A palladium-catalyzed oxidative aminocarbonylation reaction of alkynone: O -methyloximes with amines and CO in PEG-400

Article abstract of DOI:10.1039/c9gc03562a

A palladium-catalyzed oxidative aminocarbonylation of O-methyloximes and amines under aerobic conditions with PEG-400 as an environmentally benign medium was accomplished. This novel protocol provides the first straightforward synthetic method for the assembly of structurally diverse 4-carboxamide isoxazoles with high atom- and step-economy and exceptional functional group tolerance. Notably, the employment of ionic liquids as an additive, with air as the sole oxidant, without ligands is an attractive feature of the present approach.

Full text of DOI:10.1039/c9gc03562a

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A palladium-catalyzed oxidative
aminocarbonylation reaction of alkynone
Cite this: DOI: 10.1039/c9gc03562a
O-methyloximes with amines and CO in PEG-400†
Jianxiao Li, * Junsheng Yu, Wenfang Xiong, Hao Tang, Miao Hu, Wanqing Wu
and Huanfeng Jiang
*
A palladium-catalyzed oxidative aminocarbonylation of O-methyloximes and amines under aerobic con-
ditions with PEG-400 as an environmentally benign medium was accomplished. This novel protocol pro-
vides the first straightforward synthetic method for the assembly of structurally diverse 4-carboxamide
isoxazoles with high atom- and step-economy and exceptional functional group tolerance. Notably, the
employment of ionic liquids as an additive, with air as the sole oxidant, without ligands is an attractive
feature of the present approach.
Received 15th October 2019,
Accepted 2nd December 2019
DOI: 10.1039/c9gc03562a
rsc.li/greenchem
8
Carbonylation reactions have attracted considerable attention ling partners for the assembly of functionalized heterocycles.
in both academia and industry, and represent a direct and Despite these significant advances, some of these develop-
atom-economical strategy for the assembly of various high ments suffer from certain limitations, such as limited sub-
value-added carbonyl containing functionalized molecules strate scope, high pressure of CO, and/or high reaction temp-
such as carboxylic acids, esters, ketones, aldehydes, and amide erature. As a consequence, the development of efficient and
1
derivatives. Among various carbonylative reagents, CO gas as expeditious synthetic approaches under 1 atm of CO atmo-
a low cost, high reactivity, and abundant C1 building block sphere to access polyfunctionalized heterocycle skeletons, is
has been widely investigated in contemporary organic chem- more appealing.
2
istry. Since the seminal discovery of palladium-catalyzed
Isoxazoles and their derivatives represent essential hetero-
carbonylation of aryl and vinyl halides for the preparation of cycle scaffolds and are commonly prevalent in advanced
esters, amides, acyl chlorides, and aldehydes was reported by organic materials, biologically active molecules, naturally
3
9
Heck and co-workers in 1974, several noble metal catalysts occurring heterocyclic products and drugs. For instance, leflu-
4
5
(
such as Pd, Rh, Ru, and Ir), and non-noble-metal catalyzed,
nomide, a 4-carboxamide isoxazole, was reported as an anti-
6
10
and even metal-free carbonylation reactions have now been rheumatic drug. Therefore, great efforts have been devoted to
well-established and utilized for constructing various fine developing efficient and synthetic methodologies to access
chemicals, pharmaceutical molecules, and biologically active functionalized isoxazole derivatives in the past few years.
heterocycles as well. Particularly, numerous elegant transform- Traditionally, cycloisomerization reactions of alkynone
ations of intramolecular cascade carbonylation have under- O-methyloximes with various electrophiles have been con-
gone rapid development for constructing synthetically versatile firmed as an efficient and practical methodology
1
1
heterocycle scaffolds with high efficiency and selectivity during (Scheme 1a). Using this strategy, Larock and co-workers
7
recent years. In this context, the Wu and Beller group has developed an elegant method for the synthesis of a variety of
exploited a series of elegant palladium-catalyzed carbonylation 4-halo(seleno)isoxazoles with ICl, I , Br and PhSeBr as electro-
2
2
1
2
reactions of o-aminobenzamide, o-bromobenzaldehydes, and philes. In recent years, transition metal-catalyzed cascade
o-hydroxyacetophenones with a wide range of suitable coup- cyclization/functionalization has also been extensively studied
for the construction of highly functionalized isoxazole hetero-
1
3
aromatic motifs (Scheme 1b). Ryu and co-workers disclosed
Key Lab of Functional Molecular Engineering of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, P. R. China. E-mail: cejxli@scut.edu.cn, jianghf@scut.edu.cn
an elegant protocol of gold-catalyzed cascade cyclization–fluor-
14
ination for the preparation of 4-fluoroisoxazoles. Recently,
the group of Gao and Yang discovered efficient and expedi-
tious synthetic approaches for the synthesis of 4-sulfenyl isoxa-
†
Electronic supplementary information (ESI) available: Experimental details and
1
13
characterization of all compounds, and copies of H and C NMR spectra for
selected compounds. CCDC 1949919. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c9gc03562a
1
5
zoles. Very recently, we have reported a palladium-catalyzed
three-component cascade S-transfer reaction for the assembly
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a
Table 1 Optimization of reaction conditions
b
Entry
Catalyst
PdCl
Additive
Solvent
Yield /%
1
2
3
4
5
6
7
8
9
2
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
KI
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Dioxane
Toluene
N.D
N.D
12
27
36
11
18
34
26
Pd(OAc)
Pd(dppf)Cl
Pd(MeCN)
Pd(Py) Cl
Pd(MeCN)
2
2
2
Cl
2
2
2
4
(BF
[PdCl( -allyl)]
)
4 2
η
2
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Scheme 1 Methodologies for the synthesis of substituted isoxazoles.
10
11
12
Trace
Trace
48
3
CH CN
PEG-400
PEG-600
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
MeOH
of structurally diverse 4-sulfenylisoxazole derivatives in ionic 13
36
44
58
50
76
63
1
6
14
Pd(Py) Cl
2
liquids. To the best of our knowledge, there is no straight-
forward synthetic protocol for the preparation of 4-carboxa-
1
1
5
6
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
Pd(Py)
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
nBu
nBu
4
NI
4
NCl
mide isoxazoles via metal catalyzed oxidative aminocarbonyla- 17
[Cpmim]Cl
[Bmim]Cl
1
1
2
8
9
0
tion of O-methyloximes with simple amines. Inspired by our
c
d
e
f
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
[Cpmim]Cl
84 (78)
68
Trace
Trace
N.D
76
1
7
previous studies on the synthesis of heterocycle frameworks,
1
8
and Pd-catalyzed coupling reactions in ionic liquids, we 21
2
2
2
2
3
4
herein disclose the first palladium-catalyzed oxidative amino-
carbonylation of O-methyloximes with amines, delivering
straightforward and practical access to 4-carboxamide isoxa- 25
g
h
i
65
37
21
c
c
2
2
6
7
zoles with maximum atom and step economy in PEG400 with
air as the green and terminal oxidant (Scheme 1c).
We started our study by using O-methyl oxime (1a) and
propan-2-amine (2a) as the model substrates to screen the oxi-
EtOH
a
All reactions were performed with 1a (0.10 mmol), 2 (0.40 mmol),
[
Pd] catalyst (3 mol%), additives (2 equiv.), and solvent (1 mL) under
air for 12 h. [Cpmim]Cl: 1-cyanopropyl-3-methyl imidazolium chloride.
1-butyl-3-methylimidazolium chloride. PEG-400:
dative aminocarbonylation optimal conditions, and the results [Bmim]Cl:
Polyethylene glycol 400. PEG-600: Polyethylene glycol 600. N.D. = not
determined. Determined by GC using dodecane as the internal stan-
dard. The value in parentheses is the yield of the isolated product.
are summarized in Table 1. Various palladium salts were
firstly examined for this reaction, such as PdCl , Pd(OAc) , Pd
dppf)Cl , Pd(MeCN) Cl , Pd(Py) Cl , Pd(CH CN) (BF , and
PdCl( -allyl)] , and Pd(Py) Cl was found to be the most
b
2
2
c
d
(
[
2
2
2
2
2
3
4
4
)
2
50 mg 4 Å MS (molecular sieve) was used. 1 equiv. [Cpmim]Cl was
η
e
f
g
used. 10 mol% [Cpmim]Cl was used. At room temperature. Under a
2
2
2
h
N atomosphere. 10 mol% xantphos (4,5-Bis(diphenylphosphino)-9,9-
2
effective catalyst (Table 1, entries 1–7). Subsequently, different
solvents such as DMSO, 1,4-dioxane, CH CN, toluene, sphino)butane) was used.
i
dimethylxanthene) was used. 10 mol% DPPB (1,4-bis(diphenylpho-
3
PEG-400, and PEG-600 were screened, and PEG-400 proved to
be the suitable solvent (Table 1, entries 8–13). Among a variety
of additives (Table 1, entries 14–18), ionic liquids [Cpmim]Cl
showed the best efficiency to provide the aminocarbonylation cascade aminocarbonylation. As outlined in Scheme 2, a wide
product 3a in 76% GC yield (Table 1, entry 17). Notably, 3a can variety of primary amines can be successfully coupled with CO
be obtained in 84% GC yield by using 50 mg 4 Å MS (Table 1, and alkynone O-methyl oxime (1a) under the newly developed
entry 19). In addition, the amount of [Cpmim]Cl was then catalytic system. Gratifyingly, both linear chain and branch
examined; it was found that only a trace amount of the desired chain primary amines were perfectly accommodated for this
product 3a was detected by GC-MS when only 10 mol% reaction, providing the corresponding amides 3a–3d in moder-
[Cpmim]Cl was used (Table 1, entry 21). However, this reaction ate to good yields. Specifically, O-methyloximes containing the
did not work at room temperature (Table 1, entry 22). alkynyl structural moiety were also good candidates in this
Encouraged by this result, we then investigated bidentate aminocarbonylation, albeit somewhat reduced yields were
phosphine ligands, such as xantphos and DPPB; unfortu- obtained (3d). Of note, three-, four-, five- or six-membered ring
nately, a diminished yield was observed (Table 1, entries 24 amines were all nicely tolerated, and gave the desired products
and 25). Finally, other protic solvents such as MeOH, and 3e–3j in 73% to 86% yields. Furthermore, heteroaromatic
EtOH were screened, showing that the reaction is rather slug- primary amines, such as furan-2-ylmethanamine (2n), thio-
gish in these solvents (Table 1, entries 26 and 27).
phen-2-ylmethanamine (2q), and pyridin-4-ylmethanamine
With the above optimized conditions in hand, we further (2r), were compatible with the current catalytic system,
investigated the substrate scope and limitations of this affording the corresponding products 3n, 3q, and 3r in 77%,
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Scheme 3 Substrate scope of O-methyl oxime 1 derivatives. All reac-
tions were performed with 1 (0.20 mmol), 2j (0.80 mmol), Pd(Py)
3 mol%), [Cpmim]Cl (2 equiv.), 100 mg 4 Å MS, PEG-400 (2 mL) at 60 °C
for 12 h. Yields refer to isolated yield.
2 2
Cl
(
Scheme 2 Substrate scope of various amines 2. All reactions were per-
formed with 1a (0.20 mmol), 2 (4 equiv.), Pd(Py) Cl (3 mol%), [Cpmim]
2 2
Cl (2 equiv.), 100 mg 4 Å MS, PEG-400 (2 mL) at 60 °C for 12 h. Yields
refer to isolated yield. N.D. = not determined.
tuted O-methyl oximes bearing three-, or six-membered ring
groups converted smoothly to the target products 4n and 4o in
7
7%, and 75% yields, respectively. Notably, the 2-thienyl group
7
8%, and 82% yields, respectively. To our delight, 2-tetrahydro-
was found to be suitable for assembling 4p in 83% yield.
Additionally, alkynone O-methyl oximes and various R substi-
tuents were also perfectly tolerated, and delivered the desired
products 4q–4v in moderate to excellent yields. It is remark-
able that double alkyl substituted O-methyl oximes, such as
furfurylamine was tolerated well, leading to the corresponding
product 3p in 73% yield. It is worth mentioning that primary
amines containing the cyclohexenyl group could also react
smoothly to afford the products 3s in moderate yield.
Naphthalen-1-ylmethanamine (2t) also proved to be an amen-
able substrate, delivering the desired product 3t in 88% yield.
However, the optimal conditions were not compatible with aro-
matic amine (2u) and secondary amine (2v), and only a trace
amount of the target products 3u and 3v was detected by
3
1
-cyclopentylhept-2-yn-1-one O-methyl oxime (1w) and 1,3-
dicyclohexylprop-2-yn-1-one O-methyl oxime (1x) were also
accommodated in the transformation, affording the target pro-
ducts 4w and 4x in 73% and 76% yields, respectively.
Subsequently, we evaluated the substitution effect of alky-
none oxime ethers under the current catalytic system
(Scheme 4). Unfortunately, only a trace amount of the product
GC-MS analysis. The possible reason was that the pK
a
values of
aromatic amines and secondary amines were lower than those
1
9
of primary amines.
3k was detected by GC-MS with the 1,3-diphenylprop-2-yn-1-
Inspired by the above results, we turned our attention to
the investigation of various structurally diverse alkynone
O-methyl oximes (1) under the optimal conditions, and the
representative results are shown in Scheme 3. In general, both
electron-donating and electron-withdrawing substituents on
the phenyl ring of O-methyl oximes were well accommodated,
generating the desired products in moderate to good yields
one oxime 1aa as the coupling candidate. Satisfactorily, O-allyl
alkynone oxime ether (1ab), and O-benzyl alkynone oxime
ether (1ac) were compatible with this aminocarbonylation pro-
tocol, albeit in somewhat low yields.
The efficiency of this catalytic system was further demon-
strated by running the transformation on a gram-scale. For
example, when 5 mmol of 1a was utilized, 1.186 g of the
desired product 3k was obtained in 67% yield (Scheme 5).
(
4a–4f). Obviously, an electronic effect was observed in that
electron-donating groups generally gave yields higher than
electron-withdrawing ones. However, the substrate (1g) con-
taining a strongly electron-withdrawing group on the phenyl
ring failed to give the expected product 4g under the standard
conditions, instead the protonolysis product 4g′ was obtained
in 85% GC yield by GC-MS analysis. As anticipated, linear
chain O-methyl oximes showed a similar reactivity to provide
the desired products 4h–4k with yields ranging from 79% to
8
2%. The structure of the product 4l was unequivocally con-
Scheme 4 Exploration of the substitution effect of alkynone oxime
2
0
firmed by X-ray crystallographic analysis. Cycloalkyl substi- ethers.
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Scheme 5 Gram-scale transformation of 1a.
Scheme 6 The straightforward carbonylation reaction of benzylamines.
Amazingly, the straightforward carbonylation products of
benzylamines were obtained when Pd(CH CN) (BF ) was used
3 4 4 2
as the catalyst under similar reaction conditions (Scheme 6). For
Scheme 8 Proposed mechanism.
instance, simple benzylamine (2j) and 4-methoxybenzylamine
(2t) were allowed to react with CO, affording the carbonylation
products 5a and 5b in 87% and 90% yields, respectively.
desired product 3j was detected by GC-MS (eqn (4) and (5)).
To gain preliminary insights into this aminocarbonylation These observations demonstrated that 3,5-diphenylisoxazole (7)
reaction mechanism, the following control experiments were and 4-chloro-3,5-diphenylisoxazole (8) are not possible inter-
performed (Scheme 7). Firstly, no direct cyclization/amination mediates in this aminocarbonylation reaction. Finally, when
product 6 was generated when the reaction was carried out in O-benzyl oxime ether (1ac) and benzylamine were allowed to
the absence of CO (eqn (1)). Subsequently, no desired product react under the standard conditions, the desired product 3j was
3
j was observed when this aminocarbonylation coupling reac- detected in 76% yield by GC-MS analysis. Meanwhile, the for-
tion was performed under an Ar atmosphere (eqn (3)). In con- mation of benzyl chloride could be observed in the reaction
trast, 3j was formed in 87% GC yield when the reaction was system, which reveals that the cleavage C–O bond in the intra-
carried out under an O2 atmosphere (eqn (2)). The results molecular reaction mode could be confirmed (eqn (6)).
described above suggested that air or O
2
as an oxidant is indis-
On the basis of the above results and literature precedents,
pensable for this cascade aminocarbonylation. Furthermore, a plausible mechanism is illustrated in Scheme 8. Initially, in
−
when 3,5-diphenylisoxazole (7) and 4-chloro-3,5-diphenylisoxa- the presence of excess Cl , the oxygen attacks the activated
zole (8) were employed under the standard conditions, no alkyne of alkynyl oxime ethers via 5-endo cyclization after the
loss of a methyl group to afford the vinylpalladium intermedi-
1
3,18g
ate I.
palladium–carbon bond produces acylpalladium species II.
Then, ligand exchange between intermediate II and primary
Subsequently, the migratory insertion of CO into the
2
1
2
2
amines generates carbamoylpalladium intermediate III.
Finally, the reductive elimination of intermediate III gives the
aminocarbonylation products and regenerates the Pd(0)
species. The oxidation of Pd(0) to Pd(II) by air completes the
2
3
catalytic cycle. Notably, the ionic liquid plays an important
role in this transformation. It not only acts as a good solvent,
but also provides excess halide ions to eliminate hydrochloride
from acetylinic oximes 1, giving the vinyl palladium intermedi-
2
4
ate I. Furthermore, according to previous reports, they can
behave as ligands, and stabilizing agents for both the metal
2
5
active species and intermediates of the catalytic system.
Conclusions
In summary, we have successfully accomplished a practical
and efficient palladium-catalyzed protocol for the synthesis of
4-carboxamide isoxazoles via three component cascade amino-
Scheme 7 Control experiments.
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carbonylation of alkynone O-methyloximes with structurally
diverse primary amines in the presence of CO. Importantly,
the present methodology demonstrates excellent functional
group compatibility, excellent atom- and step economy, and
mild reaction conditions. Notably, O2 or air as the terminal
oxidant, an ionic liquid as the additive, and PEG400 as the
solvent make this transformation green and practical.
Moreover, the chemistry described herein represents an
efficient synthetic approach for accessing biologically impor-
tant 4-carboxamide isoxazole derivatives.
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(
b) X.-F. Wu, L. Wu, R. Jackstell, H. Neumann and
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