Rapid pseudo five-component synthesis of intensively blue luminescent 2,5-di(hetero)arylfurans via a Sonogashira–Glaser cyclization sequence

Summary 2,5-Di(hetero)arylfurans are readily accessible in a pseudo five-component reaction via a Sonogashira–Glaser coupling sequence followed by a superbase-mediated (KOH/DMSO) cyclization in a consecutive one-pot fashion. Besides the straightforward synthesis of natural products and biologically active molecules all representatives are particularly interesting due to their bright blue luminescence with remarkably high quantum yields. The electronic structure of the title compounds is additionally studied with DFT computations.


General considerations
All cross coupling reactions were carried out in oven-dried 80 mL microwave vessels (CEM Corporation) by using septa and syringes under nitrogen atmosphere.
Commercial-grade reagents were used as supplied without further purification and were purchased from Sigma-Aldrich Chemie GmbH, ABCR GmbH & Co. KG Darmstadt. The spots were detected with UV light at 254 nm and by using aqueous potassium permanganate solution. 1 H, 13 C, and 135-DEPT NMR spectra were recorded on a Bruker Avance III -600 spectrometer. CDCl 3 and DMSO-d 6  Düsseldorf. The UV-vis spectra were recorded with a Perkin Elmer Spectrometer (Lambda 19). The emission data were carried out on Perkin Elmer Luminescence Spectrometer (LS55). The quantum yield was determined as followed: ( ) ( ) [1] with p-terphenyl ( f

2,5-Di-m-tolylfuran (2c) [12]
The synthesis was carried out according to the general procedure with 436 mg (2.00 mmol) of m-iodotoluene (3c) (Sigma-Aldrich). The crude product was absorbed onto Celite® and purified by column chromatography on silica gel with n-hexane as an eluent to give 160 mg (0.65 mmol, 65%) of the desired product as colorless solid.

2,5-Di-p-tolylfuran (2d) [12]
The synthesis was carried out according to the general procedure with 436 mg (2.00 mmol) of p-iodotoluene (3d) (Sigma-Aldrich). The crude product was absorbed onto Celite® and purified by column chromatography on silica gel with n-hexane as an eluent to give 100 mg (0.40 mmol, 40%) of the desired product as colorless solid.

2,5-Bis(3-methoxyphenyl)furan (2f) [12]
The synthesis was carried out according to the general procedure with 468 mg (2.00 mmol) of m-methoxyiodobenzene (3f) (Sigma Aldrich). The crude product was absorbed onto Celite® and purified by column chromatography on silica gel with nhexane as an eluent to give 110 mg (0.39 mmol, 39%) of the desired product as beige solid.

UV-absorption and fluorescence data of compounds 2a-2o
E 0-0 was determined with the cross-section of the absorption and emission spectra ( ).

CV data of compounds 2a-2o
The oxidation potentials were determined with ( ) The HOMO energy of all compounds were calculated with the first oxidation potential (vs NHE) of the corresponding molecule.

( ( ) )
The summand 4.6 eV is the ferrocene HOMO energy value related to the vacuum

xyz-Coordinates and dihedral angles of compounds 2a-2o
The geometries of the furans were optimized in DFT calculations with the B3LYP [5] functional and the 6-311G(d,p) [6][7][8][9] basis set as implemented in the program package Gaussian09 [10]. The minima were confirmed by analytical frequency analysis. To find the global minima the DFT calculation started from the "trans" configuration with a dihedral angle of 90° (Fig. 1).