The reaction of two fully substituted or multi-substituted precursors to achieve the cyclization of the benzene ring in one step results in multi-substituted benzene or heteroaromatic rings. This reaction is highly efficient and can prepare many useful multi-substituted aromatic compounds.
The method of cyclization using alkenyl ketones and reactive alkynes has been well applied, leading to the preparation of many biologically active molecules. Recent studies have found that N-phosphoryl alkynes are excellent precursors for the preparation of multi-substituted benzene rings with high regioselectivity, making them good substrates for synthesizing indoles and nitrogen-containing aromatic rings.
Reaction Mechanism
This reaction mechanism is a “two-step reaction mechanism”, where the reaction first yields a cyclobutenone intermediate, which then opens to form the alkenyl ketone, leading to the benzene ring. Under photocatalysis, α-diazoketones undergo Wolff rearrangement to produce alkenyl ketones, which then react with N-phosphoryl alkynes in a [2 + 2] cycloaddition to generate the cyclobutenone intermediate. Subsequently, under light or thermal catalysis, a reversible four-electron electrocyclization cleavage occurs to yield alkenyl ketones, followed by a 6-π electrocyclic rearrangement to produce the final product.
Various alkenyl, aryl, and heterocyclic aromatic α-diazoketones can be used to prepare fused aromatic hydrocarbons and hybrid aromatics through this reaction. In some substrate reactions, a mixture of cyclobutenone intermediates and product aromatics is obtained, which well demonstrates the “two-step mechanism”. In such cases, heating the mixture for a period can yield phenolic aromatic products.
The characteristic of this reaction is the use of N-phosphoryl alkynes as substrates, which are significantly faster than the previously used N-sulfonyl and N-acyl substituted alkynes. This reaction employs a flow photocatalytic reactor, allowing the reaction substrates to be fully illuminated through FEP (fluorinated ethylene-propylene) tubing. The reaction is more thorough than typical illuminated reactions in reaction vessels. Moreover, the products generated through the tubing directly enter the receiving container without decomposing due to continued exposure to light.
Operational Steps
A. 2-Diazo-1-(2,4-dimethylphenyl)ethan-1-one (1).
A 250-mL, three-necked, round-bottomed flask (Note 1) equipped with a 25 x 10 mm, Teflon-coated, octagonal magnetic stir bar is placed in a glove box. Solid LiHMDS(1.81 g, 13.5 mmol, 1.1 equiv) (Note 2) was weighed out in the glove box, added to the flask, and three septa are attached. The flask is removed from the glove box and equipped with an argon inlet adapter and a 50-mL pressure-equalizing addition funnel fitted with a rubber septum. The third septum is fitted with a thermocouple temperature probe. Tetrahydrofuran (35 mL) (Note 3) is added by syringe and the solution cooled to -78 °C in a dry ice-acetone bath (Note 4). A solution of 2′,4′-dimethylacetophenone (1.82 mL, 1.81 g, 12.2 mmol, 1.0 equiv) (Note 5) in 27 mL of THF is added via the addition funnel over 15 min while the internal temperature of the reaction mixture is kept below -70 °C. The addition funnel is rinsed with two 5-mL portions of THF. The reaction mixture is stirred at -78 °C for 1 h and then 2,2,2-trifluoroethyl trifluoroacetate (1.97 mL, 2.88 g, 14.7 mmol, 1.2 equiv) (Note 6) is added rapidly in one portion via syringe. After 30 min, the yellow solution is poured into a 500-mL separatory funnel containing 120 mL of diethyl ether and 120 mL of 5% aqueous HCl solution. The aqueous layer is separated and extracted with two 60-mL portions of diethyl ether. The combined organic layers are washed with 120 mL of saturated NaCl solution, dried over 10 g of anhydrous MgSO4, and filtered through a 30-mL sintered glass Büchner funnel (medium porosity, 30 mm diameter). The MgSO4 is washed with diethyl ether (3 x 10 mL) and the combined filtrate is concentrated by rotary evaporation (20 °C, 20 mmHg) to afford 4 g of a yellow oil. This material is immediately dissolved in 15 mL of acetonitrile and transferred via glass funnel (9 cm diameter) to a 250-mL single-necked, round-bottomed flask equipped with a 25 x 10 mm, Teflon-coated, octagonal magnetic stir bar (Note 7). The original flask is rinsed with acetonitrile (2 x 5 mL), which is transferred via glass funnel (9 cm diameter) to the 250-mL flask. water (0.22 mL, 12.2 mmol, 1.0 equiv) and triethylamine(2.56 mL, 1.86 g, 18.4 mmol, 1.5 equiv) (Note 8) are added via syringe. A 50-mL pressure-equalizing addition funnel is attached. A solution of 4-acetamidobenzenesulfonyl azide (4.41 g, 18.4 mmol, 1.5 equiv) (Note 9) in 25 mL of acetonitrile is then added via the addition funnel over 15 min. The addition funnel is rinsed with 5 mL ofacetonitrile and the reaction flask is wrapped with aluminum foil. The resulting dark yellow solution is stirred at room temperature for 2 h, during which time a white precipitate appears (Figure 1).
The solution is then concentrated by rotary evaporation (40 °C, 20 mmHg) to provide 10.7-16.6 g of a thick yellow suspension. This material is diluted with 1:1 diethyl ether-hexanes (100 mL) and filtered through 5 g of Celite in a 30-mL sintered glass funnel (medium porosity, 30 mm diameter) into a 250-mL, round-bottomed flask. The solid material is washed with 1:1 diethyl ether-hexanes (3 x 15 mL), and the filtrate is then concentrated by rotary evaporation (20 °C, 20 mmHg) to yield 2.8-4.7 g of a dark yellow oil.
This material is dissolved in a minimum amount of 1:7 ethyl acetate-hexanes (ca. 6 mL) and loaded onto a column (64 mm diameter) of 200 g of silica gel (Note 10) prepared as a slurry in 1:7 ethyl acetate-hexanes. Elution with 1:7 ethyl acetate-hexanes (35 mL fractions collected in 20 x 150 mm test tubes) affords the product in fractions 21-57. These fractions are combined and the solvent is removed by rotary evaporation (20 °C, 20 mmHg). Further concentration at room temperature, 0.05 mmHg for 1 h provides 1.77 g (83%) of diazo ketone 1as a yellow crystalline solid (Notes 11 and 12).
B. Diethyl benzyl(3-hexyl-4-hydroxy-6,8-dimethylnaphthalen-2-yl)phos-phoramidate (2).
Figure shows the continuous flow photochemical reactor employed for this reaction. Fluorinated ethylene propylene (FEP) tubing, o.d. = 1.59 mm, i.d. = 0.76 mm, length = 1520 cm (Note 13), is wrapped around a quartz immersion well in tightly packed coils leaving 90 cm of tubing free at each end. The length of tubing wrapped around the well is 1340 cm (internal volume = 6.1 mL). The ends of the tubing are secured to the well with Teflon tape. The top end of the tubing is fitted with a nut, ferrule, and a thread to a female Luer adapter for attachment to a syringe as shown in Figure 2. The bottom end of the tubing is connected through a rubber septum to a 100-mL, round-bottomed flask equipped with an argon inlet needle and a needle vent.
The receiving flask is wrapped in aluminum foil (Figure 3). The immersion well is connected to recirculating tap water via PVC tubing. A Pyrex filter and a Hanovia 450 W medium-pressure mercury lamp are placed inside the immersion well (Note 14).
A 100-mL pear flask equipped with a rubber septum and argon inlet needle is charged with 2-diazo-1-(2,4-dimethylphenyl)ethan-1-one (1) (1.56 g, 8.90 mmol, 1.2 equiv), diethyl benzyl(oct-1-yn-1-yl)phosphoramidate2(2.63 g, 7.48 mmol, 1.0 equiv), and CH2Cl2 (30 mL) (Notes 15 and 16). The yellow solution is degassed via three freeze-pump-thaw (-196 °C, 0.05 mmHg) cycles. A 20-mL plastic syringe is charged with 5 mL of CH2Cl2, the syringe is connected to the Luer adapter, the lamp is turned on (Note 17), and the 5 mL of CH2Cl2 is pumped through the system at a rate of 0.58 mL/min (Note 18). Approximately one-half of the reaction mixture is transferred to a 20-mL plastic syringe and pumped through the system at a rate of 0.58 mL/min (calculated residence time = 10.4 min). The syringe is charged with the other half of the reaction solution which is then pumped through the system. The total time required for the injection of the 30-mL reaction solution is 50 min. The 100-mL pear flask is then rinsed with two 0.6-mL portions and one 9 mL portion of CH2Cl2 with each portion pumped through the tubing at a rate of 0.58 mL/min. The lamp is turned off, the aluminum foil on the 100-mL collection flask is removed, and the orange solution is concentrated by rotary evaporation (20 °C, 20 mmHg) to afford 3.97 g of dark orange oil (Note 19). This material is dissolved in 50 mL of toluene (Note 20) and the flask is equipped with a 20 x 10 mm, Teflon-coated, oval magnetic stir bar and an 11-cm Liebig condenser fitted with an argon inlet adapter. The orange solution is heated at reflux in an oil bath for 5 h and then concentrated to afford 3.70 g of orange solid (Note 21). This material is dissolved in 15 mL of CH2Cl2 and concentrated onto 10 g of silica gel. The resulting free-flowing powder is deposited onto a column (80 mm diameter) of 300 g of silica gel prepared as a slurry in 1:2 ethyl acetate-hexanes. Elution is carried out with a gradient of 1:2 to 4:1 ethyl acetate-hexanes, with a ratio of 1:2 EtOAc-hexanes used for fractions 1-81, 1:1 for fractions 82-121, 2:1 for fractions 122-151, and 4:1 for the remaining fractions. Fractions (35 mL) are collected in 20 x 150 mm test tubes. The desired product is obtained in fractions 70-164. These fractions are combined and the solvent is removed by rotary evaporation (20 °C, 20 mmHg). Further concentration at room temperature, 0.1 mmHg for 10 h provides 3.15 g (88%) of 2 as a pale yellow solid.
This content is adapted from: Org. Synth. 2016, 93, 127-145，
DOI: 10.15227/orgsyn.093.0127