However, there are disadvantages in terms of environmental impact due to the requirement of harsh conditions such as UV light irradiation. Visible light photocatalysis has proven to be a powerful tool for various organic syntheses due to its environmental strength. Irradiation with visible light is attractive because this reaction can transfer sensitive functional groups that are easily degraded by UV irradiation.
Many variants using visible light photocatalysis have been reported for significant transformations in organic synthesis. The visible light-induced [2+2] photocycloaddition of alkyne with alkene was achieved via an energy transfer mechanism. Also, conjugated diene products were directly accessible via [2+2] cycloaddition of enyne followed by an electrocyclic ring opening reaction.
In contrast to previous examples of [2+2] photocycloadditions between alkyne and alkene, this strategy was achieved under visible light irradiation. Consequently, this method is expected to play an important role in the synthesis of complex organic molecules.
Introduction
Cyclobutene
The most efficient and direct route to the preparation of cyclobutene is [2+2] cycloaddition of alkyne with alkene.3 [2+2] cycloaddition of alkyne with an alkene is broadly categorized as a photochemical reaction and catalyzed reaction. Using this approach, a lot of methods using transition metal4,5,6 and Lewis acid7,8 as catalysts have been reported. Lewis acid catalysts have been developed for the [2+2] cycloaddition.7,8 In this approach, Lewis acid-catalyzed reaction usually requires a combination of electron-rich and electron-deficient substrates.
The use of strong Lewis acid resulted in the formation of cyclobutene derivatives only with low efficiency. Unfortunately, Lewis acid-catalyzed [2+2] cycloadditions require the presence of polar functional groups on the substrates. Various transition metals have been developed for the [2+2] cycloaddition, such as gold, rhodium, cobalt and nickel.
Recently, many methodologies have been reported using a highly efficient complex of Co and Ni (Scheme 2a and b).4 The Hilt group reported cobalt-catalyzed [2+2]. UV-induced [2+2] photochemical cycloadditions are well-established methodologies for four-membered carbocyclic ring construction. [2+2] photocycloadditions proceed through energy transfer from an excited-state catalyst to the substrate.
Visible-Light Photocatalysis
The reaction mixture was washed with a saturated solution of NH4Cl and extracted with CH2Cl2. Then the reaction temperature was raised to room temperature and the mixture was stirred for 6 hours. The reaction mixture was stirred at 135°C overnight, and the heterogeneous mixture was cooled to room temperature.
The reaction mixture was stirred at room temperature until TLC indicated complete consumption of the starting material. Iodoethane (1.0 equiv) was added dropwise at -78°C, and the reaction mixture was stirred at room temperature for 1 hour. After stirring for 30 min, trimethylsilyl chloride (2.2 equiv.) was added dropwise at -78°C, and the reaction mixture was stirred at room temperature overnight.
The appropriate amine (x equiv) in solvent was added dropwise at 0°C, and the reaction mixture was stirred at room temperature. The reaction mixture was then irradiated with 12W blue LED lamp at room temperature (maintained with a cooling fan).
Results and Discussion
Visible-Light-Induced intermolecular [2+2] cycloaddition of alkyne
First, we engineered the reaction of alkyne 5a with maleimide 6a under visible-light photocatalytic conditions to form cyclobutene derivative ( Table 1 ). Our efforts began with the reaction of di-p-tolylacetylene 5a with N-methylmaleimide 6a under visible light irradiation in the presence of a variety of photocatalysts (Table 1, entries 1–10). Further optimization of the equivalents of photocatalyst and reaction concentration caused an improved yield (entry 16).
Finally, control experiments performed in the absence of either light or catalyst confirmed no product formation, supporting that the reaction was facilitated by visible light photocatalysis (entries 17 and 18). To demonstrate the reaction pathways, we compared the physical properties of maleimide 6a with those of photocatalyst, Ir[dF(CF3)ppy]2(dtbbpy)PF6 (Figure 5a). No correlation was observed between the redox potential of the photocatalysts and the formation of 7a.
Considering these results, we speculated that the reaction occurs via the energy transfer process rather than electron transfer. First, visible-light irradiation of the iridium photosensitizer produces the long-lived excited state of the iridium complex. The reaction of the di-substituted maleimide also proceeded smoothly to provide the desired product 7e , suggesting that steric hindrance was tolerated in the reaction.
In the case of N -arylmaleimide, electron-withdrawing substituent gave the desired cyclobutene 7f in moderate yields. First, we aimed to form alkyne 5h to prove that substrates with other alkyl substituents, besides cyclohexanol, continued to give the desired product. The mono-thionation of N-methylmaleimide proceeded smoothly using Lawesson's reagent to give the desired product 6j.
The desired alkene 6k was obtained through bromination of 4′-methylacetophenone followed by sodium sulfonate-mediated reaction. Furthermore, we hoped to afford allene derivatives, the structure of which was of great importance in [2+2] cycloaddition reactions.19 The desired allene 6l was prepared starting from vinyl bromide 11. Disappointingly, we found that these substrates (5i, 6j-l ) did not provide the desired cyclobutene products except the alkyne 5h.
Visible-Light-Induced synthesis of conjugated diene derivatives
Additionally, sulfonamide 8k was formed by reaction of the appropriate aniline with sulfonyl chloride, which was prepared from 4-methylstyrene ( Scheme 17c ). The progress of the reaction was monitored by thin layer chromatography (TLC) using Merck TLC Silica gel 60 F254 plates (0.2 mm thickness). The mixture was cooled to room temperature and the solvent was evaporated under reduced pressure.
The reaction mixture was diluted with water and extracted with CH 2 Cl 2 , and the combined organic layer was washed with brine. The reaction mixture was quenched by adding a saturated solution of NH4Cl and extracted with ethyl acetate. The mixture was stirred at room temperature for 3 hours and the solvent was evaporated under reduced pressure.
The reaction mixture was quenched by the addition of saturated NH 4 Cl solution and extracted with Et 2 O. Jones reagent (2.5 M in water, 3.0 equiv) was added dropwise and the mixture was stirred at room temperature for 1 hour. The mixture was stirred at room temperature until TLC indicated complete consumption of the starting material.
The mixture was stirred at room temperature until TLC indicated complete consumption of starting material. After completion of the reaction, as indicated by TLC, the solution was concentrated under reduced pressure.
Conclusions
Experimental
A solution of PPh3 (3.0 equiv) in CH2Cl2 was added dropwise, keeping the temperature below 0 °C. To a solution of the appropriate acid (1.0 equiv) and Et3N (1.2 equiv) was dissolved in solvent (x M) under nitrogen and cooled to 0 °C. Sulfuryl chloride (2.0 equiv, per styrene) was added dropwise to stirred anhydrous DMF (2.0 M) at 0 °C under a nitrogen atmosphere.
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A copper-catalyzed ligand-free cross-coupling reaction of alkynes with aryl iodides and vinyl halides. General synthesis of alkenyl-substituted benzofurans, indoles, and isoquinolones by cascade palladium-catalyzed heterocyclization/oxidative coupling Heck. Catalytic Transformations of Alkynes to α-Alkoxy or α-Aryl Enolates: Mannich Reactions with Cooperative Catalysis and Evidence for Nucleophilic-Directed Chemoselectivity.
