workers reported a highly
efficient one-pot enantioselective synthesis of spiropyrazo-
lones by sequential organo- and silver catalysis (Sche-
me 1b).[5] Very recently, the same group also synthesized
spiropyrazolones by NHC-catalyzed asymmetric [3+2] annu-
lation reactions (Scheme 1c).[6]
P
yrazolones and their derivatives are important molecular
scaffolds that widely occur in natural products, biologically
active molecules, and pharmaceutical agents.[1] 4-Spiro-5-
pyrazolones and pyrazole-5-thiones have been shown to
possess valuable biological properties, as exemplified by
their miticidal activity and their use as PPARa antagonists
and inhibitors of type-4 phosphodiesterases (Figure 1).[2]
Figure 1. Selected bioactive spiropyrazolones.
Scheme 1. Asymmetric synthesis of five-membered-ring 4-spiro-5-pyra-
zolones. Bn=benzyl.
Therefore, the development of methods that provide efficient
access to these core structures has been the subject of
intensive research. Over the last several years, many elegant
studies have been published for the asymmetric synthesis of
4-spiro-5-pyrazolones bearing a six-membered ring.[3] How-
ever, only a handful of reports on the efficient synthesis of
optically active 4-spiro-5-pyrazolones with a five-membered
À
In recent years, the direct functionalization of inert C H
bonds has been recognized as an effective way to construct
stereogenic centers owing to the rapid development of
suitable chiral catalysts and ligands.[7] In this regard, transi-
tion-metal complexes supported by chiral cyclopentadienyl
(Cp) ligands have been established as powerful and robust
[*] Dr. J. Zheng, S.-B. Wang, Prof. Dr. C. Zheng, Prof. Dr. S.-L. You
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
University of Chinese Academy of Sciences
Chinese Academy of Sciences
À
catalysts for asymmetric C H functionalization since the
introduction of a biotinylated [Cp*RhIII] complex by the
groups of Ward and Rovis as well as C2-symmetric chiral
CpxRh complexes by Cramer and co-workers.[8] The
C2-symmetric chiral CpxRh complexes, in particular, have
found broad applications in asymmetric catalysis.[9] Notably,
Lam and co-workers reported an elegant RhIII-catalyzed
345 Lingling Lu, Shanghai 200032 (China)
E-mail: slyou@sioc.ac.cn
Homepage: http://shuliyou.sioc.ac.cn/
Prof. Dr. S.-L. You
Collaborative Innovation Center of Chemical Science and Engineer-
ing (Tianjin), Tianjin 300072 (China)
À
asymmetric synthesis of spiroindenes by C H functionaliza-
tion and spiroannulation with alkynes.[10] Very recently, we
reported the synthesis of a new class of chiral Cp ligands
(SCp) based on the privileged 1,1’-spirobiindane scaffold, and
their rhodium complexes (SCpRh) were found to be excellent
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700021.
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À
catalysts for the synthesis of axially chiral biaryl com-
We then examined the generality of this C H function-
alization/annulation reaction under the optimized reaction
conditions (Table 1, entry 8). First, the reactions of various
pyrazol-5-ones (1) were investigated using 1,2-diphenyl-
ethyne (2a) as the reaction partner (Table 2). In general,
pounds.[11] In line with our interest in asymmetric C H
À
functionalization processes, we envisioned that five-mem-
bered-ring 4-spiro-5-pyrazolones could be obtained in an
À
enantioselective fashion by Rh-catalyzed C H functionaliza-
tion of 4-aryl-5-pyrazolones and subsequent annulation with
alkynes (Scheme 1d). Herein, we report the results of this
study.
Table 2: Substrate scope with substituted pyrazol-5-ones.[a]
Our investigations began with the optimization of the
reaction conditions with 1-cyclohexyl-3-methyl-4-phenyl-1H-
pyrazol-5(4H)-one (1a) and 1,2-diphenylethyne (2a) as the
model substrates (Table 1). The proposed reaction proceeded
Table 1: Optimization of the reaction conditions.[a]
Entry
Catalyst
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
(R)-K1
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMF
80
99
99
95
75
97
98
99
74
99
94
96
95
90
88
96
95
97
97
96
(S)-K2a
(S)-K2b
(S)-K2c
(S)-K2d
(S)-K2a
(S)-K2a
(S)-K2a
(S)-K2a
(R)-K1
toluene
tAmylOH
tAmylOH
tAmylOH
[a] Reaction conditions: 1 (0.3 mmol), 2a (0.2 mmol), (S)-K2a
9[d]
10
t
(5 mol%), and Cu(OAc)2 (2 equiv) in AmylOH (2 mL) at 808C for 1 h,
unless otherwise noted. Yields of isolated products are given. The
ee values were determined by HPLC analysis on a chiral stationary phase.
[b] On 0.1 mmol scale. [c] In 1,4-dioxane at 608C. PMB=para-methoxy-
phenyl.
[a] Reaction conditions: 1a (0.15 mmol), 2a (0.10 mmol), catalyst
(5 mol%), and Cu(OAc)2 (2 equiv) in the indicated solvent (1 mL) at
808C for 1 h, unless otherwise noted. [b] Yields of isolated products are
given. [c] Determined by HPLC analysis on a chiral stationary phase.
[d] With 2.5 mol% of (S)-K2a. Cy=cyclohexyl.
excellent enantioselectivities along with good to excellent
yields were achieved with differently substituted pyrazol-5-
ones. The substituent on the pyrazol-5-one N atom (R2) was
not limited to Cy, and substrates with tBu or Bn substituents in
this position also furnished the corresponding products in
excellent enantioselectivity (95% ee for 3ba and 96% ee for
3ca). To our delight, ethyl or carboxylic ester groups were
also tolerated as the R1 substituent, and the desired spiroin-
dene products (3da and 3ea) were obtained in 89 and 99%
yield, respectively, with 98% ee each. The introduction of
either electron-donating (3-Me or 4-OMe) or electron-with-
drawing groups (3-Cl, 4-Br, 4-F, or 4-CN) on the aryl ring was
well tolerated, and the corresponding spiro products 3 fa–3ka
were afforded in good to excellent yields (52–99%) with high
levels of enantioselectivity (86–96% ee). Notably, a 4-thienyl-
substituted pyrazol-5-one provided the desired product 3la in
96% ee, albeit in low yield. An analogous pyridine-based
smoothly in the presence of the BINOL-derived CpxRh
complex (R)-K1 (5 mol%) as the catalyst and Cu(OAc)2
(2 equiv) as the oxidant in 1,4-dioxane. The desired five-
membered-ring 4-spiro-5-pyrazolone 3aa was afforded in
reasonable yield (80%) and excellent enantiomeric excess
(94% ee; entry 1). To our delight, 3aa was obtained in
quantitative yield (99%) and excellent enantiopurity (96%
ee) when the SCpRh catalyst (S)-K2a was used (entry 2). The
use of diverse other SCpRh complexes did not afford better
results (entries 3–5). Various solvents were well tolerated, and
tAmylOH was found to give the best results (99% yield, 97%
ee; entry 8). Notably, reducing the loading of (S)-K2a from
5 mol% to 2.5 mol% led to a diminished yield of 3aa (74%;
entry 9). With tAmylOH as the solvent, (R)-K1 (5 mol%) also
gave excellent results in terms of the yield and enantioselec-
tivity (entry 10).
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substrate was well tolerated, and the corresponding product
To demonstrate the synthetic utility of our method, we
3ma was afforded in good yield and enantioselectivity.
Next, various alkynes were explored in the asymmetric
spiroannulation with 1a (Table 3). Symmetric alkynes bearing
conducted the reaction between 1a and 2a on gram scale,
which led to the smooth, quantitative formation of 3aa in
98% ee. The absolute configuration of spiroindene 3aa was
determined to be S by X-ray crystallographic analysis (see the
Supporting Information for details).[12]
Moreover, the five-membered-ring 4-spiro-5-pyrazolone
products were subjected to several further transformations
(Scheme 2). The novel spiropyrazole-5-thione 4 was synthe-
Table 3: Substrate scope with various alkynes.[a]
Scheme 2. Product transformations. DIBAL-H=diisobutylaluminum
hydride.
[a] Reaction conditions: 1a (0.3 mmol), 2 (0.2 mmol), (S)-K2a
t
(5 mol%), and Cu(OAc)2 (2 equiv) in AmylOH (2 mL) at 808C for 1 h,
sized in 99% yield by treating spiropyrazolone 3aa with
Lawessonꢀs reagent (Scheme 2). The amide group in 3aa
could be reduced with DIBAL-H to afford pyrazoline
derivative 5 in 95% yield. The indene double bond in 3aa
was hydrogenated to give compounds 6 and 6’ as a pair of
diastereomers (1:1 d.r.). Compound 7, with a free NH group,
unless otherwise noted. Yields of isolated products are given. The
ee values were determined by HPLC analysis on a chiral stationary phase.
[b] Combined yield of the two regioisomers. [c] On 0.1 mmol scale.
[d] The diastereomeric ratio (d.r.) was determined by HPLC analysis.
t
aromatic groups with various electronic properties were
tolerated well, and the corresponding spirocyclic products
(3ab–3af) were obtained in good to excellent yields (77–
99%) with excellent enantioselectivity (91–97% ee). Inter-
estingly, 5-decyne was also a suitable coupling partner,
furnishing the desired product (3ag) in 56% yield with 96%
ee. Notably, alkyl and (hetero)aryl alkynes also smoothly
participated in this transformation, providing spiroindenes
3ah–3ak in excellent yields (92–99%), very high enantiose-
lectivity (94–96% ee), and moderate to excellent regioselec-
tivity (4.3:1– > 19:1 r.r.). To our delight, ethyl 3-phenyl-
propiolate was also a viable substrate, and the corresponding
spiroindene product (3al) was isolated in good yield and
enantio- and regioselectivity (65% yield, 83% ee, > 19:1 r.r.),
which further increased the product diversity. Moreover, an
alkyne-substituted estrone derivative was employed, and the
corresponding product (3am) was obtained in 73% yield,
99:1 d.r., and > 19:1 r.r., providing proof of concept that this
method can be applied for the late-stage modification of
a complex bioactive molecule.
was obtained upon removal of the Bu group in 3ba. In all
cases, the reactions proceeded without any noticeable loss of
enantiopurity.
Based on previous reports,[13] a putative catalytic cycle for
the reaction is proposed in Scheme 3 with 1a and 2a as
representative substrates. Pyrazolone A isomerizes into its
aromatic form B, whose hydroxy group is deprotonated by the
À
Rh catalyst. Subsequently, intermediate I undergoes C H
bond activation to give rhodacycle II. The eight-membered-
ring rhodacycle III,[14] which might be in equilibrium with the
six-membered-ring isomer IV, results from alkyne coordina-
tion and migratory insertion, which might be the enantiose-
lectivity-determining step according to a previous DFT study
on a related system[13a] (see the Supporting Information for
a working model for the stereochemistry). Alternatively,
p-oxaallyl rhodium intermediate V might also be involved.
Finally, reductive elimination gives the desired spiropyrazo-
lone, and the released RhI species is concomitantly oxidized
to the active RhIII catalyst by Cu(OAc)2, completing the
catalytic cycle.
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Scheme 3. Proposed catalytic cycle.
À
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In conclusion, we have described the first asymmetric
synthesis of five-membered-ring spiropyrazolones from read-
ily available pyrazolones and alkynes by a C H functional-
ization/annulation process. The use of a chiral SCpRh catalyst
provided excellent enantioselectivity, reactivity, and regiose-
lectivity in this cascade reaction. This method enables the
transformation of a wide range of substrates into highly
enantioenriched spiropyrazolones containing all-carbon qua-
ternary stereogenic centers.
À
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Acknowledgements
We thank the National Key Research and Development
Program of China (2016YFA0202900), the National Basic
Research Program of China (2015CB856600), the NSFC
(21421091 and 21332009), the Program of Shanghai Subject
Chief Scientist (16XD1404300), and the Chinese Academy of
Sciences for generous financial support.
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Conflict of interest
À
racemic C H bond functionalization/annulation reactions with
The authors declare no conflict of interest.
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Keywords: alkynes · asymmetric catalysis ·
C-H functionalization · pyrazolones · rhodium
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Manuscript received: January 2, 2017
Revised: February 27, 2017
Final Article published: && &&, &&&&
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Communications
Rhodium Catalysis
J. Zheng, S.-B. Wang, C. Zheng,
S.-L. You*
&&&& — &&&&
Asymmetric Synthesis of
Spiropyrazolones by Rhodium-Catalyzed
2
À
C(sp ) H Functionalization/Annulation
Enantioenriched five-membered-ring 4-
alkynes. The use of a chiral SCpRh
catalyst (see scheme) led to excellent
enantioselectivity, reactivity, and regiose-
lectivity in this cascade process.
Reactions
spiro-5-pyrazolones were synthesized by
2
À
the rhodium-catalyzed C(sp ) H func-
tionalization of 4-aryl-5-pyrazolones fol-
lowed by a [3+2] annulation reaction with
6
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