Radical-induced ring-opening and reconstruction of cyclobutanone oxime esters

Article abstract of DOI:10.1039/c8cc10109d

The first example of intramolecular ring-opening and reconstruction of cyclobutanone oxime esters via selective C-C bond cleavage leading to the synthesis of 3,4-dihydronaphthalene-2-carbonitriles in the presence of a cheap copper catalyst has been reported. The protocol is distinguished by mild and safe reaction conditions that exclude ligands, oxidants, bases, toxic cyanide salts and tolerates a wide scope of cyclobutanones without compromising their efficiency and scalability. The alternative visible-light-driven photoredox process for this coupling reaction was also uncovered.

Full text of DOI:10.1039/c8cc10109d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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DOI: 10.1039/C8CC10109D
Journal Name
COMMUNICATION
Radical-Induced
Ring-Opening
and
Reconstruction
of
Cyclobutanone Oxime Esters
Received 00th January 20xx,
Accepted 00th January 20xx
Panpan Wang,a,b,c Binlin Zhao,b,c Yu Yuan, and Zhuangzhi Shi
a*
b*
DOI: 10.1039/x0xx00000x
www.rsc.org/
a) Zard's precedent work:
The first example of intramolecular ring-opening and
reconstruction of cyclobutanone oxime esters via selective C-C
bond cleavage leading to the synthesis of 3, 4-
dihydronaphthalene-2-carbonitriles in the presence of a cheap
copper catalyst has been reported. The protocol is distinguished
by mild and safe reaction conditions that avoid ligand, oxidant,
base, toxic cyanide salt and exhibited wide scope of
cyclobutanones without compromising its efficiency and
scalability. The alternative visible-light-driven photoredox process
for this coupling reaction was also uncovered.
NC
COOMe
+
SET
O
O
N
77%
COOMe
O
N
S
S
Py
500 W tungsten lamp
b) Recent developments:
OBz
N
CN
Ar
+
SET
Ar
The dissociation and recombination of C-C bond cleavage is of
fundamental interest and plays an important role in the synthesis of
complex organic molecules. Due to the presence of multiple C–C
Copper catalysis, Shi
Photocatalysis, Xiao, Chen
1
c) This design:
bonds in an organic compound with subtle difference in activation
barrier, controlling the site-selectivity represents a key challenge.
Cyclobutanones are a privileged simple scaffold widely applied in
the synthesis of pharmaceuticals, natural products and bioactive
NOBz
CN
Ar
SET
Ar
H
2
molecules. Considerable attention has been turned to the selective
Copper catalysis
Photocatalysis
3
functionalization of cyclobutanones and the derivatives via C–C
4
bond cleavage over the years. Since the pioneering work of Zard
5
Figure 1 Radical Induced Ring-Opening and Reconstruction of
and Uemura, an array of C–C bond β-scission reactions in
6
Cyclobutanone Oximes.
cyclobutanone oximes and other derivatives via iminyl radicals
have been extensively exploited to construct carbon-carbon and
7
carbon-heteroatom bonds by transition-metal catalysis and visible- photocatalysis under visible-light (Figure 1b). Herein, we report the
8
light photocatalysis.
rational design of a series of 2-benzylidenecyclobutan-1-one
derivatives for ring-opening and reconstruction via C–C bond
cleavage to synthesize various 3, 4-dihydronaphthalene derivatives
(Figure 1c). Notably, Dong and coworkers recently uncovered a
general approach to the catalytic activation of C–C bonds in simple
cyclopentanones and some cyclohexanones undergoing ring-
opening and reconstruction by combination of a rhodium pre-
catalyst, an N-heterocyclic carbene ligand and an amino-pyridine
co-catalyst, in which the in situ formed cycloketimine act as a key
As early in 1994, Zard group reported a sequential ring
opening/addition of cyclobutanone oximes and methyl acrylate
using stiochiometric radical initiators or UV irradiation (Figure 1a).
In 2017, we and Xiao group independently extended this elegant
chemistry to intermolecular olefination of cyclobutanone oxime
esters with alkenes, triggered by copper catalysis, or
4
c
9
10
1
1
1
2
intermediate. This radical-induced strategy is based on a novel
fragmentation-rearrangement sequence of cyclobutanone oxime
esters using copper catalysis or photocatalysis.
a. College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
25002, China, E-mail: yyuan@yzu.edu.cn
State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China, E-mail:
shiz@nju.edu.cn
These authors contributed equally.
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, characterization of starting materials and products and
crystallographic data. See DOI: 10.1039/x0xx00000x
2
b.
Under the inspiration of our copper chemistry, we first prepared
-((E)-benzylidene)cyclobutan-1-one O-benzoyl oxime (1a) as a
2
c.
model substrate and copper salts was chosen as the radical
activator to investigate the possibility of the ring-opening and
reconstruction reactions as shown in Table 1. To our delight, we
†
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Table 1 Optimization studiesa
NOBz
Journal Name
Table 2 Substrate Scopea
View Article Online
CN
NOBz
Cu(OTf)2 (5 mol %)
DOI: 10.1039/C8CC10109D
Ar
Ar
R2
R1
cat [Cu]
CN
THF, 70 o
C
H
solvent, Ar, 70 o
C
R2
R1
1
2
1
a
2a
Entry
1
Subatrates 1
Products 2
Yield
82%
T
time
(h)
Yield
entry
catalyst
solvent
NOBz
(oC)
(%)b
CN
1b
1c
2b
1
2
3
4
5
6
7
8
9
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
1,4-dioxane
DMF
THF
70
70
70
70
70
70
70
50
90
70
70
70
12
12
12
12
12
12
12
12
12
3
53
54
Me
Me
Me
Me
NOBz
CN
2
2c
2d
81%
65%
78
ether
THF
trace
32
NOBz
NOBz
CN
CN
Cu(acac)2
3
4
1d
1e
MeO
BnO
MeO
BnO
CuSO4
THF
45
CuF2
THF
51
2e 92%
Cu(OTf)2
THF
38
Cu(OTf)2
THF
45
NOBz
CN
1
1
1
0
1
2
Cu(OTf)2
THF
88 (85)
68
c
5
1f
2f
53%
MeS
MeS
Cu(OTf)2
-
THF
24
3
NOBz
NOBz
CN
THF
0
6
7
1g
1h
1i
2g
2h
2i
83%
89%
81%
66%
a
F
1
a (0.2mmol), 5 mol% of catalyst in solvent (0.1 M) under Ar at 70
C. GC-MS yield using dodecane as internal standard. isolated yield.
F
o
b
c
CN
obtained the product 3,4-dihydronaphthalene-2-carbonitrile (2a) in
3% yield, when the reaction was performed in the presence of
Cl
5
Cl
o
Cu(OTf)
2
(5.0 mol %) in 1,4- dioxane at 70 C for 12 hours (entry 1).
NOBz
NOBz
CN
CN
A brief evaluation of solvents showed that THF was superior to
other investigated solvents (entry 2), and the yield of desired
product 2a was improved to 78% (entry 3). Other copper salts such
8
Br
Br
as Cu(acac)
2
, CuSO
4
or CuF
2
led to lower yield of 2a compared with
(entries 5-7). Under these reaction
9
1j
2j
the performance of Cu(OTf)
2
F3C
F3C
conditions, changing the temperature failed to enhance the
efficiency of the transformation (entries 8-9). Critical to the
promotion of this reaction was to shorten the reaction time to 3
hours, in which 85% yield of 2a was obtained (entry 10). The
decrease in yield with an increase in time is likely due to the
decomposition of product 2a under the reaction conditions (entry
NOBz
CN
1
0
1k
1l
2k
2l
72%
78%
O
O
NOBz
CN
11
S
1
1). Finally, control experiment confirmed the coupling process did
S
not occur in the absence of the copper catalyst (entry 12).
CN
NOBz
With the optimized reaction conditions in hand, the scope of this
copper-catalyzed C-C bond cleavage was then examined (Table 2).
In general, a wide range of cyclobutanone derivatives containing
electronically varied groups reacted smoothly, yielding the
corresponding products, in modest to excellent yields. Substrates
with different aryl moieties bearing electron-neutral, donating and
withdrawing substituents including methyl (1b-c), ether (1d-e),
1
2
1m
2m 66%
(93:7)
NOBz
CN
Ph
Ph
2n 35%b
13
1n
1o
Ph
Bn
NOBz
CN
Bn
3
mercapto (1f), F (1g), Cl (1h), Br (1i), and CF (1j) groups at the
Ph
Ph
2o 55%b
14
ortho, and para position underwent facile ring-opening and
reconstruction. It is note to worth that some hetero aromatic rings
including furan (1k) and thiophene (1l) were also compatible well
with the mild conditions and the corresponding products were
obtained in good yields with excellent regioselectivity (2k-2l). In
case of a β-naphthalene substituted substrate 1m, we could furnish
the cyclization product 2m in high yields with good regioselectivity.
Next we tested other substrates with varieties on the
cyclobutanone rings. The cyclobutanone oximes with a phenyl,
benzyl and dimethyl groups on the cyclobutanone rings were
NOBz
CN
15
1p
2p
6
7%b
Me
Me
Me Me
a
Unless otherwise noted the reactions were carried out on a 0.2 mmol scale
(5 mol%) in THF (2 mL) at 70 C under Ar
atmosphere for 3 hours . Yields of the isolated yields. For 12 hours.
o
of 1 in the presence of Cu(OTf)
2
b
2
| J. Name., 2012, 00, 1-3
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prepared and applied under the standard conditions delivering the anion B and Cun+1 species. Subsequently, radical A was transformed
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3
,4-dihydronaphthalene-2-carbonitrile derivatives in 35% to 67% to a distal radical C driven by the ring straiDnOvIi:a10a.1β0-3s9c/iCss8iCoCn1o0f10C9-DC
yields (2n-2p).
bond. Then an intramolecular radical cyclization happened between
the carbon radical and the aromatic ring providing a reduction
potential intermediate D. Finally, D was oxidized to a cyclization
Arylnitriles are ubiquitous in bioactive natural products,
pharmaceutically relevant molecules and electronic materials.13
Furthermore, the nitrile group plays an important role in the area
of synthesis chemistry owing to its robust transformable
n+1
cation E by Cu to form the desired product, and the regenerated
n
Cu was put into the next catalytic cycle.
1
4
HOBz
properties. Having established this strategy for generating the
,4-dihydronaphthalene-2-carbonitrile frameworks, which are
2
1
OBz
3
OBz B
B
valuable precursors for the construction of arylnitriles. This is
documented in Scheme 4 by the highly selective DDQ oxidation of
CN
Cun+
X
N
2
e , 2k and 2l to give the corresponding arylnitriles 4, 5 and 6 in
E
excellent yields respectively.
Cu(n+1)
X
A
CN
CN
BnO
BnO
CN

DDQ (3.0 equiv)
CH3COOH (5.0 equiv)
CN
2e
4, 96%
D
toluene, 90 oC, 20 h
CN
CN
C
Figure 2 Proposed catalytic cycle
X
X
In conclusion, we reported the first carbon-radical cyclization
enabled by copper-catalyzed selective C-C bond cleavage of
cyclobutanone oximes. This method was applied for the synthesis
of 3,4-dihydronaphthalene-2-carbonitriles containing diverse
2
k, X = O
l, X = S
5
, X = O, 97%
2
6, X = S, 95%
Scheme 1. Synthetic Applications and Transformations
To get insight into the possible mechanism of copper-catalyzed fuctional groups. Moreover, these products could be efficiently
intramolecular radical cyclization. TEMPO radical quenching converted into varieties of arylnitriles. The primary mechanistic
experiment was carried out with the oxime 1a. The radical experiments suggested that a radical-mediated sequence was
cyclization product 2a was inhibited and considerable amount of involved in this transformation. A photo-induced strategy was also
distal carbon-TEMPO adduct 3 was formed (Scheme 2a). This result developed for an alternative approach for this ring-opening and
suggested that the reaction occurred via a radical-mediated reconstruction.
pathway, and the distal carbon radical was the important
We thank the “1000-Youth Talents Plan”, the “Jiangsu
intermediate in this transformation. We also tried the
Specially-Appointed Professor Plan”, NSF of China (Grant
photocatalytic reactions to find an alternative reaction system to
2
167020084), Jiangsu Provincial Nature Science Foundation
realize the intramolecular C-C bond cleavage/C-C bond formation.
We tested three oximes with different redox potentials under blue
LEDs (5 W) irradiation with fac-[Ir(ppy) ] as the photocatalyst giving
3
(SBK2016021885), the Top-notch Academic Programs Project
of Jiangsu Higher Education Institutions, and the Priority
different yields of the desired products respectively (Scheme 2b). Academic Program Development of Jiangsu Higher Education
The performance of the conversion was closely associated with the Institutions for financial support.
different redox potentials of the substrates and the 4-CF Ph
3
substituted oxime was chosen as a good candidate exhibiting well
compatibility with the photo-induced reaction system.
Conflicts of Interest
(a) TEMPO trapping experiment:
There are no conflicts to declare.
Me
Me
BzON
"
standard conditions"
.5 equiv TEMPO
CN
O
N
+
Me
1
Me
Notes and references
CN
3, 80%
1
a
2a, < 5%
1
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O
O
N
CN
1
.0 mol% fac-[Ir(ppy)3]
DMF, rt, Ar
1
2a
Ar = Ph, 1a
trace
45%
61%
Ar = 4-NO2Ph, 1a'
Ar = 4-CF3Ph, 1a''
2
Scheme 2. Mechanistic Experiments
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the single electron transfer (SET) reduction process with a release of
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