Photocatalytic Arylation of Alkenes, Alkynes and Enones with Diazonium Salts

Teaching old dogs new tricks: Visible light photoredox catalysis improves the classic Meerwein arylation protocol significantly and allows the light-controlled arylation of alkenes, alkynes and enones by diazonium salts.

addition of diethyl ether. Further purification was achieved by repeating the procedure of dissolving the solid in acetone and precipitating the product by addition of diethyl ether, until the product was obtained as a white solid. The aryl diazonium salt 1 was filtered, washed three times with diethyl ether and dried in vacuo.

General procedure for photocatalytic arylation reactions
A 5 mL reaction vessel with a magnetic stirring bar was equipped with Ru(bpy) 3 Cl 2 ·6H 2 O (1 mol%, 1.5 mg), aryl diazonium tetraflouroborate 1 (1 eq., 0.2 mmol), unsaturated compound 2 (5 eq., 1.0 mmol) and dry DMSO (1 mL). The mixture was degassed by "freeze-pump-thaw" technique (three cycles) and irradiated with blue high power LEDs ( max = 455 ± 15 nm, P = 3 W) for two hours at a temperature of 20°C. The yield of the reaction was determined via pathway a) or b). a) GC-analysis: The reaction mixture was filtered, and a sample for GC analysis was prepared by diluting the reaction mixture (500 µL) with DMSO (250 µL) and a stock solution of naphthalene (250 µL = 15 mg/mL in DMSO) as the standard. The yield was determined by integration of the peaks in the gas chromatogram.

b) Isolation and purification:
The reaction mixture was diluted with water (4 ml) and extracted with diethyl ether (3 x 5 mL). The combined organic layers were concentrated in vacuum. Traces of water were removed by lyophilization. Purification of the crude product was achieved by preparative thin-layer chromatography on silica gel 60 F254 using a mixture of petroleum ether and ethyl acetate as eluent. S4

Kinetics
Monitoring of the reaction kinetics for the [Ru(bpy) 3 ] 2+ -mediated arylation of styrene showed that initially the trans isomer is formed as the major product. A partial isomerization to the cis isomer was observed upon irradiation with blue light ( max = 455 ± 15 nm) leading to a product distribution of 5:2 (trans:cis) after two hours of reaction time. Further irradiation does not lead to an increase of product formation. Nitrogen evolution starts as soon as the light is switched on and ends immediately after removal of the radiation source ( Figure 1). In case of eosin Y as the photocatalyst, the yields are generally lower and the trans isomer is formed exclusively. The nonappearance of photo isomerization can be explained by the lower energy of the green light (high power LED, max = 520 ± 15 nm) used to excite eosin Y.

Mechanistic Model and Trapping Experiments
Based on the results of our coupling and trapping experiments as well as the consideration of literature-known data, a mechanistic model for the visible light-mediated arylation of unsaturated compounds catalyzed by [Ru(bpy) 3 ] 2+ was developed (Scheme 1). [3,4,5,6] [Ru(bpy) 3  the radical with TEMPO and the resulting adduct 7 has been detected. The highly reactive aryl radical attacks unsaturated compound 2 forming adduct radical 5 which is stabilized by its benzylic position. Again, a trapping experiment with TEMPO led to product 8 indicating the existance of a radical species. Intermediate radical 5 can be oxidized to the carbenium ion 6 either by re-donating an electron to the photocatalyst and simultaneously closing the catalytic cycle or by reducing another diazoinium salt molecule 1 initiating a radical chain mechanism. A control experiment that was performed by irradiating the mixture for ten minutes followed by stirring for another two hours in the dark showed that the radical chain must be very short or does even not occur since the yield did not increase after switching off the light. Starting from carbenium ion 6, the product 3 is formed after deprotonation. The existence of 6 has been proved by a trapping experiment with MeOH as the nucleophile leading to product 9.
In all, a vinylic H-atom has been replaced by an aryl residue. The utilization of BF 4 as a nonnucleophilic counter-ion is important to avoid addition products of type 8. However, the systematic addition of a nucleophile may be useful for the reaction because it is able to form a second bond in this type of reaction. Nevertheless, the nucleophile must not be too basic since the presence of strong bases leads to decomposition of the diazonium salts.