Stable hexacenes through nitrogen substitution.

The stabilization of larger acenes is a demanding task both synthetically as well as conceptually to be solved in different ways. Neither unsubstituted hexacene nor its higher homologues are stable, but their existence can be demonstrated in suitable matrixes. In the case of pentacene, two strategically attached TIPS-ethynyl groups suffice to fully stabilize and solubilize this material (TIPS = triisopropylsilyl). In larger acenes, however, two TIPS-ethynyl groups do not provide enough stabilization to furnish long-term persistent representatives. Anthony et al. demonstrated that even sterically encumbered hexacenes (with two tris(trimethylsilyl)silylethynyl substituents) react in solution under butterfly dimerization with a half-life of around 20 minutes. Only the introduction of four more aryl groups in lateral positions increases the stability of higher acenes so far that the Wudl and Chi groups could obtain persistent heptacene derivatives with a half-life of up to one week in solution. But even here, formation of endoperoxides is observed after some time. The laterally attached phenyl groups lead to isolation of the p systems with respect to their next neighbors, as evidenced by single-crystal structure analysis. We demonstrate herein that nitrogen atoms introduced into the acene skeleton give persistent disubstituted heterohexacenes, which are stable even when stored for longer periods of time. The palladium-catalyzed coupling of 1 with 2 in the presence of the ligand L gave the tetrazaacene 3 after oxidation with MnO2 in good yields (Scheme 1). [8, 9] The dichlorobenzoquinoxaline 4 also couples in good yields to give 5 (Scheme 2) However, attempts to oxidize 5 by MnO2, IBX = 2-iodoxybenzoic acid, N-bromosuccinimide (NBS), potassium chromate, pyridinium chlorochromate (PCC), or Cu(OAc)2 were fruitless. Difficult-to-separate product mixtures formed, but not the desired acene. This behavior was not entirely unexpected, as Kummer and Zimmermann had already unsuccessfully attempted to oxidize 6, readily obtained by co-melting of diaminonaphthalene and dihydroxyanthracene at 220 8C, using chloroanil or PbO2. Azahexacenes remained unknown. To maximize the shielding effect of the TIPS groups, it might be better to attach them in the center of the molecule. Consequently, 7a,b were coupled to 2 (Scheme 3). The Pdcatalyzed coupling works very well in the presence of L and furnishes 8a,b in good to excellent yields (92 %, 56%). Both are dehydrogenated by MnO2 into the azaacens 9 a,b in 56% and 74% yield, but it is not clear why this reaction does not work for 5. The heteroacenes 9a and 9b are greenish black crystalline powders, stable under laboratory conditions both as solids and in solutions, which display a green-yellow color. By NMR spectroscopy we could not detect endoperoxide formation or dimerization of 9 to butterfly cycloadducts. In our case, triisopropylsilyl and even the triethylsilyl groups are sufficient to stabilize and solubilize the hexacene skeleton, but 9b is considerably less soluble than 9a. To extend this chemistry, we prepared 7c in a multistep synthesis starting from diaminoScheme 1. Palladium-catalyzed synthesis of 3. dba = dibenzylideneacetone.