Diastereoselective Trapping of Transient Carboxylic Oxonium Ylides with α,β-Unsaturated 2-Acyl Imidazoles

Article abstract of DOI:10.1002/adsc.202000701

By developing a diastereoselective reaction of cyclopropene carboxylic acids with α,β-unsaturated 2-acyl imidazoles, we reported here a Michael-type trapping of transient carboxylic oxonium ylides. This transformation provides a direct approach for the construction of valuable γ-butenolide derivatives in good yields (60–99%) with high diastereoselectivities (up to >95:5 dr) under mild reaction conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.202000701

Title: Diastereoselective Trapping of Transient Carboxylic Oxonium
Ylides with α,β-Unsaturated 2-Acyl Imidazoles
Authors: Mengchu Zhang, Tianyuan Zhang, Dan Zhang, and Wenhao
Hu
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10.1002/adsc.202000701
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: xxxxxx((will be filled in by the editorial staff))
Diastereoselective Trapping of Transient Carboxylic Oxonium Ylides with
α,β-Unsaturated 2-Acyl Imidazoles
Mengchu Zhang,a Tianyuan Zhang,a Dan Zhang a,* and Wenhao Hua,*
a
Guangdong Key Laboratory of Chiral Molecule and Drug Discovery, School of Pharmaceutical Sciences, Sun Yat-Sen
University, Guangzhou 510006, China
E-mail: zhangdan5@mail.sysu.edu.cn; huwh9@mail.sysu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract: By developing a diastereoselective reaction of
cyclopropene carboxylic acids with α,β-unsaturated 2-
acyl imidazoles, we reported here a Michael-type
trapping of transient carboxylic oxonium ylides. This
transformation provides a direct approach for the
construction of valuable γ-butenolide derivatives in
good yields (60-99%) with high diastereoselectivities
(up to >95:5 dr) under mild reaction conditions.
introduction of an acyloxy moiety or construction of
lactone skeletons.
Recently, we demonstrated that transient protic
oxonium ylides (Scheme 1b, B), formed from metal
carbenes and protic heteroatoms, could be trapped by
various electrophiles.[4] In this field, a wide range of
novel reactions have been developed by our group[5]
and others,[6] which has proven to be an efficient
methodology for constructing molecular complexity
and diversity in atom- and step-economic fashions.
However, compared to the well-studied trapping
process of protic ammonium and oxonium ylides, the
interception of carboxylic oxonium ylides remains
largely undeveloped.[7] This may be attributed to the
higher acidity of carboxylic acids that would promote
the proton transfer process and compromise the
stability of this intermediates.[3c]
To facilitate the trapping process, the development
of a new strategy to generate stabilized carboxylic
oxonium ylides should be a key approach. As
emerging synthons, 3-carbonylcyclopropenes have
been used for the synthesis multisubstituted O-
heterocycles.[8] Typically, transition metal-induced
cycloisomerization reactions of cyclopropene
carboxylates readily give rise to 2-alkoxyfurans, in
which the carbon-carbon double bond conjugated
cyclic carbonyl ylides were proposed as the key
intermediate (Scheme 1c, path c).[8-9] While the use of
cyclopropene carboxylic acids as substrates could
result in γ-butenolides.[10] Recently, we revealed that
cyclopropene carboxylic acids could readily form
cyclic carboxylic oxonium ylides, which displayed
higher stability than those generated from diazo
compounds and carboxylic acids (A in Scheme 1) due
to stabilizing effect from conjugated C=C moiety, and
thus could be trapped by electrophiles with reactive
C=X (X = O, NAr) functionality (Scheme 1c, path d)
via 1,2-addition.[11] Herein, we reported the first
conjugate addition of carboxylic oxonium ylides
Keywords: Ylides; Carbenes; Conjugate addition;
Green
chemistry;
Cyclopropene;
γ-Butenolide
derivatives
Transition-metal-catalyzed reactions of carbenes,
such as insertion and cycloaddition reactions, play a
pivotal role in carbon-carbon and carbon-heteroatom
bond formation.[1] Transient onium ylides, in situ
generated from reaction of a metal carbene with a
heteroatom in the presence of transition-metal
catalysts, are generally considered as the key
intermediates in the carbon-heteroatom bond
formation process.[1a][1f] Interception of these
intermediates could lead to many useful
transformations. In this field, carbonyl ylides, one of
the most common and important species generated by
the interaction of carbenes with the oxygen atom of
carbonyl compounds (aldehydes, ketones, esters, and
amides), could be trapped by a diverse set of
dipolarophiles via 1,3-dipolar cycloaddition reactions.
This well-developed transformations afford powerful
approaches for the construction of a variety of highly
functionalized compounds (Scheme 1a, path a).[2] On
the other hand, when using carboxylic acids as the
carbonyl nucleophiles, the reaction would result in
carboxylic oxonium ylides (Scheme 1b, A), which
exhibit distinctly different reactivity and could
undergo a rapid proton transfer rather than 1,3-dipolar
cycloaddition (Scheme 1a, path b).[3] Interception of
the carboxylic oxonium ylides with electrophiles
would be a desirable task that could offer a direct
1
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10.1002/adsc.202000701
Advanced Synthesis & Catalysis
Scheme 1. The chemistry of carbonyl ylides.
Table 1. Optimization of the reaction conditions.a
M-cat. (x)
Solvent
T/℃
t/h
72
Yield/%b
dr c
/
Entry
1
JohnPhosAu(MeCN)SbF6 (5)
CH2Cl2
25
0
AgOTf (10)
Cu(OTf)2 (10)
Rh2(OAc)4 (5)
Rh2(esp)2 (2)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
25
25
25
25
72
72
72
72
0
8
17
82
/
2
3
4
5
>95:5
91:9
>95:5
Rh2(esp)2 (2)
Rh2(esp)2 (2)
DCE
PhCl
25
25
72
72
81
88
>95:5
>95:5
6
7
Rh2(esp)2 (2)
Rh2(esp)2 (2)
Rh2(esp)2 (2)
Rh2(esp)2 (2)
Rh2(esp)2 (2)
THF
25
25
25
40
40
72
72
72
6
27
76
87
>95:5
>95:5
>95:5
>95:5
>95:5
8
9
10
11
12e
EtOAc
toluene
toluene
toluene
90 (89d)
78
6
a Unless otherwise indicated, reaction conditions: 1a (0.15 mmol), 2a (0.10 mmol), metal catalyst, solvent (1.5 mL). dr: diastereomeric
ratio; JohnPhos: di-tert-butyl-(2-phenylphenyl)phosphane; Rh2(esp)2: bis[rhodium(α,α,α′,α′-tetramethyl-1,3-benzenedipropionic acid)];
DCE: 1,2-dichloroethane; THF: tetrahydrofuran.
b The yields were determined by crude 1H NMR using 1,3,5-trimethoxylbenzene as the internal standard.
c Anti/syn; the dr values were determined by 1H NMR spectroscopy of the crude reaction mixture.
d Isolated yield.
e 0.12 mmol 1a.
2
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10.1002/adsc.202000701
Advanced Synthesis & Catalysis
Subsequently, with using Rh2(esp)2 as catalyst, the
reaction was evaluated in a variety of solvents (entries
6-10). The less toxic toluene was chosen for further
optimizations. Since the reaction required long time
(72 hours), to enhance the reactivity, the reaction was
with electron-deficient olefins (Scheme 1c, path d),
which affords direct construction of γ-butenolide
derivatives, the privileged structures shared in
numerous pharmaceuticals and natural products.[12]
We commenced the study with the reaction of
cyclopropene carboxylic acid (1a) and α,β-
unsaturated 2-acyl imidazole (2a). In the initial
attempt, some frequently used metal catalysts in
o
warmed to 40 C. The results indicated that reaction
time could be reduced down to 6 hours and 3aa could
be obtained in 90% (entry 11). In addition, a reduced
stoichiometry in cyclopropene carboxylic acid 1a
resulted in a lower yield (entry 12). Therefore, the
conditions listed in Table 1, entry 11 were selected as
the optimal ones.
cyclopropene
chemistry,[13]
including
JohnPhosAu(MeCN)SbF6, Cu(OTf)2, and AgOTf,
were tested to promote the reaction, but almost no
coupling reaction took place or very low conversion
was observed (Table 1, entries 1-3). When we turned
to rhodium catalysts,[14] the results revealed that
Rh2(OAc)4 provided desired product 3aa in 17% yield,
while Rh2(esp)2 significantly improved the yield to
82% and excellent diastereoselectivity (entries 4-5).
With the optimal reaction conditions in hand, the
scope of α,β-unsaturated 2-acyl imidazoles was firstly
investigated (Scheme 2a). We were delighted to
observe that halide substituents (F, Cl, Br, I) at para-
1
Scheme 2. Scope of substrates. Isolated yields are provided. The dr values were determined by H NMR spectroscopy
referring to the crude reaction mixture. [a] 3.0 equiv. of 1a was used.
3
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10.1002/adsc.202000701
Advanced Synthesis & Catalysis
position (3aa-3ad), meta-position (3ae-3af), and
We then investigated the catalytic asymmetric
ortho-position (3ag-3ah) were well tolerated and the reaction of 1a and 2a by using several chiral rhodium
corresponding trapping products were obtained in catalysts. As outlined in Scheme 4, Rh2(S-PTTL)4
good to high yields (61-89%). In the case of could afford 3aa in 97% yield, while Rh2(S-NTTL)4
substrates containing a strong electron-withdrawing and Rh2(S-DOSP)4 were less effective and low yields
group (EWG; CN, NO2) at para-position, the of desired product were obtained. All chiral rhodium
corresponding products (3ai-3aj) could be isolated in catalysts tested gave high diastereoselectivities, but
excellent yields (96-98%). When switching the resulted in racemic products. These results suggested
substituent from the electron-withdrawing groups to
the electron-donating one (EDG; MeO), only
moderate yield (38%) of product (3ak) was isolated
owing to the lower reactivity of substrate, but the
yield could be enhanced to 92% by increasing the
amount of 1a to 3.0 equiv. Similarly, other substrates
without an EWG also could lead to corresponding
products (3al-3ao) in high yields (up to 99%) by this
modification of reaction conditions. Furthermore,
heterocyclic substrates also worked well and provided
excellent results (3ap-3aq). Notably, high
diastereoselectivities (>95:5 dr) were obtained for
most substrates, and the relative configuration was
determined by X-Ray single crystal analysis of
3aa.[15]
The generality of cyclopropene carboxylic acids
was then assessed under the reaction conditions
(Scheme 2b). Substrates bearing both EDGs and
EWGs at m- and p-positions proceeded smoothly in
the reaction and afforded substituted γ-butenolide
derivatives (3ba-3bj) in good to excellent yields with
high diastereoselectivities (>95:5 dr for most cases).
As a limitation, the cyclopropene carboxylic acids
with an ortho-bromide phenyl or a benzyl group
didn’t work and no reaction took place, probably due
to the steric hindrance and low reactivity, respectively.
To illustrate the synthetic utility of this
transformation, a gram-scale synthesis was performed
with a reduced loading of Rh2(esp)2 (1.0 mol%),
which provided 3ai in 91% yield (1.09 g) with 94:6 dr
(Scheme 3). Furthermore, the imidazole moiety could
be removed and transformed into methyl ester under
the conditions of MeOTf/MeCN and MeOH/DBU,
sequentially, in 87% yield.
Scheme 4. Attempted asymmetric reactions. The yields
were determined by crude 1H NMR using 1,3,5-
trimethoxylbenzene as the internal standard; the dr value
1
was determined by H NMR spectroscopy of the crude
reaction mixture; the ee value was determined by HPLC
analysis using a chiral stationary phase. [a] 4 mol%
Rh2(esp)2 was used to activate the cyclopropenes.
that the rhodium catalysts are not involved in the step
of nucleophilic addition to α,β-unsaturated 2-acyl
imidazole, and the non-metal associated ylide
intermediate is more likely. Alternatively, We also
tried the asymmetric reaction using in situ-generated
chiral Lewis acids to activate α,β-unsaturated 2-acyl
imidazole,[16] including chiral bisoxazoline/Zn(OTf)2
complex
(Box-Zn),
chiral
pyridine-2,6-
bisoxazolines/Zn(OTf)2 complex (PyBox-Zn), and
chiral N,N'-dioxide/Sc(OTf)3 complex, but the
strategy also failed to induce asymmetric reaction.[17]
Moreover, to further explore the reaction pathway,
we carried out the control experiment using
preformed γ-butenolide as nucleophile (Scheme 5).
Firstly, in the absence of electrophile 2a, Rh2(esp)2
could transform cyclopropene carboxylic acid 1a into
insertion product 6 in 40% yield as an off-white solid.
Scheme 3. Gram-scale synthesis and transformation of
product 3ai. The dr value was determined by H NMR
spectroscopy of crude mixture.
1
4
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10.1002/adsc.202000701
Advanced Synthesis & Catalysis
When 6 and 2a were subjected to the reaction synthesis is also performed and the imidazole moiety
conditions, 3aa was not observed, and both 6 and 2a of the product could be removed. Importantly, this
stayed unreacted. The result indicated that 6 was not study presents the first trapping of carboxylic
the reactive nucleophile, and an active intermediate, oxonium ylides via Michael addition, and provides a
such as carboxylic oxonium ylide, was the more new approach for the development of new
likely one.
transformations via interception of transient
carboxylic oxonium ylides.
Experimental Section
Experimental procedure for the synthesis of 3aa: 1-
phenylcycloprop-2-ene-1-carboxylic acid 1a (48 mg, 0.30
mmol, 1.5 equiv.), (E)-3-(4-bromophenyl)-1-(1-methyl-1H-
imidazol-2-yl)prop-2-en-1-one 2a (58.2 mg, 0.20 mmol,
1.0 equiv.) and Rh2(esp)2 (3.0 mg, 2.0 mol%) were loaded
in an oven-dried test tube with a stirring bar. Toluene (3.0
o
mL) was then added, and the mixture was stirred at 40 C
(oil bath). Upon completion of the reaction monitored by
TLC (about 6 h), the mixture was concentrated to give a
1
Scheme 5. Control experiment.
residue which was subjected to H NMR spectroscopy for
the determination of diastereoselectivity (dr value).
Purification of the crude products by flash chromatography
on silica gel (eluent: petroleum ether/EtOAc = 10/1~3/1)
afforded (S*)-5-((S*)-1-(4-bromophenyl)-3-(1-methyl-1H-
imidazol-2-yl)-3-oxopropyl)-3-phenylfuran-2(5H)-one
(3aa) as a white solid (79.8 mg, 89% yield).
According to the control reaction and previous
studies on 3-carbonylcyclopropenes[8-11] as well as the
ylide trapping process,[5] a reaction mechanism is
outlined in Scheme 6. Rhodium-induced ring-opening
of cyclopropenes 1 generates electrophilic vinyl
carbene C, followed by intramolecular attack by the
tethered carboxylic group to give cyclic carboxylic
oxonium ylides D. Release of the rhodium catalyst
will lead to the non-metal associated ylide E and its
resonance structure F, which is captured by the
electron-deficient olefins 2 via Michael addition to
result in the γ-alkyl γ-butenolides 3.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 21901259), Guangdong Basic
and Applied Basic Research Foundation (No.
2020A1515011116), and the Program for Guangdong
Introducing Innovative and Entrepreneurial Teams (No.
2016ZT06Y337).
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In summary, we have developed a rhodium-
catalyzed trapping process of transient carboxylic
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6
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Advanced Synthesis & Catalysis
COMMUNICATION
Diastereoselective Trapping of Transient
Carboxylic Oxonium Ylides with α,β-
Unsaturated 2-Acyl Imidazoles
Adv. Synth. Catal. Year, Volume, Page – Page
Mengchu Zhang, Tianyuan Zhang, Dan Zhang* and
Wenhao Hu*
7
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