A Conjugated Microporous Polymer for Palladium‐Free, Visible Light‐Promoted Photocatalytic Stille‐Type Coupling Reactions

The Stille coupling reaction is a versatile method to mainly form aromatic C—C bonds. However, up to now, the use of palladium catalysts is necessary. Here, a palladium‐free and photocatalytic Stille‐type coupling reaction of aryl iodides and aryl stannanes catalyzing a conjugated microporous polymer‐based phototcatalyst under visible light irradiation at room temperature is reported. The novel coupling reaction mechanism occurs between the photogenerated aryl radical under oxidative destannylation of the aryl stannane, and the electron‐activated aryl iodide, resulting into the aromatic C—C bond formation reaction. The visible light‐promoted Stille‐type coupling reaction using the polymer‐based pure organic photocatalyst offers a simple, sustainable, and more economic synthetic pathway toward palladium‐free aromatic C—C bond formation.


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(Quantachrome Instruments). The polymers were degassed at 100 °C for 24 h under vacuum before analysis. Data was obtained using QuadraWin software from Quantachrome Instruments. Pore size distributions and pore volumes were calculated from the adsorption branches of the isotherms using Quenched Solid Density Functional Theory (QSDFT, N2, evaluating carbon adsorbent with slit pores). The BET surface area was obtained based on data points received from 0<P/P0<0.25 and the non-local density functional theory (NLDFT) equilibrium model was employed as the BET model fitting. The quantum mechanical calculations were performed with the Gaussian 09 program suite on a cluster system. The thermodynamic data of 4-iodonitrobenzene were obtained by applying a vibrational calculation on pre-optimized geometries on the semi-empirical level with the PM6 method at 1 atm and 298.15 Kelvin. The HOMO/LUMO molecular orbitals were calculated by applying density functional theory level with the Becke, three-parameter, Lee-Yang-Parr B3LYP hybrid functional and a 6-31G(d) split valence basis set. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was conducted on a Jobin-Yvon Activa M spectrometer.
Time-resolved photoluminescence (TRPL) on a nanosecond timescale was taken with a Streak Camera System (Hamamatsu C4742) in slow sweep mode. The excitation wavelength of 400 nm was provided using the frequency-doubled output of a commercial titanium:sapphire amplifier (Coherent LIBRA-HE, 3.5 mJ, 1 kHz, 100 fs). The data was fitted with a biexponential decay function.

Bromination of azulene
According to the literature 1 500 mg (3.9 mmol, 1 eq.) azulene was dissolved in 50 ml THF and cooled to 0 °C. 1.53 g (8.6 mmol, 2.2 eq.) of N-bromosuccinimide were dissolved in 30 ml THF and cooled. The cold NBS solution was added dropwise under exclusion of light over a time period of one hour. The mixture was allowed to warm up to room temperature and stirred for 24 hours. The crude mixture was extracted by dichlormethane and washed several times with water. The combined organic phases were dried and concentrated.

Synthesis of 1,3-bis(phenyl)azulene (Az-Ph 2 )
A 25 ml Schlenk tube was filled under argon with 100 mg 1,3-dibromoazulene (0.35 mmol, 1 eq), 86 mg phenylboronic acid (0.70 mmol, 2 eq), 25 mg Pd(PPh 3 ) 4 (6 mol%). After dissolving the mixture in 4 ml THF, 1.5 ml of aqueous potassium carbonate (341 mg, 2.47 mmol, 7 eq) was added. The reaction mixture was left at reflux temperature while it was stirring about 24 h. After cooling down to room temperature, the reaction mixture was washed with brine and water, and extracted three times with 100 ml of dichloromethane. The organic phase was filtered over Celite® to remove the catalyst residue. After drying over MgSO 4. the solvent was removed under reduced pressure. The crude product was purified S4 with column chromatography with hexane as eluent. 36 mg (37%) of dark blue solid was obtained. 1
After 24 hours additional 67 mg 1,3-dibromoazulene (0.23 mmol, 0.5 eq) was added to the reaction mixture. After another 24 hours, the last step was repeated and again 67 mg 1,3dibromoazulene (0.23 mmol, 0.5 eq) was added to the reaction mixture, while keeping the reaction for 24 hours at 90°C. Finally the reaction mixture was refluxed at 145 °C for further 24 hours. Each addition step was accompanied with a change of color from pale green, dark blue, to black. After cooling down to room temperature, the reaction mixture was filtered, washed several times with brine, water, and several times with dichloromethane and THF.
Additional 61 mg phenylboronic acid (0.5 mmol) were added as endcapping agent. The mixture was again refluxed for 6 h before cooling down to room temperature. The reaction mixture was washed with brine and water, and extracted three times with 100 ml of dichloromethane. The organic phase was filtered over cellite to remove traces of precipitations of the catalysts. After drying over MgSO 4 the solvent was removed under reduced pressure. The crude product was purified with column chromatography. The final product was precipitated in cold methanol. 83 mg (82%) of dark green powder was obtained. After the reaction was finished the catalyst was removed by filtration and the raw product was purified by column chromatography with hexane/ethylacetate (5:1 volume ratio) as eluent.

Radical trapping experiment with N-tert-butyl-α-phenylnitrone (PBN)
A 20 ml vial was filled under argon atmosphere with 5 mg P-Az-B, 65 µl 2- while it was stirring for 24 hours. After the reaction was finished the catalyst was recovered by membrane filtration and the raw product was purified by column chromatography with hexane/ethylacetate (5:1 volume ratio) as eluent. The same batch of P-Az-B was reused in 5 consecutive runs by the above mentioned procedure.  Figure S1: Solid state CP-MAS 13 C-NMR spectrum of P-Az-B at rotation frequency of 10 kHz (side bands*).

Apparent Quantum Yield Measurements
In order to determine the apparent quantum yield, the photocatalytic Stille coupling was conducted by irradiating 5 mg P-Az-B, 50 mg (0.2 mmol) 4-iodonitrobenzen, and 63µl 2-(tributylstannyl)furan with a blue LED (460 nm, 0.26 W/cm 2 ). The conversion was determined after 1 hour by column chromatography. The illumination area was 6.76 cm 2 and the LED intensity was measured by a Coherent Lab-Max energy meter. The apparent quantum yield was estimated by following equation: