C?C bond acylation of oxime ethers via NHC catalysis

Article abstract of DOI:10.1021/acs.orglett.0c04248

An N-heterocyclic carbene (NHC)-catalyzed strategy has been developed to address the issue of using toxic transitional metals in the field of C?C bond activation. The novel reaction mode enables an efficient docking between the cyanoalkyl from the cycloketone oxime derivative and the acyl group from the aldehyde, affording ketonitrile in moderate to good yields, which is one kind of useful building block for synthesizing nitrogen-containing pharmacophores.

Full text of DOI:10.1021/acs.orglett.0c04248

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Letter
C−C Bond Acylation of Oxime Ethers via NHC Catalysis
Hai-Bin Yang* and Dan-Hong Wan
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ABSTRACT: An N-heterocyclic carbene (NHC)-catalyzed strategy
has been developed to address the issue of using toxic transitional
metals in the field of C−C bond activation. The novel reaction mode
enables an efficient docking between the cyanoalkyl from the
cycloketone oxime derivative and the acyl group from the aldehyde,
affording ketonitrile in moderate to good yields, which is one kind of
useful building block for synthesizing nitrogen-containing pharma-
cophores.
Scheme 1. Strategies of the Radical-Promoted C−C Bond
Activation
n contrast with C−H bond functionalization as a method of
Imodifying the periphery of molecules, C−C bond
functionalization provides a powerful tool to modify their
core framework. It endows the medicinal chemistry commun-
ity with a new tactic for discovering lead compounds from
known bioactive molecules via late-stage skeletal diversifica-
tion, a concept referred to as “scaffold hopping”.1 This
appealing chemistry is prevailingly achieved under the catalysis
of a transitional metal (Rh, Ru, Pd) via a two-electron
pathway,2 but the radical-promoted C−C bond cleavage
affords a unique and complementary strategy.3 The essence
of synthetic protocols through an open-shelled intermediate is
harnessing the β-scission of a radical at the exocyclic α-position
to afford an alkyl radical that is trapped by a radicalphile;
meanwhile, the targeted functional group is introduced
(Scheme 1A, 1 → 2). However, toxic transitional-metal
catalysts such as nickel are usually required to enable these
radical process.4 In this scenario, it is highly desirable to
develop a new strategy to break the C−C bond in a green and
sustainable manner.5
It is well known that N-heterocyclic carbene (NHC)
catalysis can access umpolung reactivity of carbonyls as acyl
anions, enolates, and homoenlates, which leads to the reaction
with electrophiles through ionic pathways.6 On the contrary,
NHC catalysis that proceeds via single-electron transfer (SET)
processes involving the coupling of radical intermediates is also
known, although it is far less explored in organic synthesis.7 As
early as in 2008, a SET process between a Breslow
intermediate and TEMPO was proposed in the oxidative
esterification of aldehyde reported by Studer.8 Rovis, Chi, Ye,
and Sun developed diverse NHC-catalyzed β-, γ-, or ε-
functionalizations of enals via radical/radical coupling9 or
radical addition10 mechanisms thereafter. Although great
progress has been made, these reactions mainly realized the
carbon−carbon and carbon−heteratom bond formation at the
remote position of the carbonyl group through an open-shelled
radical intermediate. Very recently, Nagao and Ohmiya
demonstrated a novel C−CO bond construction strategy via
a persistent ketyl-type radical 3 that resulted from the
oxidation of the Breslow intermediate (Scheme 1 B).11 Scheidt
disclosed that the reduction of the acyl azolium intermediate
could also provide an analogous ketyl-type radical, which was
Received: December 24, 2020
Published: January 12, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04248
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1049−1053
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applied to the ketone synthesis.12 Following their works,
diverse coupling reactions between carbonyl compounds and
radical precursors have been developed for synthesizing
functionalized ketones.13 Inspired by these excellent prece-
dents involving the ketyl-type radical 3, we considered that the
goal of cleaving an inert C−C bond in a green and sustainable
manner would be attained via NHC catalysis. Moreover, the
cyclic oxime derivative is an excellent platform for C−C bond
activation reactions.14 We envisaged that a ring-opening
coupling between a cyclic oxime derivative and an aldehyde
could occur under the catalysis of NHC, affording ketonitrile,
which is one kind of useful building block for constructing
nitrogen-containing pharmacophores such as piperidine,
pyridine, pyridin-2(1H)-one, and 2-aminothiophene (Scheme
1C).15
3). At first, a number of aromatic aldehydes were examined.
Benzaldehyde, which bears a substituent such as chlorine at the
meta position, was an effective coupling partner, affording the
corresponding product 6b in 80% yield. When unsubstituted
benzaldehyde was employed as the substrate, ketonitrile 6c was
obtained in 74% yield. Benzaldehydes, which bear substituents
at the para position, including acetyl and cyano, were tolerated,
and the reaction provided compounds 6d and 6e in 62 and
53% yields. Treating oxime ether 4c with fused aromatic
aldehydes such as 2-naphthaldehyde and 2-quinolinecarbox-
aldehyde, the corresponding products 6f and 6g were,
respectively, obtained in 80 and 69% yields. An electron-
deficient heteroaryl aldehyde such as nicotinaldehyde is a
better coupling partner than an electron-rich heteroaryl
aldehyde such as 2-furfural, and the corresponding products
6h and 6i were, respectively, delivered in 88 and 43% yields.
However, the current reaction condition does not work for
other types of aldehydes such as aliphatic and α,β-unsaturated
aldehydes. Next, we focused on investigating the scope of
oxime ethers 4. The reaction proceeded smoothly to provide
ketonitriles 6j−n in 50−82% yields for various cyclopentanone
oxime ethers 4 bearing a scope of para-, meta-, and ortho-
substituted phenyl rings at the endocyclic α-position. Mean-
while, both electron-donating groups, such as methyl and
methoxy, and electron-withdrawing groups, such as fluoro and
ester, were tolerated in this mild catalytic system. Remarkably,
the cyclopentanone oxime ethers bearing other types of
aromatic substituents such as 2-naphthyl and benzo[b]-
thiophen-5-yl were also compatible with this catalytic system,
affording compounds 6o and 6p in 87 and 51% yields.
Furthermore, cyclobutanone oxime ethers were more reactive
substrates, and the coupling products 6q and 6r after the ring-
opening process were obtained in 82 and 65% yields at room
temperature. Although the oxidative radical cyanomethylation
of allylic alcohol provides an effective strategy for synthesizing
1, 5-ketonitrile derivatives, this protocol will not produce a
regioisomer due to a mechanistic discrepancy (Ar3 ≠ Ar2).17
Compared with the four-membered ring and five-membered
ring counterparts, this organocatalyzed ring-opening acylation
became less favorable for cyclohexanone-derived oxime ether,
affording compound 6s in 54% yield at 80 °C (6c and 6q vs
6s). An attempt to construct quaternary stereogenic center
through the ring-opening benzoylation of 2-methyl-2-phenyl-
cyclopentan-1-one-derived oxime ether was unsuccessful, and
the corresponding product could not be detected via 1H NMR
analysis.
We initiated our investigation by examining the reaction of
2-phenylcyclopentan-1-one oxime derivative 4 and 3-methox-
ybenzaldehyde 5a in the presence of 20 mol % thiazolium C1.
Preliminary condition optimization showed that the reaction
was very sensitive to the leaving group of 2-phenylcyclopentan-
1-one oxime derivative 4 (Scheme 2). When O-benzoyl oxime
Scheme 2. Evaluation of the Effects of the Leaving Group
4a (R = Bz) was employed as a substrate, the corresponding
product 6a was obtained in 6% yield with 51% of starting
material 4a remaining, which indicated that the reactivity of 4a
was not strong enough for this thiazolium-based carbene
catalytic system. It is foreseeable that decreasing the electron
density of the aryl motif in the leaving group can increase the
reductive potential of the substrate; then, the reactivity will be
greatly improved. Next, we synthesized O-perfluorobenzoyl
oxime 4b, and performing the reaction with 4b provided 6a in
30% yield. However, the reaction mass balance was not ideal,
and we speculated that oxime ester was unstable under basic
conditions. Therefore, we considered using the more stable
oxime ether 4c as the substrate, where the 2,4-dinitrophenol
motif is also a good redox-active leaving group.16 Nevertheless,
a SET process between the nitro group and a Breslow
intermediate might occur to produce a persistent N−O radical,
which might interrupt the designed reaction.9 To our delight,
the reaction proceeded smoothly to provide 6a in 90% NMR
yield. After a further screening of other parameters such as
thiazolium C, temperature, solvent, and base, we established
the optimal conditions using 10 mol % of carbene precursor
C1 and 1.0 equiv of Cs2CO3 as a base in DCE at 60 °C to
provide product 6a in 82% isolated yield. (For details, see the
Supporting Information.)
To demonstrate the synthetic potential of the current
methodology, a formal synthesis of CWJ-a-5, which is a potent
topoisomerase I inhibitor, was conducted (Scheme 4).18
Treating oxime derivative 4d with benzaldehyde under
standard conditions, the reaction proceeded smoothly to
afford compound 6t in 70% yield, which could be converted to
CWJ-a-5 according to the reference method.19 Considering the
ease availability of substituted aromatic aldehydes, this route is
convenient for synthesizing CWJ-a-5 analogues.
A possible mechanism for this organocatalyzed ring-opening
acylation of cyclic oxime derivatives is depicted in Scheme 5.20
The catalytic cycle begins with the addition of the NHC to the
benzaldehyde in the presence of cesium carbonate. The
resulting Breslow intermediate 8 (Ered = −0.7 to −1.0 V vs
SCE)21 undergoes a SET process with cyclic oxime ether 4c
(acetophenone-derived analogue, Ered = −0.55 V vs SCE)16a to
form the key intermediates 10 and 9. Unlike the other N-
With the optimal condition in hand, we investigated the
scope of this organocatalyzed ring-opening cross-coupling
between the oxime derivative and the aryl aldehyde (Scheme
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a
Scheme 3. NHC-Catalyzed C−C Bond Activation: Scope of Ketonitrile Products
a
Reaction was performed with carbene catalyst precursor C1 (10 mol %), aryl aldehyde 5 (0.3 mmol, 1.5 equiv), cyclic oxime ether 4 (0.2 mmol,
b
c
1.0 equiv), and cesium carbonate (1.0 equiv) in DCE at 60 °C. All yields are isolated yields. Isolated yield on a 1.0 mmol scale. Carried out at
room temperature. Performed with 20 mol % of C1 at 80 °C.
d
Scheme 4. Formal Synthesis of CWJ-a-5
Scheme 5. Reaction Mechanism for NHC-Catalyzed C−C
Bond Cleavage
centered radicals, iminyl radical 10 is a σ-type species with its
unpaired electron in the orbital on the N atom in the nodal
plane of the CN π-system.22 The hyperconjugation effect
between the unpaired-electron-occupied orbit on the N atom
and the σ orbital of the α-C−C bond would lead to a rapid
radical-type ring-opening process, affording the nucleophilic
alkyl radical 11. The cross-coupling between the transient
radical 11 and the persistent radical 9 gives reaction species 12,
which undergoes an intramolecular fragmentation to yield
product 6c and regenerate carbene catalyst.
To further understand the reactivity of the iminyl radical in
NHC catalysis, we explored the reaction outcome for oxime
ether, where α-C−C bond cleavage is an unfavorable process
(Scheme 6). Using benzophenone-derived oxime ether 4e as a
substrate, 32% of N-coupled product 13 was detected in the
presence of 20 mol % C1.23 The N-acylation product 13 is
possibly formed as follows: (1) radical/radical cross-coupling
between iminyl radical 14 and captodative radical 9 and (2)
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Scheme 6. Acyl-Transfer Mechanism of Noncycloketone-
Derived Oxime Ether
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04248.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra of all new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Hai-Bin Yang − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, China; orcid.org/0000-0002-8977-4963;
Email: haibiny@gdut.edu.cn
Author
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Ed. 2008, 47, 8727−8730.
Dan-Hong Wan − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, China
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was sponsored by financial support from the 100
Young Talents Programme of Guangdong University of
Technology (no. 220413292).
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