VISIBLE LIGHT IN THE SYNTHESIS OF 3,3-BICARBAZOLES

EL AÍN, MARÍA ALEXIA; BUDÉN, MARÍA E.; ROSSI, ROBERTO A.

Ghent

Simposio; European Symposium on Organic Chemistry (ESOC 2021); 2021

Bicarbazoles (BiCz) are very important heterocycles employed in organic electronics and photonics, sensing and memory devices because of their advantageous characteristics, such as low ionization potentials, excellent glassy nature (Tg), and blue luminescence. Synthesis of symmetric bicarbazoles has always been achieved by transition-metal-based C−C intermolecular coupling reactions, starting from two carbazole moieties. The other commonly exploited method to introduce a C−C bond between electron-rich arene carbazole is the oxidative coupling. Generally, this approach leads to several side products (oligomers, higher oligomers, or polymers) through possible active sites.In this report, we present a synthetic and mechanistic study of novel iterative double cyclization intramolecular SRN1 reactions from diarylmines bearing two aryl halide moieties. This cyclization affords symmetric and unsymmetric bicarbazole compounds in very good yields. The synthetic strategy employed for their preparation was a Buchwald-Hartwig arylation of anilines followed by an intramolecular SRN1 reaction, in the presence of base and visible light, at room temperature.Initially, the cyclization reaction was optimized using N4,N4´-bis(2-bromophenyl)-[1,1´-biphenyl]-4,4´-diamine (1a) as model substrate (Scheme 1). Screening of nature base in different equivalents, solvent, source lamp irradiation, time, concentration, etc. was carried out. Results show that using 1a as substrate, KOtBu (6 equiv.), DMSO, blue LED, in 1 h are necessary for full conversion, obtaining the product (2a) in 66% yield (RMN). Similar results were obtained starting from the chloride analogue (1b), affording the product 2a in 65% isolated yield. Different diarylamines were explored, obtaining a variety of bicarbazoles in good to very good yields (40 ? 79%, R=CH3, OCF3, CF3, Z=CH, N).Substrate 1a does not react with an excess KOtBu in the dark. These results indicate that a photoinduced electron transfer is needed for the reaction to occur. The mechanism is non-trivial and we propose that radicals are intermediates. The regiochemistry was studied using computational calculations, employing the DFT method, B3LYP functional and 6-31G as base.