Palladium-catalyzed ionic liquid-accelerated oxidative annulation of acetylenic oximes with unactivated long-chain enols

Article abstract of DOI:10.1039/d0gc02037k

A palladium-catalyzed ionic liquid-accelerated oxidative cascade annulation/functionalization of acetylenic oximes with unactivated long-chain enols under aerobic conditions was accomplished. This newly developed protocol provides the rapid and straightforward synthetic strategy for the assembly of structurally diverse isoxazole architectures under mild conditions with high atom- and step-economy and exceptional functional group tolerance. Notably, the ionic liquid acts as not only a solvent in the reaction but also provides the excess halide ions to eliminate hydrochloride from acetylenic oximes. Moreover, this catalytic system could be recycled up to eight times and reused without significant loss of the catalytic activity.

Full text of DOI:10.1039/d0gc02037k

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Palladium-catalyzed ionic liquids-accelerated oxidative annulation
of acetylinic oximes with unactivated long-chain enols
Miao Hu,a Zidong Lin,a Jianxiao Li,*a,b Wanqing Wua and Huanfeng Jiang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A palladium-catalyzed ionic liquids-accelerated oxidative cascade stereoselectivities; (iii) Pre-functionalized precursors and/or
annulation/functionalization of acetylinic oximes with unactivated activated reaction substrates. As consequence, the
a
long-chain enols under aerobic conditions was accomplished. The development of expeditious and eco-friendly synthetic
newly developed protocol provides the rapid and straightforward approaches under mild reaction conditions for the
synthetic strategy for the assembly of structurally diverse construction of privileged organic architectures with
isoxazoles architectures under mild conditions with high atom- unactivated substrates is still highly desirable.
and step-economy and exceptional functional group tolerance.
Notably, the ionic liquid acts as not only a solvent in the reaction,
but also provides the excess halide ions to eliminate hydrochloride
from acetylinic oximes. Moreover, this catalytic system could be
recycled up to eight times and reused without significant loss of
catalytic activity.
Transition-metal-catalyzed coupling reactions have been
identified as the powerful and fascinating synthetic
methodologies for the preparation of various value-added fine
chemicals.[1] In this context, palladium-catalyzed synthetic
approaches have also been well-established for the assembly
of synthetically versatile complicated molecules in step- and
atom-economical manners.[2] And,
a wide array of Pd
heterogeneous catalysts were completely investigated. In the
recent years, N-heterocyclic carbenes (NHCs) as the ancillary
ligands have been widely applied in palladium-catalyzed
synthetic transformations.[3] More importantly, owing to the
strong σ-donating and weaker π-accepting abilities of NHCs, a
library of stable and remarkable reactive NHC-Pd complexes
were synthesized, which can be further applied in carbon-
carbon and carbon-heteroatom bonds formation process.[4]
Despite these significant achievements, some of the elegant
strategies still have certain limitations: (i) toxicity, and/or
volatility solvent; (ii) lower chemo-, regio-, and
Scheme 1. Methodologies for the synthesis of functionalized isoxazoles
Isoxazoles architectures represent a privileged heterocycle
scaffolds extensively found in drugs, advanced materials, and
naturally occurring molecules.[5] For examples, Oxacillin was
used as
a a result, many
β-lactam antibiotic.[6] As
representative methodologies have been devoted to exploring
for the construction of functionalized isoxazoles building
blocks in the past few years, including [3+2] cycloaddition of
alkynes with nitrile oxides, transition metal-catalyzed C-H
functionalization of isoxazoles, condensation reactions, and
metal-catalyzed cycloisomerization reaction.[7] Moreover, the
biological activity studies demonstrated that highly substituted
isoxazoles displayed remarkable biological and therapeutic
activities.[8] In particular, the isoxazoles building blocks
containing vinyl or allyl moiety can significantly improve their
biological activities.[9] Therefore, much attention has been paid
to construct these heterocyclic scaffolds. For example, Chen
a
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
b
Guangdong Provincial Key Laboratory of Luminescence from Molecular
Aggregates, South China University of Technology, Guangzhou 510640, China
† Electronic Supplementary Information (ESI) available: [experimental details and
characterization of all compounds, copies of 1H and 13C NMR spectra for selected
compounds]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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and co-workers developed an elegant palladium-catalyzed the different types of NHC-Pd catalysts were also estimated,
View Article Online
DOI: 10.1039/D0GC02037K
cascade cyclization-alkenylation of alkynone O-methyl oximes and NHC-Pd-2 proved to be a better catalyst, whereas using
with activated olefins for the construction of 4-alkenyl other types of NHC-Pd, such as NHC-Pd-1 or NHC-Pd-3 gave
isoxazole derivatives (Scheme 1a).[10] However, the electron- poor results (Table 1, entries 13 and 14). Neither the neutral
rich alkene such as vinyl acetate was not amenable to this ionic liquid [Bmim]Cl nor the strong acidic ionic liquid
protocol. Additionally, Miyata and co-workers disclosed a nice [PrSO3Hmim]Cl were the suitable accelerating regents for this
gold-catalyzed cascade cyclization/Claisen-type rearrangement chemical transformation (Table 1, entries 15 and 16).
reaction of alkynyl oxime ethers for the preparation of 4-allyl Additionally, the reaction was almost completely inhibited at
isoxazoles (Scheme 1b).[11] In 2019, our group described a room temperature(Table 1, entry 17).[19] Finally, the recycling
Pd(II)-catalyzed one-pot tandem cyclization/alkylation reaction experiments of this approach showed that the catalytic
of internal alkynes with allylic or homoallylic alcohols for the systems (catalyst, ionic liquids, and H2O) could be typically
synthesis of β/ω-isoxazole aldehydes/ketones.[12] Despite the recovered and reused for subsequent reactions for eight times
significance of this protocol, the equivalent oxidant chloranil, with no appreciable decrease in yields (Table 1, entry 18).[20]
and two additives such as KI and TBAB were required. After
Table 1. Optimization of Reaction Conditions.a
that, we also reported
a palladium-catalyzed cascade
annulation/allylation of alkynyl O-methyloximes with allyl
halides (Cl, Br, I) to access fully substituted isoxazoles.[13]
However, this protocol needed the activated allyl halides as
the allyl source. The unactivated long-chain enols were not
compatible with the present synthetic strategy. Moreover, the
substrate scope of structurally diverse allyl halides were
limited. Very recently, we also successfully developed a
palladium-catalyzed oxidative aminocarbonylation reaction of
O-methyl oximes with amines and CO in PEG-400 for the
assembly of 4-carboxamide isoxazoles.[14] To the best of our
knowledge, there is no straightforward and green synthetic
methodologies for the assembly of 4-allyl isoxazoles using
unactivated enols as the alkenyl sources with eco-friendly
manner. In the recent years, ionic liquids as the
environmentally friendly solvents have gained considerable
attention.[15] Based on our continuing studies on Pd-catalyzed
coupling of alkynes with unactivated enols,[16] and Pd-
catalyzed coupling reactions in ionic liquids chemistry,[17] we
herein described a NHC-palladium-catalyzed ionic liquids-
accelerated oxidative cascade annulation/functionalization of
acetylinic oximes with unactivated long-chain enols for the
preparation of 4-allyl and 4-carbonyl isoxazoles with maximum
atom and step economy (Scheme 1c). Notably, the unique
chemical properties of the ionic liquids lead to the different
reaction pathway.
Entry
1
2
3
4
5
6
7
8
changes from the 'standard conditions'
none
Yield/%b
92 (88)
N.D
N.D
15
22
10
28
trace
trace
62
47
69
52
78
trace
54
N.D
66
Without [C2O2mim]Cl
Pd(dppf)Cl2 instead of NHC-Pd-
2
Pd(CH3CN)4(BF4)2 instead of NHC-Pd-
2
[PdCl(ŋ-allyl)]2 instead of NHC-Pd-
Pd(MeCN)2Cl2 instead of NHC-Pd-
2
2
Pd(Py)2Cl2 instead of NHC-Pd-
2
nBuN4Cl instead of [C2O2mim]Cl
Toluene instead of H2O
DMF instead of H2O
9
10
11
12
13
14
15
16
17
18c
CH3CN instead of H2O
PEG-400 instead of H2O
NHC-Pd-
NHC-Pd-
1
3
instead of NHC-Pd-
instead of NHC-Pd-
2
2
[Bmim]Cl instead of [C2O2mim]Cl
[PrSO3Hmim]Cl instead of [C2O2mim]Cl
room temperature instead of 110 oC
none
We started our studies by choosing acetylinic oxime (1a
)
and allylic alcohol (2a) as the model substrates to optimize the
reaction conditions, and the results are summarized in Table 1.
Gratifyingly, under the combination of NHC-Pd-
2
(0.05
o
mol%)/[C2O2mim]Cl (2 equiv) in H2O in the open air at 110 C
for 12 h, the oxidative cascade annulation/allylation product
3a was produced in 92% GC yield (Table 1, entry 1). The
employment of acidic ionic liquid [C2O2mim]Cl as the additives
significantly increased the yield of this cascade approach.
Various palladium salts were then screened, such as
Pd(dppf)Cl2, Pd(CH3CN)4(BF4)2, Pd(Py)2Cl2, Pd(MeCN)2Cl2, and
[PdCl(ŋ-allyl)]2, gave inferior results as compared with the NHC-
Pd catalysts (Table 1, entries 3-7). Notably, water as the green
and clean reaction medium proved to be the suitable solvent
in this transformation.[18] Other solvent systems such as
toluene, DMF, CH3CN, and PEG-400 seemed to be less efficient
for this cascade process (Table 1, entries 9-12). Subsequently,
a All reactions were performed with 1a (0.10 mmol), 2a (0.12 mmol), [Pd] catalyst
(0.05 mol %), additives (2 equiv), solvent (1 mL) at 110 oC for 12 h. [Bmim]Cl: 1-
butyl-3-methylimidazolium chloride. [C2O2mim]Cl: 1-carboxymethyl-3-methyl-
imidazolium chloride. [PrSO3Hmim]Cl: 1-propylsulfonate-3-methylimidazolium
b
chloride. N.D. = not determined. Determined by GC using dodecane as the
c
internal standard. The value in parentheses is the yield of isolated product.
Reused for eight times.
Having established the optimal reaction conditions, the
substrate scope and limitations of various acetylinic oximes of
this cascade annulation/allylation were then investigated, and
the results are summarized in Table 2. Generally, both
electron-withdrawing (Cl, Br, CN) and electron-donating (SCF3)
substituents on the phenyl ring of acetylinic oximes (R1) were
2 | J. Name., 2012, 00, 1-3
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perfectly accommodated, affording the desired products in
good to excellent yields (3a 3e). Moreover, the thienyl group
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DOI: 10.1039/D0GC02037K
-
was also amenable for this transformation, delivering the
corresponding allylation product 3f in 93% yield. Particularly,
the cycloalkyl and alkyl acetylinic oximes 1g and 1h were
compatible with the current protocol, and provided the
desired products 3g and 3h in 89% and 86% yields, respectively.
In addition, the substrate scope of acetylinic oximes (R2) were
then investigated. Similarly, both aryl and alkyl substituted
acetylinic oximes were all nicely tolerated under the optimized
reaction conditions, furnishing the desired products 3i
82% to 94% yields with high regioselectivities.
Table 2. Substrate scope of various acetylinic oximes 2 a
-3q in
a
All reactions were performed with 1 (0.20 mmol), 2 (1.2 equiv.), NHC-Pd-2 (0.05
mol %), [C2O2mim]Cl (2 equiv), H2O (2 mL) at 110 oC for 12 h. Yields referred to isolated
yield. b 16 h.
Table 4. Substrate scope of unactivated long-chain enols a
a
All reactions were performed with 1 (0.20 mmol), 2a (1.2 equiv.), NHC-Pd-2 (0.05
mol %), [C2O2mim]Cl (2 equiv), H2O (2 mL) at 110 oC for 12 h. Yields referred to isolated
yield.
a
All reactions were performed with 1 (0.20 mmol), 2 (1.2 equiv.), NHC-Pd-2 (0.05
mol %), [Cpmim]BF4 (2 mL) at 90 oC for 6 h. Yields referred to isolated yield. b 10 h.
Furthermore, the limitations and scopes of various
unactivated long-chain enols were then evaluated (Table 3).
Satisfactorily, a series of long-chain 1, n-enols such as but-3-
en-1-ol (2b), pent-4-en-1-ol (2c), hex-5-en-1-ol (2d), and hept-
6-en-1-ol (2e) are suitable coupling partners, and delivered the
Satisfyingly, when ionic liquid [Cpmim]BF4 (1-cyanopropyl-3-
methyl imidazolium tetrafluoroborate) was used as the
medium, this cascade annulation/alkylation could also be
extended to the synthesis of a highly functionalized 4-carbonyl
isoxazole derivatives (Table 4). In general, both alkyl and aryl
substituted enols were all compatible with the standard
conditions, generating the desired products in moderate to
good yields. Noteworthy, the heteroaromatic groups such as
the 2-furyl and 2-thienyl groups also worked well and
produced the corresponding products 5g and 5h in 81% and
80% yields, respectively. However, only a complex mixture 5i
was obtained when the pyridyl enol was employed under the
standard conditions. Of note, acetylinic oximes containing H
and CH3 substituents were all nicely tolerated, furnishing the
desired products 5j and 5k in 85% and 88% yields, respectively.
In particular, the styryl-substituted oxime was successfully
compatible with the current catalytic system, giving the target
product 5m in 85% yield. It is remarkable that the unactivated
desired products 4a-4d with yields ranging from 76% to 90%.
Particularly, the branched chain olefins such as 2-methylprop-
2-en-1-ol (2f), and 2-bromoprop-2-en-1-ol (2g) were also
perfectly compatible with this catalytic condition. However,
internal enol but-2-en-1-ol was not amenable to the current
chemical transformation, and failed to deliver the desired
product 4g
.
Table 3. Substrate scope of unactivated long-chain enols a
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long-chain enols such as but-3-en-1-ol and pent-4-en-1-ol were
Based on the current experimental results and the related
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also accommodated in this cascade chemical transformation, literature precedents,[21] a plausible mecDhOaIn: i1s0m.10i3s9d/De0sGcrCib02e0d37inK
leading to the target products 5n and 5o in 74% and 68% yields, Scheme 4. Initially, in the presence of excess Cl-, the oxygen
respectively.
attacks the alkyne of alkynyl oxime via 5-endo cyclization gives
[10]
Encouraged by the obtained results, a scale-up experiment
the vinylpalladium intermediate
I
.
Subsequently, alkene
of the cascade annulation/allylation was conducted under the
similar reaction condition. As shown in Scheme 2, when 10
mmol of 1a was utilized, 1.98 g of the desired product 3a was
migratory insertion to the vinylpalladium intermediate
affords Pd-alkyl intermediate II. After that, protonation the
I
hydroxyl group of long-chain enols with the aid of acidic ionic
liquid [C2O2mim]Cl forms the Pd-alkyl species III [22]
.
Then, β-
obtained in 76% yield with 0.05 mol% of NHC-Pd-2 as the
heteroatom elimination produces the desired products and 4.
3
catalyst, 2 equiv. of [C2O2mim]Cl as the additive in 20 mL H2O
at 110 oC for 18 h.
Notably, the acidic ionic liquid [C2O2mim]Cl not only acts as a
proton source for protonating the hydroxyl group, but also
provides excess halide ions to eliminate hydrochloride from
oximes. Alternatively, Pd-alkyl intermediate II undergoes rapid
β-hydride elimination and reinsertion to change the position of
the palladium on the alkyl chain (chain-walking), generating
Pd-alkyl intermediate IV [23]
.
Subsequently, β-hydride
elimination of Pd-alkyl species IV gives vinyl alcohol, which
undergoes keto-enol tautomerism to furnish the target
Scheme 2. The gram-scale experiment
product 5.
To figure out the plausible mechanism of this cascade
annulation/allylation, several control experiments were then
performed, and the representative results are summarized in
Particularly noteworthy was that the ionic liquid [Cpmim]BF4 not
only acts as the solvent in the cascade annulation/alkylation
reaction, but also provides the alkaline system to absorb
hydrofluoroboric acid from acetylinic oximes. More importantly,
owing to the strong chelating ability of the nitrile group of the ionic
liquid [Cpmim]BF4, which can behave as ligands and stabilizing
agents for both the metal active species and intermediates of the
catalytic system.[24]
Scheme 3. When 3,5-diphenylisoxazole (
6) or 4-chloro-3,5-
diphenylisoxazole ( ) was employed under the standard
7
conditions, however, no the desired product 3a was detected
by GC-MS analysis (Scheme 3, eq 1 and eq 2). These results
suggested that neither 3,5-diphenylisoxazole (
6) nor 4-chloro-
3,5-diphenylisoxazole ( ) are possible intermediates in this
7
cascade transformation. Furthermore, when oximes (1a) and
allyl chloride (2h) were allowed to react under the standard
conditions, the desired product 3a was detected in 54% yield
by GC-MS analysis. Finally, when O-benzyl oxime ether (1q
)
and allyl alcohol (2a) were allowed to react under the standard
conditions, the desired product 3a was detected in 80% yield
along with the 74% yield of benzyl chloride by GC-MS analysis
(eq 4). These observations demonstrated that the acidic ionic
liquid [C2O2mim]Cl provided the chloride ion for the formation
of BnCl.
Scheme 4. Proposed mechanism
Conclusions
In summary, we have successfully developed a palladium-
catalyzed ionic liquids-accelerated oxidative cascade
annulation/functionalization of acetylinic oximes with
unactivated long-chain enols under aerobic conditions. The
present methodology provides the rapid and straightforward
synthetic strategy for the assembly of structurally diverse
isoxazoles architectures under mild conditions with high atom-
Scheme 3. Control experiments
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lower NHC-Pd loading, are some of significant advantages of
this protocol, which also meets the requirements of Green
Chemistry.
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We thank the National Natural Science Foundation of China
(21971072), the Pearl River S&T Nova Program of Guangzhou
(201906010072), the Ministry of Science and Technology of
the People’s Republic of China (2016YFA0602900), the
Fundamental Research Funds for the Central Universities
(2018ZD16), the Open Fund of Guangdong Provincial Key
Laboratory of Luminescence from Molecular Aggregates
(2019B030301003) for financial support.
W. Yang, S. Yang, L. Huang, W. Wu, Y. Sun and H. Jiang,
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with unactivated long-chain enols see the Supporting
Information
20 For details of the recycling of the cascade annulation
/allylation catalytic system see the Supporting Information.
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