Polycyclic aromatic azomethine ylides: a unique entry to extended polycyclic heteroaromatics† †Electronic supplementary information (ESI) available. CCDC 1022817 and 1022816. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc02793k Click here for additional dat

Based on polycyclic aromatic azomethine ylides, a metal-free “cycloaddition-planarization-sequence” is proposed, providing a unique entry to nitrogen-containing polycyclic aromatic hydrocarbons.

A solution of the respective 2,6-dibromoaniline 24a -24c (3.26 mmol, 1.00 eq.) and 1hydroxy-3H-2,1-benzoxaborole (1.30 g, 9.78 mmol, 3.00 eq.) in a mixture of toluene (80 mL), ethanol (16 mL) and 2 M potassium carbonate solution (31 mL) was purged with argon for 30 min. After tetrakis(triphenylphosphine)palladium(0) (0.38 g, 0.32 mmol , 10 mol%) was added, the mixture was refluxed in a preheated oil bath (110 °C) overnight. The reaction mixture was allowed to reach room temperature and the organic layer was separated. The aqueous phase was extracted with diethylether (50 mL, three times) and the combined organic layers were washed with brine and dried over magnesium sulfate afterwards. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica (hexane:ethyl acetate; 1:2) to afford the title compound 25a -25c. NH  The crude product was redissolved in toluene (anhydrous, 10 mL) and heated to 90 °C under argon. Then a solution of triphenylcarbenium tetrafluoroborate (0.54 g, 1.63 mmol, 1.10 eq.) in anhydrous acetonitrile (5 mL) was added dropwise. Stirring is continued for 30 min and solvents were removed under reduced pressure. The residue was dissolved in a minimum amount of DCM (~10 mL) and precipitated in hexane (250 mL). The crude product was washed with a mixture of hexane and DCM (100mL, 9:1) and precipitated again to obtain the title compounds (20a -20c) as yellow solids. Yields over two steps are given in Figure S2.   (5) A dry and inert 25 mL Schlenk tube was charged with initial cycloaddition product 4 (0.10 g, 0.21 mmol, 1.00 eq.) and DDQ (0.06 g, 0.28 mmol, 1.30 eq.). The Schlenk tube was sealed with a septum and evacuated and refilled with argon three times. Afterwards anhydrous toluene (2 mL, purged with argon) was added and the reaction mixture was stirred for 30 min.
The reaction mixture was filtered over a small plug of silica and the product was eluted with a mixture of ethyl acetate/DCM (2/1) and pure ethyl acetate till no fluorescence was observed in the filtrate. After removal of the solvents, the title compound 5 was obtained as a slight yellow powder (0.081 g, 0.17 mmol, 82%).  In a dry and inert 25 mL Schlenk tube the respective precursor 20a -20c (0.10 mmol, 1.00 eq.) and the corresponding dipolarophile 6 -14 (0.12 mmol, 1.20 eq.) were dissolved (20b)/suspended (20a, 20c) in DCM (anhydrous, Ar bubbled, 4 mL). Under vigorous stirring triethylamine (anhydrous, Ar bubbled, 0.25 mL, ~12.00 eq) was added in one shot. The reaction was stirred for several minutes and transferred to a round bottom flask afterwards.

General procedure for the cycloaddition-planarization sequence for N-PAHs 15b -19b
Solvents and residual triethyl amine were removed under reduced pressure to obtain the crude product. DDQ (30 mg, 0.13 mmol, 1.30 eq.) was added and the flask was sealed with a septum and evacuated and refilled with argon three times. Toluene (4 mL, anhydrous, argon purged) was added via a syringe. The reaction was quenched by addition of water (10 mL) after 30 min and purified either by filtration over Alox (5,   In a dry and inert 25 mL Schlenk tube precursor 20a (0.05 g, 0.12 mmol, 1.00 eq.) and the dipolarophile 14 (0,02 g, 0,12 mmol, 1.00 eq.) were suspended in anhydrous toluene (4 mL).
Under vigorous stirring anhydrous triethylamine (80 µL, 4.80 eq) was added and stirring was continued for 60 minutes. DDQ (83 mg, 0.37 mmol, 3.00 eq.) was added to the flask. The reaction was quenched after 60 minutes by the addition of water (10 mL). The crude product was filtered and washed with water and recrystallized from ethanol to afford the title compound (0.05 g, 0.10 mmol, 77% yield). In a dry and inert 25 mL Schlenk tube the precursor 20a -20b (0.24 mmol, 2.00 eq.) and

General procedure for the twofold-cycloaddition-planarization sequence for N-PAHs
dipolarophile 13 (0.013 g, 0.12 mmol, 1.00 eq.) were dissolved in anhydrous dichloromethane      Details of the crystal data and a summary of the intensity data collection parameters for 15a and 16 are listed in Table S1  The crystals structures were visualized using Mercury 3.3.

4) UV-vis absorption spectra General information
Solution UV-vis absorption spectra were recorded in anhydrous solutions at a concentration of 10 -5 moll -1 in THF.