N-heterocyclic carbene-catalyzed formal [3+2] annulation of alkynyl aldehydes with nitrosobenzenes: A highly regioselective umpolung strategy

Article abstract of DOI:10.1021/cs400602v

N-Heterocyclic carbene-catalyzed formal [3+2] annulation of alkynyl aldehydes and nitrosobenzenes has been reported. This transformation provided the novel C-X bond formation under mild conditions in moderate to satisfactory yields. The catalytic protocol allows for a rapid construction of 2,5-disubstituted isoxazol-3(2H)-ones and 2,3-disubstituted isoxazol-5(2H)-ones from the same materials via a highly regioselectively umpolung stratgy.

Full text of DOI:10.1021/cs400602v

Letter
pubs.acs.org/acscatalysis
N‑Heterocyclic Carbene-Catalyzed Formal [3+2] Annulation of
Alkynyl Aldehydes with Nitrosobenzenes: A Highly Regioselective
Umpolung Strategy
Runfeng Han,† Jing Qi,† Jixiang Gu,† Donghui Ma,† Xingang Xie,† and Xuegong She*,†,‡
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P. R. China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: N-Heterocyclic carbene-catalyzed formal [3+2]
annulation of alkynyl aldehydes and nitrosobenzenes has been
reported. This transformation provided the novel C−X bond
formation under mild conditions in moderate to satisfactory
yields. The catalytic protocol allows for a rapid construction of
2,5-disubstituted isoxazol-3(2H)-ones and 2,3-disubstituted
isoxazol-5(2H)-ones from the same materials via a highly
regioselectively umpolung stratgy.
KEYWORDS: N-heterocyclic carbene, [3+2] annulation, regioselective, umpolung, alkynyl aldehydes, nitrosobenzenes
N-Heterocyclic carbene (NHC)-catalyzed umpolung of alde-
hydes has been investigated thoroughly over the past decade.1
For this, benzoin condensation2 and Stetter reaction3 are the
most common types. Also, umpolung of enals via a
homoenolate intermediate (a3-d3 umpolung) was another
useful umpolung strategy and had become increasingly
attractive for C−C bond4 and C−X bond5 formation, while
investigation of the a3-d3 umpolung of alkynyl aldehydes via an
allenolate intermediate was mainly focused on the β-
protonation followed by esterification6 or other transforma-
tions.7 However, it is a challenge to construct a C−C or C−X
bond via a3-d3 umpolung of alkynyl aldehydes probably because
of its low nucleophilicity. Recently, Snyder et al. and our group
reported NHC-catalyzed/Lewis acid-mediated conjugate um-
polung of alkynyl aldehydes for the synthesis of the tricyclic
framework of the securinega alkaloids8 and butenolides,9
Figure 1. Isoxazol-3(2H)-one and isoxazol-5(2H)-one skeletons in the
respectively. However, highly regioselective umpolung of
alkynyl aldehydes has rarely been investigated.
materials and biologically active compounds.
Isoxazol-3(2H)-one and isoxazol-5(2H)-one are regioiso-
meric skeletons of some of the synthetic biologically active
compounds and materials (Figure 1).10 Thus, their synthesis
attracted our attention, and we are likely to synthesize them
from aldehydes and nitrosobenzenes. This would be directly
related to the regioselectivity of alkynyl aldehydes. In the
literature, there are several excellent works about the annulation
of aldehydes and nitrosobenzenes. In 2008, Ying and co-
workers reported a NHC-catalyzed C−N bond formation via
which α,β-unsaturated aldehydes and nitrosobenzenes gave N-
phenylisoxazolidin-5-ones, followed by an acid-catalyzed
esterification and Bamberger-like rearrangement in one pot
leading to N-methoxyphenyl-protected β-amino acid esters (eq
1, Scheme 1).5a Then, Cheng and co-workers reported NHC-
catalyzed annulation of o-vinylarylaldehydes with nitrosoben-
zenes proceeded via a cascade azabenzoin and oxo-Michael
addition to produce multifunctional 2,3-benzoxazinones in
good to excellent yields (eq 2, Scheme 1).11 Recent work by
Ma’s group reported the NHC-catalyzed cyclization of β-
halopropenals and arylnitroso compounds to 2,3-disubstituted
isoxazol-5(2H)-ones (eq 3, Scheme 1).5f All of this work led to
only one type of product skeleton, and the regioselective type
Received: July 25, 2013
Revised: September 24, 2013
Published: October 23, 2013
© 2013 American Chemical Society
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Scheme 1. Representative Annulations of Nitrosobenzenes
with Aldehydes
Table 1. Screening of Reaction Conditions for the Synthesis
of 2,5-Disubstituted Isoxazol-3(2H)-one
a
b
entry catalyst
base
additive
t (min)
yield (%)
trace
c
1
A
B
C
D
E
F
G
E
E
E
E
E
E
E
E
E
E
E
E
E
DBU
−
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
60
60
60
60
60
2
DBU
−
−
−
−
−
−
−
−
−
−
−
−
not determined
3
DBU
trace
28
4
DBU
5
DBU
48
6
DBU
50
7
DBU
29
8
K2CO3
DABCO
tBuOK
Et3N
4
d
9
trace
trace
trace
trace
trace
28
10
11
12
13
14
15
16
17
18
19
20
Cs2CO3
NaOAc
e
TBD
−
DBU
DBU
DBU
DBU
DBU
DBU
CH3OH
CH3OH
EtOH
iPrOH
tBuOH
H2O
65
78
74
has not been developed. Inspired by these pioneering works, we
considered how to obtain the two regioisomers by controlling
the reaction conditions, electronic and steric effects, or other
factors. However, if such controls could be efficiently utilized, it
would become an ideal platform for the divergent synthesis of
different types of skeletons from the same reactants. Combined
with our group works, regioselective umpolung (a3-d3 and a1-
d1) of alkynyl aldehydes followed by formal [3+2] annulation
attracted our attention; however, this strategy has rarely been
reported. Herein, we disclose the novel NHC-catalyzed formal
[3+2] annulation of alkynyl aldehydes with nitrosobenzenes by
a highly regioselective umpolung strategy (eq 4, Scheme 1).
There have been excellent contributions from Bode’s group
and Smith’s group recently about catalyst selection.12 Because
the key issue of the two pathways lies in the generation of either
d1-synthon or d3-synthon from alkynyl aldehyde and NHC
catalyst, we envisioned that the steric hindrance of catalysts
could control the reaction pathways and therefore overcome
the competition problem and thus exclusively lead to either of
the desired products. Less steric NHCs could be beneficial for
the a1-d1 umpolung, while steric catalysts might be favored for
the a3-d3 umpolung (eq 4, Scheme 1). In view of the difficulty
of the two pathways, our investigation began with a1-d1
umpolung. Initial screening revealed that a catalytic amount
of base was not effective. Then 3-phenylpropiolaldehyde 1c and
4-chloronitrosobenzene 2a were treated with 10 mol %
thiazolium salts A−C and 1.5 equiv of DBU; under these
conditions, 2,5-disubstituted isoxazol-3(2H)-one 3c was
obtained in very low yield (Table 1, entries 1−3). Fortunately,
the product was isolated in 28% yield in the presence of
triazolium-based NHC precatalyst D, and other triazolium salts
E−G gave higher yield than NHC salt D (Table 1, entries 4−
7). After an extensive evaluation of other bases, we found that
the yield decreased even in the presence of economical NHC
precursor E (Table 1, entries 8−14). A further survey of other
solvents (CH2Cl2, toluene, Et2O, CH3CN, EtOAc, CHCl3,
acetone, DCE, and 1,4-dioxane) was not effective. On the basis
70
70
59
a
Reaction conditions: 1c (0.3 mmol), 2a (0.2 mmol), catalyst (10 mol
%), base (0.3 mmol, 1.5 equiv), THF (2.0 mL). Isolated yield. DBU
1,8-diazabicyclo[5.4.0]undec-7-ene. DABCO = 1,4-
diazabicyclo[2.2.2]octane. TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene.
b
c
d
=
e
of the analysis of the reaction mechanism, to facilitate the 5-
endo-dig cyclization of the intermediate, different types of
alcohols were tested as inexpensive and easily accessible
additives for the promotion of the reaction. Compared to the
reaction time and the variety and amount of alcohols, CH3OH
(5.0 equiv) was an appropriate proton source and 3c was
isolated in 78% yield (Table 1, entries 15−19). To our surprise,
the yield was only slightly smaller than that of CH3OH even
though H2O was used as an additive (Table 1, entry 20).
Finally, 10 mol % NHC precursor E, DBU as a base, and 5.0
equiv of CH3OH used as a proton source in THF were chosen
as an optimal condition for evaluating the substrate scope.
As shown in Scheme 2, a broad range of β-substituted alkynyl
aldehydes and nitrosobenzenes could readily participate in this
reaction under the optimized condition. For alkynyl aldehydes
bearing β-aryl substituents, electron-rich and unsubstituted
reactants had little influence on the reaction yields (Scheme 2,
3a−c and 3m), but electron-poor substrates gave moderate
yields (Scheme 2, 3d). The same situation was discovered in
the m-substituted phenylpropioaldehydes (Scheme 2, 3e−3g),
as well. Surprisingly, o-substituted phenylpropioaldehydes
demonstrated different results, of which both EDG- and
EWG-substituted substrates gave slightly higher yields, and it
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a
Scheme 2. Substrate Scope for the a1-d1 Umpolung
Table 2. Screening of Reaction Conditions for the Synthesis
of 2,3-Disubstituted Isoxazol-5(2H)-one
a
b
entry
catalyst
solvent
t (min)
yield (%)
1
2
H
I
J
J
J
J
J
J
J
J
J
J
THF
60
60
60
60
60
60
60
60
60
60
60
20
7
THF
9
3
THF
20
4
DCM
toluene
CH3CN
CHCl3
Et2O
7
5
15
6
13
7
mixtures
mixtures
31
8
9
DMF
DMSO
DMF
DMF
10
30
c
11
12c
42
42
a
Reaction conditions: 1c (0.3 mmol), 2a (0.2 mmol), catalyst (10 mol
b
%), base (0.3 mmol, 1.5 equiv), solvent (2.0 mL). Isolated yield.
c
When 20 mol % J was used.
a
reaction time was shortened to 20 min (Table 2, entry 12).
Unfortunately, further screening of other bases (such as K2CO3,
DABCO, Et3N, Li2CO3, Cs2CO3, TBD, NaOAc, and DIPEA)
did not further improve the yield. Additionally, other attempts
to change the loading of the catalyst, the reaction temperature,
the amount of bases, and the method as described in ref 4m to
further improve the yield proved to be futile possibly because of
the low nucleophilicity of the allenolate intermediate.
Under the optimized condition, a variety of substituted
alkynyl aldehydes and 4-chloronitrosobenzene were used for
investigating the substrate generality of this method (Scheme
3). Generally, the yields were not high and EDG-substituted 3-
phenylpropiolaldehydes were adaptive for the conversion. It is
obvious that the yield was lower than in the former case when
EDG-substituted alkynyl aldehydes were replaced by the
reverse electronic property group, thus indicating a relatively
strong electronic effect associated with the aromatic ring.
Compared to the homoenolate intermediate, the allenolate
intermediate is typically less reactive, and optimization of the
yield for a3-d3 umpolung is often more difficult. It is
understandable that the yields were not as high as that of the
a1-d1 umpolung pattern. Additionally, we could show that steric
different β-substituted alkynyl aldehydes on the aromatic ring
were all well tolerated in this reaction.
The possible mechanism of the regioselective umpolung
strategy was revealed as follows (Scheme 4). Initially, NHCs
were produced in the presence of bases, followed by formation
of Breslow intermediate K. Acyl anion equivalent (L) (cycle I)
was generated when triazolium-based catalyst E was used and
then reacted with 4-chloronitrosobenzene for C−N bond
formation. After the NHC catalyst had been released, product
3c was obtained by 5-endo-dig cyclization of intermediate M
followed by proton transfer and the structure of product 3c was
unambiguously assigned on the basis of X-ray crystallographic
Reaction conditions: 1 (0.75 mmol), 2 (0.5 mmol), catalyst (10 mol
%), base (0.75 mmol, 1.5 equiv), THF (5.0 mL), isolated yield.
was revealed that the yields may not be affected by the electron
or steric effect (Scheme 2, 3h−3j). Also, other nitrosobenzenes
gave high yields (Scheme 2, 3k and 3l). Substitution patterns
on both the aldehydes and nitrosobenzenes affected the yields
to some extent. We were also pleased to obtain the
corresponding products in moderate yields even when
heterocyclic substituted propioaldehydes were used (Scheme
2, 3n and 3o). Significantly, β-alkyl-substituted alkynyl
aldehydes were well tolerated and gave the products in good
to moderate yields with either 4-CH3- or 4-Cl-substituted
nitrosobenzenes (Scheme 2, 3p−3w). This demonstrates that
the substrate scope of this catalytic system is remarkably broad.
On the other hand, a3-d3 umpolung of enals was another
interesting area for constructing C−C and C−X bonds, but
there are only a few examples of a3-d3 umpolung of alkynyl
adehydes. Recently, our group developed the novel C−C bond
formation of a3-d3 umpolung of alkynyl adehydes and obtained
butenolides.9 After that, this area attracted our attention, and
we tried to construct the C−X bond in a similar manner. To
avoid the unwanted formation of the a1-d1 umpolung product,
the steric imidazolium-derived NHCs might be superior to
other NHC scaffolds. To test this hypothesis, we investigated 3-
phenylpropiolaldehyde 1c and 4-chloronitrosobenzene 2a as
model substrates, and after using the steric imidazolium-based
NHC precatalysts H−J, we found J was the optimal one even
though 4a was isolated in only 20% yield (Table 2, entries 1−
3). Then other solvents were tested, and it was shown that
DMF was appropriate for this transformation (Table 2, entries
4−10). The yield was slightly increased when the amount of
catalyst loading was 20 mol % (Table 2, entry 11) and the
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a
Scheme 3. Substrate Scope for the a3-d3 Umpolung
Scheme 5. Deuterium Experiment That Aimed to Determine
the Role of Additive
However, we could not obtain the deuterium isotope effects of
CD3OD and CH3OH because more than one kind of proton
source existed in the system. Finally, it is obvious that the 5-
endo-dig cyclization was promoted by CH3OH to some extent.
Also, to prove that this highly regioselective umpolung
strategy is effective, control experiments were performed using
1H NMR analysis of the products (Scheme 6). It can be noted
a
Reaction conditions: 1 (0.75 mmol), 2 (0.5 mmol), catalyst (20 mol
%), base (0.75 mmol, 1.5 equiv), DMF (5.0 mL), isolated yield.
Scheme 4. Possible Mechanism for the Regioselective
Umpolung
Scheme 6. Controlled Experiments for the Regioselective
Umpolung
that the ratio of the two regioisomers was more than 99:1 and
30:1 under each condition, and this revealed our highly
regioselective umpolung strategy was successful.
analysis (see the Supporting Information). On the other hand,
a3-d3 umpolung (O) occurred via the allenolate intermediate,
followed by reaction with 4-chloronitrosobenzene (cycle II).
Finally, product 4a was formed by lactonization of P.
In summary, we have developed a highly regioselective
umpolung reaction using alkynyl aldehydes as nucleophiles in
which regioselectivity was controlled by the reaction conditions.
This catalytic protocol allows for a rapid construction of
isoxazol-3(2H)-ones and isoxazol-5(2H)-ones from the same
materials under mild conditions in moderate to acceptable
yields. Further investigations of the use other electrophiles and
the regioselective umpolung strategy are being pursued in our
laboratory.
To verify the role of CH3OH and the possible mechanism of
the a1-d1 umpolung strategy, deuterium phenylpropiolaldehyde
1c-D was used with 4-chloronitrosobenzene 2a in the absence
of CH3OH, and 3ca and 3cb were obtained in a 1:1 ratio
(Scheme 5, eq 1). It was revealed that the aldehydic deuterium
was transferred to the expected position as shown in 3cb. On
the other hand, theoretically, 3cb would be more than 90% in
the two products, but we could obtain only 50% 3cb from the
reaction, which may be due to the participation of a trace
amount of H2O in the system (it would be clarified as shown in
Scheme 5, eq 4; 30% of the proton comes from H2O and
[DBU-H]+). This confirmed our hypothesis that there may be
three kinds of proton sources from N to 3c, and they were
[DBU-H]+, aldehyde, and H2O. Interestingly, the amount of
3ca was increased when CH3OH was added as another proton
source (Scheme 5, eq 2); this may be due to the fast proton
transfer when CH3OH is added as a proton source. The proton
of CH3OH tracked to the position in 3cb, which was shown in
eq 3 of Scheme 5. Therefore, cyclization would not be fast
enough in the absence of CH3OH, and the yield was decreased.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization, and crystallographic
data for the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: shexg@lzu.edu.cn.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for the financial support by the NSFC (211252
07, 21102062, and 21072086), the MOST (2010CB833203),
PCSIRT (IRT1138), the FRFCU (lzujbky-2013-49 and
lzujbky-2013-ct02), and Program 111.
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