Electrochemical [4+2] Annulation-Rearrangement-Aromatization of Styrenes: Synthesis of Naphthalene Derivatives

Article abstract of DOI:10.1002/anie.201902315

We report the first electrochemical strategy to synthesize functionalized naphthalene derivatives through [4+2] annulation—rearrangement–aromatization from styrenes under mild conditions. The electrolysis does not require metals, oxidants and high valence substrates, indicating the atom and step-economy ideals. The dehydrodimer produced through [4+2] cycloaddition of 4-methoxy α-methyl styrene is isolated and proved to be the key intermediate for the following oxydehydrogenation to form carbon cation, which undergoes rearrangement–aromatization to afford the final products. This reaction represents a powerful access to construct multi-substituted naphthalene blocks in a single step.

Full text of DOI:10.1002/anie.201902315

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Accepted Article
Title: Electrochemical [4+2] Annulation-Rearrangement-Aromatization
of Styrenes: Synthesis of Naphthalene Derivatives
Authors: Ma Yueyue, Lv Jufeng, Liu Chengyu, Yao Xiantong, Yan
Guoming, Yu Wei, and Jinxing Ye
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902315
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10.1002/anie.201902315
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Electrochemical [4+2] Annulation-Rearrangement-Aromatization
of Styrenes: Synthesis of Naphthalene Derivatives
Yueyue Ma, Jufeng Lv, Chengyu Liu, Xiantong Yao, Guoming Yan, Wei Yu, and Jinxing Ye*
Abstract: We report the first electrochemical strategy to synthesize
functionalized naphthalene derivatives via [4+2] annulation-
rearrangement-aromatization from styrenes under mild conditions.
The electrolysis does not require metals, oxidants and high valence
substrates, indicating the atom and step-economy ideals. The
dehydrodimer produced through [4+2] cycloaddition of 4-methoxy α-
methyl styrene is isolated and proved to be the key intermediate for
the following oxydehydrogenation to form carbon cation, which
undergoes rearrangement-aromatization to afford the final products.
This reaction represents a powerful access to construct multi-
substituted naphthalene blocks in a single step.
electrocatalytic DA reaction assisted by aromatic redox tag
between styrene and diene.[9b] Most of these works only involved
the [4+2] annulation to afford non-conjugated products. Lei et al.
developed an oxidative [4+2] cycloaddition between styrenes and
alkynes under photoredox/cobaloxime dual catalytic system to
construct naphthalene motifs.[8d] Especially, Farid et al. reported
a photo-induced cycloaddition reaction of 1, 1-diarylethylenes and
yielded [4+2] cyclodimers, dehydrodimers and [2+2] cyclodimers
(Scheme 1, a).[8a] Inspired by this work, we hypothesized that the
aromatic motifs might be constructed through further
rearrangement-aromatization of dehydrodimer employing
electrochemical method (Scheme 1, b)[10]
. In this work,
electrochemical [4+2] annulation-rearrangement-aromatization of
styrenes is reported to access naphthalene derivatives under mild
conditions in absence of the high valence substrates, metals and
oxidants are required.
Functionalized naphthalenes and its derivatives are valuable
structural skeletons among functional materials, pharmaceuticals，
and natural products.[1-3] To regioselectively access this structures
is always the targets in organic synthesis. The most prevalent
and synthetically useful method is Diels-Alder (DA) reactions,
followed by the sequenced oxidative aromatization.[4]
Furthermore, due to the absence of stoichiometric oxidants,
dehydro or dehydrogenative Diels-Alder reactions (DDA), in
which the diene or alkene are replaced by high-valence
substrates, reveals its remarkable advantages in the synthesis of
sophisticated aromatics.[5,6] But, its applications are also limited
by its pre-dehydrogenative highly saturated substrates and
thermal conditions, where the heat, microwaves, and transition
metals are indispensable to initiate DA reactions. Therefore,
developing a green and mild method to access multi-substituted
aromatics from easily obtained reactants is an appealing and
challenging task for chemists.
Scheme 1. Radical cation triggered DA reactions
There is no doubt that the radical-cation cycloaddition has
enormously enriched the scope of thermal DA reaction. It not only
proceeds at more rapid reaction rates than the thermal pericyclic
reaction with excellent regio- and chemo-selectivity, but also
occurs between two electron-rich dienes.[7] Particularly, photo-
and electro-initiated radical cation DA reactions of electron-rich
olefins have been extensively reported.[8,9] For instance, Yoon et
al. developed a visible light mediated radical cation DA reaction
of two electron-rich olefins using low loadings of Ruthenium (II)
polypyridyl complexes.[8c] Chiba et al. established an
In view of the relatively low oxidation potential (Ep/2 = 1.35 V)
and high nucleophilicity,[11] 4-methoxy α-methyl styrene (1) was
chosen as a model substrate to optimize the electrolysis
conditions. After screening a wide range of reaction systems, in
an undivided cell equipped a carbon felt (CF) anode and Ni plate
cathode, the desired product 2 was prepared in 67% yield using
CH3CN as the solvent, hexafluoroisopropanol (HFIP) as the co-
solvent, tris(4-bromophenyl)amine (I, TBPA) [12] as mediator and
0.02 M Et4NOTs as electrolyte under a constant current of 10 mA
(details see SI, Table s1). It is worth to note that the concentration
of Et4NOTs was much less than 0.1 M, which is the normal
concentration used in practice. In the absence of TBPA, the yield
dropped to 41%. The electrode material is pivotal to achieve the
desired product. And the carbon felt was irreplaceable as the
anode and Ni was the best as the cathode, even similar result
obtained using Pt plate (65% yield). Replacing CF with other
carbon material as anode, such as reticulated vitreous carbon
(RVC) and graphite, only a trace of 2 was produced. Furthermore,
the electrolysis failed when HFIP was replaced by AcOH or MeOH
as co-solvent, which involved the cathodic reduction to generate
H2 and base in situ and may also stabilize the intermediate radical
[ ]
*
Y. Ma, J. Lv, C. Liu, X. Yao, Prof. Dr. J. Ye. Engineering Research
Centre of Pharmaceutical Process Chemistry, Ministry of Education;
Shanghai Key Laboratory of New Drug Design; School of Pharmacy,
East China University of Science and Technology,130 Meilong Road,
Shanghai 200237, China. E-mail: yejx@ecust.edu.cn
G. Yan, W. Yu. Shanghai Zhongxi Sunve Pharmaceutical Co., Ltd.
No. 158 Minle Road, Fengxian District, Shanghai 201419, China.
This work was partially supported by National Natural Science
Foundation of China (21272068, 21572056), Program for New
Century Excellent Talents in University (NCET-13-0800), the
Fundamental Research Funds for the Central Universities, and 111
project (B07023).
cation [13]
.
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10.1002/anie.201902315
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tolerated in the homo-cycloaddition. Due to the selectivity of
migrating groups, chemoselectivity was found in individual
examples, such as 6, 8, 10, 11 and 13. Its ratios were dependent
to the substrates and had no explicit relation with its structure.
Scheme 2. Substrate scope for the homo-cycloaddition. Reaction conditions:
Carbon felt anode, Ni plate cathode, constant current =10 mA, 1 (0.4 mmol), I
(0.04 mmol), Et4NOTs (0.16 mmol), HFIP (100 µL) and 8 mL CH3CN. [a] Yield
of isolated two isomers. [b]: Ratio of the two isomers, determined by 1HNMR.
[c]: Catalyzed by III. [d] Reaction performed under N2 atmosphere. [e]:
Catalyzed by II, constant potential = 5 mA. [f]: 8 mL acetone, 0.05M tBu4NBF4.
[g]: 8 mL acetone, 0.05M tBu4NBF4, 56 oC. [h] 8 mL THF/CH3CN (3:1), 0.05 M
tBu4NBF4.
Scheme 3. Substrate scope for the cross-cycloaddition. Reaction conditions:
Carbon felt anode, Ni plate cathode, constant current =5 mA, 31 (0.2 mmol), 30
(0.6 mmol), nBu4NBF4 (0.2 mmol), HFIP (50 µL), 4 mL CH3CN/THF (1:3). [a]
Yield of isolated products. [b]: Electrolysis performed at 45oC. [c]: Electrolysis
performed at reflux.
With the optimal condition in hand, the scope of the homo-
cycloaddition was investigated. A variety of primary alkyl groups
were tested and smoothly produced the desired products with
synthetically useful yields (Scheme 2, 2-6). Secondary alkyl
groups, including chain, cyclic, and aromatic groups, also
underwent this conversion with moderate yields (Scheme 2, 7-13).
However, when it was replaced by tertiary alkyl, the product of
interest was not discovered. Perhaps its steric bulk restrained the
formation of the crowded dehydrodimer (Scheme 2, 14). With
respect to the migration groups containing heteroatoms, such as
tetrahydropyran, carbamates and ester, the electrolysis
demonstrated excellent yields and current efficiency (Scheme 2,
15-18). For para, meta disubstituted, 3, 4-methylenedioxy
substituted styrenes and vinylnaphthalene, the process exhibited
single regioselectivity (Scheme 2, 19-21). In the synthesis of 21,
the formed insoluble substances caused the passivation of the
electrodes and decreased the yield. For non-α-substituted 4-MeO
styrene, it was catalyzed by III to yield the expected unmigrated
product 22. Because the oxidation potential of 23 (Ep/2 = 1.45 V)
is lower than its substrate (Ep/2 = 1.57 V), 23 is more prone to
oxidation than its reactants even under indirect electro-catalysis,
leading to the decreasing yields.
Our following task was to explore the hetero-cycloaddition
between different styrenes. After a series of tests using 1 and α-
methyl styrene (29) as substrates, we found that the cross [4+2]
annulation suffered from the production of 2, due to the lower
oxidation potential and stronger nucleophilicity of 1 than those of
29. To avoid the homo-cycloaddition and nonselective oxidation,
the styrene as cation radical precusor must be oxidized first and
not proceed homo-cycloaddition. Therefore, 4-methoxy α-tert-
butyl styrene (30) and 29 were chosen as the model substrates.
Thus, 31 was formed in 68% yield under the direct electrolysis
with an electrolyte solution of nBu4NBF4 in CH3CN/THF(1:3)
(details see SI, Table s1).
Next, the substrates scope of the cross-cycloaddition was
explored with respect to the 4π compounds. The electrolysis
exhibited well tolerance with a host of steric substitutes at α
position, such as neopentyl, diphenylmethyl, and adamantyl
(Scheme 3, 31-37). When it was replaced by the cyclic secondary
alkyl groups, the selectivity for homo-dimerization was gradually
surpassed over cross-cycloaddition with the decreasing of ring
member. By slowly adding 4π compounds and using 4 equiv 29,
the selectivity was significantly improved and 38-44 were
obtained in moderate to good yields with trace of homodimers
produced. Other less electron-rich diene were also suitable
substrates for this protocol (Scheme 3, 45-47). It is worth to note
that 46-47 without large steric group were also formed in 43% and
64% yields under the standard conditions. Furthermore, exocyclic
double bonds also underwent smooth cycloaddition with 30 and
provided the ring expansion product 48 in 53% yield and chrysene
For some weak electron-donating groups, such as phenyl,
tertiary butyl, and ethyl, poor yields were obtained even under the
catalysis of IV. But, the direct electrolysis provided relatively high
yields (Scheme 2, 24-26). α-Methyl styrene and 1,1-diphenylene
also gave the desired products in moderate yields (Scheme 2, 27-
28). As the process was affected by the electron-donating ability
of phenyl substituents, the electron-deficient styrenes were not
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10.1002/anie.201902315
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49 in 26% yield, which derived from further dehydro-aromatization. electrochemical behavior of 68 was studied by CV, and two
A variety of dienophiles bearing various electronic properties were
investigated and broad compatibility was demonstrated. In
general, the reactivity of dienophiles depends on the electron-
donating ability of the substituents. In the presence of electron-
donating groups, 50-52 were prepared in moderate to good yields.
Due to the lower nucleophilicity of electron-deficient stryrenes, its
electrolysis was performed at 45 °C to provide the desired
products in lower yields (Scheme 3, 53-57). Besides serving as
4π compound, 1,1-diphenylethylene also can function as 2π
compound in the reaction with 30 (Scheme 3, 47, 58). For non-α-
substituted styrenes with much lower nucleophilicity than α-
methyl styrene,[14] the transformation was also proceeded
smoothly and gave 1-aryl-substituted naphthalenes in moderate
yields (Scheme 3, 59-63).
anodic peaks were detected at 1.45 V and 1.90 V respectively,
representing the loss of two electrons in sequence. The CV curve
of 0.1 M Et4NOTs/CH3CN containing I and 68 indicated the
oxidation of 68 could also be mediated by I, just like the interaction
of I and 1 (details see SI, E3).
Scheme 5. The synthesis of intermediate and its transformation
Based on these,
a
proposed mechanism for the
electrochemical [4+2] annulation-rearrangement-aromatization
was outlined in Scheme 6 using 1 as model substrate. The
reaction is initiated by the anodic oxidation of TBPA to form a
stable radical-cation TBPA (TBPA•+). Then it oxidizes the
electron-rich styrene 1 through SET to give styrene radical cation
69 and TBPA (details see SI, E1). As a nucleophilic agent, 1
attacks radical cation 69 to yield 70 through radical cyclization [8b]
,
which is deprotonated to generate radical 72. Subsequent
electron and proton loss afford dehydrodimer 68. Similarly,
oxidized by TBPA•+, 6 losses one electron and proton to afford
carbon radical 73, which further oxidized to 74. By rearrangement,
the secondary carbon cation transforms into more stable tertiary
carbon cation 75. Compared with methyl, para-anisole migration
is favored due to its electron-rich property. Finally, the
oxydehydrogenation aromatization of 75 affords the desired
product 2.
Scheme 4. Gram-scale synthesis and product transformations
We also explored the synthetic utility of our method through
scale-up experiment and product transformations (Scheme 4).
For instance, we electrolyzed 15 mmol of 1 and obtained 2 in 63%
yield, with little decrease in yield and current efficiency in
comparison to the smaller scale. Meanwhile, 92% of I was also
recovered. Based on the previous works [15] in the Ni-catalyzed C-
O bond activation, 2 was converted to 64 and 65 in 75% and 83%
yield.[15c,15g] The cross-cycloaddition is also scalable and
demonstrates great synthetic value in constructing polycyclic
aromatic compounds, such as benzophenanthridines[16] and
benzofluorenones[17]. Compared with the 68% of yield and 4.7 F
mol-1 of current efficiency in 0.2 mmol scale, 1.85 g 2 was also
furnished in 61% yield and 4.9 F mol-1 under the electrolysis of 10
mmol of 30. The transformation of methyl group to formyl group
was achieved via the subsequent bromination and hydrolysis of
31. This important intermediate underwent TBHP-promoted
intramolecular carbonylation to provide 66 in 74% yield and
transformed into 67 in 80% yield via intermolecular nitrogenation.
It
provides
a
metal-free
protocol
to
synthesize
benzophenanthridine derivatives and benzofluorenones from
simple styrenes by three steps.
Scheme 6. Proposed mechanism
In summary, we have developed a straightforward protocol to
synthesis multi-substituted naphthalenes via electro-induced
[4+2] annulation-rearrangement-aromatization of styrenes. The
typical features of the reaction are its scalability, absence of high
valence substrates, oxidants and metals, and significant synthetic
value in constructing polycyclic aromatic compounds. Mechanism
Monitored by GC, the compound 68 was detected under the
homo-cycloaddition and its concentration increased first and then
descended (details see SI, G). So, 68, which is isolated in 50%
yield (Scheme 5, a), might be the intermediate of the reaction.
Using 68 as starting material, 2 was afforded in 62% yield
(Scheme 5, b), which further proved our hypothesis. The
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Chem. Rev. 2017, 117, 13230-13319; g) J. E. Nutting, M. Rafiee, S. S.
Stahl, Chem. Rev. 2018, 118, 4834-4885.
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this process.
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Keywords: metal- and oxidant-free • [4+2] annulation • styrenes
• rearrangement-aromatization • naphthalene derivatives
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Entry for the Table of Contents
COMMUNICATION
Yueyue Ma, Jufeng Lv, Chengyu Liu,
Xiantong Yao, Guoming Yan, Wei Yu,
and Jinxing Ye*
Page No. – Page No.
Electrochemical [4+2] Annulation
Rearrangement
Styrenes: Synthesis of Naphthalene
Derivativese
Aromatization
of
v
A novel electrochemical protocol to construct multi-instituted naphthalenes without oxidants, metals and high valence reactants is
reported. The dehydrodimer produced by electro-induced [4+2] annulation is the key intermediate, which undergoes
oxydehydrogenation and rearrangement-aromatization to yield final product.
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