a Reaction conditions: 1a (0.10 mmol),2a (0.15 mmol), Pd2(dba)3 (5
mol%) and ligand (11 mol%) in 1.0 mL of solvent under an
N2 atmosphere at 25 °C for 1 h.b Yield of 3a was determined by1H NMR spectroscopic analysis of the crude product
with 1,3,5-trimethoxybenzene as an internal standard.c Pd(PPh3)4 (10
mol%).d Pd(PPh3)4 (5
mol%).e Pd(PPh3)4 (2.5
mol%).f Pd(PPh3)4 (2.0
mol%). g Isolated yield of 3a on a 0.2
mmol-scale reaction.
Under the optimal reaction conditions in hand, the substrate scope of
benzofuran-derived azadienes 1 was investigated and the results
were summarized in Scheme 3. Benzofuran-derived azadienes 1bearing different N -protecting groups participated in the
reaction smoothly, leading to the corresponding products 3b(77% yield) and 3c (90% yield), respectively. In addition,
azadienes 1 bearing various electron-withdrawing groups
(fluoro, chloro, bromo, and cyano) or electron-donating (methyl and
methoxy) at the para , meta or ortho position of the
aryl ring were well accommodated. The target products3d -3n were delivered in 79-96% yields. It was
noteworthy that 2-Cl and 2-Me-substituted substrates 1l and1m underwent the reaction smoothly, affording the target
products 3l (80% yield) and 3m (79% yield),
respectively, which is presumably attributed to the steric effect.
Naphthyl and furanyl moieties also facilitated the formation of3o (92% yield) and 3p (90% yield), respectively. In
a similar fashion, the reactions of benzofuran-derived azadienes1 bearing various substituents (bromo, methoxy, and chloro) at
the C4-C7 positions of the benzofuran ring also efficiently proceeded to
afford the target products 3q -3u in 83-92% yields.
Scheme 3 Substrate scope for Pd-catalyzed [4 + 4]
cycloaddition of benzofuran-derived azadienes 1 and dimethyl
malonate derivative 2a .
Then, the protocol generality was explored by extending the substrate
scope of homo-TMM donors 2 (Scheme 4). The analog of2a , that is, substituted malononitrile 2b , exhibited a
good reactivity to form the target products 3v (91% yield) and3w (75% yield), respectively. Compared to the symmetrical
malonate-type substrates, the cyano-substituted ester enolate2c obviously deteriorated the reaction efficiency, leading to3x in a moderate yield (41% yield). Unfortunately, the more
sterically hindered t -butyl ester counterpart 2d was
used, and only a trace amount of the desired 8-membered product3y was detected. The bis(phenylsulfonyl)methane derivative2e also proved to be unreactive in the [4 + 4]
cycloaddition reaction. The molecular structure of compound 3awas further confirmed by the X -ray single crystal
crystallographic determination (CCDC 2235088, see the SI for details).
Scheme 4 Substrate scope for Pd-catalyzed [4 + 4]
cycloaddition of benzofuran-derived azadienes 1 and dimethyl
malonates derivatives 2 .
To further demonstrate the versatility and effectiveness of this
catalytic system, a gram-scale reaction of compound 3a with a
2.5 mol% catalyst loading was performed, providing an acceptable yield
(91% yield) (Scheme 5a). Subsequently, the 4-toluenesulfonyl (Ts) group
of 3a could be removed in the presence of Mg in MeOH, yielding
the corresponding benzofuro[3,2-b ]azocine 4 in 86%
yield.[19]
Scheme 5 Demonstration of the synthetic utility.
Furthermore, the catalytic asymmetric [4 + 4] cycloaddition version
has also been explored, and several chiral ligands were screened (see
Table S1 for the details). The reaction provided moderate
enantioselectivity (61% ee) with
Pd2(dba)3 as the catalyst in the
presence of chiral ligand L* (Scheme 6).
Scheme 6 The asymmetric [4 + 4] cycloaddition.
A mechanism was proposed to illustrate the Pd-catalyzed [4 + 4]
cycloaddition (Scheme 7). First, the initial oxidative addition of a
catalytically active Pd(0) species with substrate 2a and the
deproton process by the tert -butoxy anion forms the key
PdII-π-allyl complex intermediate A . Then,
the attack by the carboanion of intermediate A to azadiene1a provides aromatization intermediate B .
Subsequently, the addition of a nitrogen anion to π-allyl-palladium in
aromatization intermediate B affords the eight-membered ring3 by intramolecular cyclization and regenerates the active
palladium catalyst for the next catalytic cycle.
Scheme 7 Proposed mechanism.
Conclusions
In summary, we have developed an efficient Pd-catalyzed [4 + 4]
cycloaddition of homo-TMM all-carbon 1,4-dipoles with benzofuran-derived
azadienes, affording various benzofuro[3,2-b ]azocines in good
to excellent yields (up to 96% yield) with exclusive
regioselectivities. The reaction proceeded with broad substrate scope
and excellent functional group tolerance. The high-order cycloaddition
of homo-TMM all-carbon 1,4-dipoles provided a convenient and mild route
to the synthesis of 8-membered rings. Further investigations on the
asymmetric [4 + 4] cycloaddition are currently underway in our
laboratory.
Experimental
Under a nitrogen atmosphere,
benzofuran-derived azadienes 1 (0.2 mmol) and
Pd(PPh3)4 (15.4 mg, 0.005 mmol) were
added sequentially into a flame-dried Schlenk tube equipped with a
magnetic stir bar. The tube was evacuated and back-filled with nitrogen
for three times. Then the anhydrous DCM (2.0 mL) was added viasyringe sequentially and the resulting mixture stirred at 25 ℃. Then,
TMM precursors 2 (0.3 mmol) was added. After completion, the
mixture was concentrated and purified by column chromatography
(petroleum ether/ dichloromethane
= 4:1) to give the corresponding cycloadducts 3 .
Supporting Information
The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement
We are grateful for financial support from National Key Research and
Development Program of China (2021YFA0804900), the NSFC (22171082,
21971062), the Leading Innovative and Entrepreneur Team Introduction
Program of Zhejiang (No. 2022R01007), and Shanghai Frontiers Science
Center of Optogenetic Techniques for Cell Metabolism.
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