Visible-Light-Driven Dearomatization Reaction toward the Formation of Spiro[4.5]deca-1,6,9-trien-8-ones

Article abstract of DOI:10.1021/acs.orglett.9b04283

A visible-light-driven regioselective dearomative cyclization between 2-benzyl-2-bromomalonate and alkynes under mild conditions leading to the formation of spiro[4,5]decanes has been developed. In the presence of H₂O, a variety of 2-benzyl-2-b

Full text of DOI:10.1021/acs.orglett.9b04283

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Letter
Visible-Light-Driven Dearomatization Reaction toward the
Formation of Spiro[4.5]deca-1,6,9-trien-8-ones
Wuheng Dong, Yao Yuan, Xiaomin Xie, and Zhaoguo Zhang*
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ABSTRACT: A visible-light-driven regioselective dearomative
cyclization between 2-benzyl-2-bromomalonate and alkynes
under mild conditions leading to the formation of spiro[4,5]-
decanes has been developed. In the presence of H2O, a variety of 2-
benzyl-2-bromomalonates smoothly undergo 5-exo-dig radical
dearomative cyclization with alkynes to afford the corresponding
spiro[4,5]decanes in moderate to good yield in a step-economical
manner under oxidant-free conditions.
Scheme 1. Radical Spirocyclization onto an Aromatic Ring
for the Synthesis of Spiro[4,5]decane Derivatives
piro[4,5]decane skeletons are found in many synthetic and
1
Snatural molecules that exhibit diverse biological activities,
such as anticancer,1a antivirus,1d and antibacterial properties.1b
Thus the development of step-economical methods for
efficient access to the spiro-cycle with a sterically hindered
quaternary carbon center from readily accessible starting
materials is of great importance. Dearomative cyclization
reactions of arenes have developed into a straightforward and
powerful strategy to construct these molecular scaffolds,2
including acid-catalyzed cyclization,3 transition-metal-catalyzed
cyclization,4 electrophilic cyclization,5 and radical cycliza-
tion.1f,6 In 1994, Santi developed an efficient coupling of
diethyl benzyl malonates and alkynes for the synthesis of
spiro[4,5]decatriene and tetrahydronaphthalene derivatives
using a stoichiometric amount of Mn(OAc)3. The regiose-
lectivity greatly depended on the substituents on the aromatic
unit, and the main product, spiro[4,5]decatriene, was obtained
only when the substrates bore 4-F or 4-OMe (Scheme 1a).6b
Remarkably, visible-light-driven photoredox catalysis has
evolved into an effective and impactful synthetic tool, which
has paved new ways for the exploitation of radical approaches
owing to its sustainable and green features.7 Among them,
many elegant examples of intra- or intermolecular dearomative
cyclization protocols for the azaspirocycles under the
irradiation of visible-light have been reported.8 Our group
has recently achieved an efficient intermolecular dearomative
cyclization of 2-bromo-1,3-dicarbonyl compounds and alkynes
for the synthesis of spiro[4,5]decane skeletons by using a
photoredox catalyst (Scheme 1b).9 Then, we developed new
access to the synthesis of spirocycle skeletons by applying H2O
as an external oxygen source via C−Br bond cleavage under
mild reaction conditions (Scheme 1c).10
aromatic ring. From the synthetic point of view, substrates
bearing no substituents on the aromatic ring have the
advantages of atom economy and step economy. As such,
the development of direct and simple methods for the general
synthesis of these spirocarbocycles form readily available
starting materials still remains a highly demanding task. Herein
Received: November 28, 2019
However, most of the aforementioned accomplishments
have been limited to the substrates bearing some para
substituents, such as hydroxyl, alkoxyl, or halogen, on the
https://dx.doi.org/10.1021/acs.orglett.9b04283
© XXXX American Chemical Society
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A

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we report a visible-light-driven regioselective dearomative
cyclization of 2-benzyl-2-bromomalonate with alkynes in the
presence of H2O to give the corresponding spiro[4,5]decanes
via the intermediate spiro[4.5]deca-1,6,9-trien-8-ol, followed
by hydrogen abstraction and oxidation (Scheme 1d).
To substantiate the aforementioned reaction design, we
embarked on the investigation with the reaction of phenyl-
acetylene 1a and diethyl 2-benzyl-2-bromomalonate 2a in the
presence of fac-Ir(ppy)3 (1 mol %) in a mixture solvent
(DMA/H2O 1:1) under 7 W blue LED irradiation. Gratify-
ingly, the reaction took place at room temperature, and the
desired product, spiro[4,5]decane 3aa, was isolated in 52%
yield along with the 6-endo-dig cyclization byproduct 5aa in
32% yield (Table 1, entry 1). Next, we investigated the
were added, and 3aa was observed in verified yield. In these
cases, the intermediate spiro[4.5]deca-1,6,9-trien-8-ol 4aa was
also present by H NMR (Table 1, entries 9−13). Moreover,
reducing the amount of 2a also lead to the decreased yield of
target product 3aa (Table 1, entries 14 and 15). Control
experiments disclosed that both the photosensitizer and visible
light were indispensable in this transformation (Table 1,
entries 17 and 18). Finally, the experiments with on/off light
suggested that the chain propagation is not a main mechanistic
pathway. (See the Supporting Information.)
1
With the optimized conditions in hand, we first examined
the scope of alkynes for this visible-light-driven dearomative
cyclization reaction (Scheme 2). Gratifyingly, a broad
a
Scheme 2. Substrates Scope of Alkynes
a
Table 1. Optimization of Reaction Conditions
3aa yield
4aa yield
5aa yield
b
b
b
entry
solvent
(%)
(%)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
DMA/H2O 1:1
DMA/H2O 5:1
DMA/H2O 10:1
DMA/H2O 30:1
DMF/H2O 5:1
THF/H2O 5:1
MeOH/H2O 5:1
CH3CN/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
DMA/H2O 5:1
52
86 (85)
0
0
3
4
0
0
0
0
7
32
11
16
18
25
46
72
85
11
11
11
11
13
35
17
13
0
c
78
63
68
54
16
0
81
71
77
71
68
34
62
85
0
d
e
14
4
f
g
11
14
19
13
1
h
i
a
j
Conditions: 1 (0.3 mmol), 2a (197.5 mg, 0.6 mmol), fac-Ir(ppy)3
k
(4.2 mg, 1 mol %), DMA/H2O 5:1 (3 mL), irradiation with 7 W blue
b
LEDs at rt for 12 h; isolated yields. 1a (510.68 mg, 5.0 mmol), 2a
l
0
0
(3.29 g, 10 mmol), fac-Ir(ppy)3 (7.0 mg, 0.1 mol %), DMA/H2O 5:1
m
0
0
(50 mL), irradiation with 7 W blue LEDs at rt for 36 h.
a
Conditions: 1a (30.6 mg, 0.3 mmol), 2a (197.5 mg, 0.6 mmol),
catalyst (4.2 mg, 1 mol %), solvent (3 mL), irradiation with 7 W blue
b
LEDs at rt for 12 h. 1H NMR yields were reported using benzyl ether
spectrum of aryl acetylenes bearing diverse substituents,
electron-donating and electron-withdrawing, smoothly under-
went the transformation to produce the corresponding
spiro[4.5]deca-1,6,9-trien-8-ones in fair to good yield (Scheme
2, 3aa−ia). To our delight, the substituents at the ortho or
meta position for the aromatic ring did not significantly affect
the reaction efficiency, and the target products were obtained
in moderate yield (Scheme 2, 3ja−ka). Remarkably, the aryl
alkyne with an estrone skeleton was also well accommodated in
this transformation to give the corresponding product 3la,
which may be applied in the late-stage functionalization of
pharmaceutical molecules. In addition, our protocol could be
applied to aliphatic alkyne, albeit in lower yield (Scheme 2,
3ma and 3na). Finally, the product 3pa could be obtained in
85% yield when the heteroaryl acetylene (thiophene) was used
as the reaction partner. To showcase the scalability of this
method, a 5.0 mmol scale reaction of 1a and 2a was also
carried out under the optimized conditions to give the target
product 3aa in 78% yield with a catalyst loading of 0.1 mol %.
Next, the scope of 2-bromo-1,3-dicarbonyls (2) was
explored. We found that the reaction outcome could be easily
c
d
as an internal standard. Isolated yield. Na2CO3 (1.2 equiv) was
e
f
added. K2CO3 (1.2 equiv) was added. Li2CO3 (1.2 equiv) was
added. NaHCO3 (1.2 equiv) was added. 2,6-Lutidine (1.2 equiv)
was added. 1.2 equiv. 2a. 1.5 equiv. 2a. 2.5 equiv. 2a. Without
photocatalyst. Reaction was carried out in the dark.
g
h
i
j
k
l
m
influence of the amount of water for the reaction (Table 1,
entries 1−4). To our delight, the isolated yield of 3aa could be
increased to 85% when a mixed solvent of DMA/H2O with a
5:1 (v/v) ratio was used (Table 1, entry 2). Subsequently,
solvent screenings showed that DMA was superior to other
solvents (Table 1, entries 5−8). To our surprise, when MeOH
was used as the solvent, the yield of byproduct 5aa was
increased as high as 72% (Table 1, entry 7). Intriguingly, the
yield of 5aa was further increased to 85%, and no desired
product 3aa was observed when the reaction was performed in
MeCN (Table 1, entry 8). We inferred that different solvents
might affect the regioselectivity by favoring the types of radical
cyclization (5-exo-dig vs 6-endo-dig). A range of various bases,
such as Na2CO3, K2CO3, Li2CO3, NaHCO3, and 2,6-lutidine,
B
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influenced by the nature of the phenyl substituents (Scheme
3). As expected, the substrate with ortho-halogens (F, Cl, Br)
Scheme 3. Substrate Scope of 2-Bromo-carbonyl
a
Compounds
Figure 1. Time−concentration profile of the model reaction.
a
Conditions: 1a (30.6 mg, 0.3 mmol), 2 (0.6 mmol), fac-Ir(ppy)3 (4.2
mg, 1 mol %), DMA/H2O 5:1 (3 mL), irradiation with 7 W blue
LEDs at rt for 12 h; isolated yields. Na2CO3 (38.2 mg, 0.36 mmol)
was added.
To further probe into the mechanism, a series of verification
experiments were performed, as shown in Scheme 4. First, in
b
Scheme 4. Verification Experiments
afforded the desired spirocycles under the optimized reaction
conditions in 60−67% yield. Interestingly, the intermediate
spiro[4.5]deca-1,6,9-trien-8-ol (4) was isolated in 24−39%
yield instead of the dihydronaphthalene byproduct 5, which
may be ascribed to the decreased nucleophilicity of the
position meta to an electron-withdrawing group. However, the
yields of the corresponding products were improved by adding
1.2 equiv of Na2CO3 (Scheme 3, 3ab−ad). When the substrate
2e with an ortho-iodo group was treated with 1a, 3ae was
obtained in 58% yield, and the dehalogenation product (3aa)
was also isolated in 13% yield. Meanwhile, the electron-
donating methyl substituent could deliver the corresponding
product 3af in high yield. However, the electron-withdrawing
ester group on the phenyl ring produced a poor yield of the
product 3ag. Furthermore, moderate yields were still achieved
from the substrates with a meta-methyl, -fluorine or -chloro
group (Scheme 3, 3ah−aj). Compared with 1f (84% yield for
3af), the substrate 1k, which was functionalized with two
ortho-methyl groups, afforded only a moderate yield of 3ak
(55%) due to the steric hindrance. In addition, 2l proved to be
a viable substrate that gave the corresponding product 3al in
54% yield. Moreover, the substrate with a naphthalene ring was
also applicable to this system, delivering the desired product
3am in 81% yield.
To gain insight into this protocol, a time−yield profile of the
coupling of phenylacetylene 1a and diethyl 2-benzyl-2-
bromomalonate 2a utilizing the optimized conditions was
recorded by 1H NMR spectroscopy in Figure 1. The substrates
were rapidly consumed to form spiro[4.5]deca-1,6,9-trien-8-ol
4aa in 43% yield in 2.0 h, along with the desired product 3aa
and the byproduct 5aa in 35 and 11% yield, respectively. Then,
spiro[4.5]deca-1,6,9-trien-8-ol 4aa gradually converted into
spiro[4,5]decane 3aa in 12 h, which indicated that 4aa was the
key reaction intermediate.
the presence of H218O, the photocatalytic intermolecular
dearomative cyclization of phenylacetylene 1a and diethyl 2-
benzyl-2-bromomalonate 2a afforded the 18O-lableling product
3aa in 80% yield, suggesting that the oxygen atom of the
carbonyl group should come from H218O (Scheme 4a). In
addition, the reaction of spiro[4.5]deca-1,6,9-trien-8-ol 4aa
and 2a under optimized conditions yielded product 3aa and
diethyl 2-benzylmalonate,which reveals that the following
oxidation of alcohol 4aa to ketone 3aa might undergo a
hydrogen abstraction (Scheme 4b).
On the basis of the control experiments and related reports,9
we thus proposed a possible mechanism for this photocatalytic
regioselective dearomative cyclization (Scheme 5). First, 2-
benzyl-2-bromomalonate 2a can be reduced by the excited
IrIII* species to generate the radical A. Subsequently, A reacted
with phenylacetylene 1a to produce the spirocycle inter-
mediate C via an intramolecular 5-exo-dig radical cyclization
sequence. The spirocycle radical C could be oxidized by an IrIV
metal complex to carbocation D. Then, the carbocation D
reacted with H2O to afford the spiro[4.5]deca-1,6,9-trien-8-ol
4aa, which underwent a hydrogen abstraction by the radical
intermediate A to give the radical intermediate E. Finally, the
C
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Scheme 5. Proposed Mechanism
ACKNOWLEDGMENTS
■
We acknowledge the financial support provided by the
National Natural Science Foundation of China. We also
express gratitude for the support and valuable suggestions from
the Instrumental Analysis Center of Shanghai Jiao Tong
University.
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In conclusion, we have developed an efficient visible-light-
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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.9b04283.
Preparation of substrates, general procedure, character-
1
ization data, and H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Zhaoguo Zhang − Shanghai Jiao Tong University,
Shanghai, China, and Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai,
China; orcid.org/0000-0003-3270-6617;
Email: zhaoguo@sjtu.edu.cn
Other Authors
Wuheng Dong − Guangxi University of Science and
Technology, Liuzhou, China, and Shanghai Jiao Tong
University, Shanghai, China
Yao Yuan − Shanghai Jiao Tong University, Shanghai,
China
Xiaomin Xie − Shanghai Jiao Tong University, Shanghai,
China; orcid.org/0000-0002-5798-291X
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
D
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