Reversible Boron-Nitrogen Coordination in Organic Synthesis – From Bidentate Lewis Acid Catalysis to Materials

Abstract

The reversible coordination of the lone pair from a nitrogen atom into the free p-orbital of a boron atom can be used in various fields of organic synthesis and organic materials. This coordination is utilized in organic synthesis in the field of Lewis acid catalysis, for example in Friedel-Crafts and aza-Diels-Alder reactions, which clearly demonstrates its significance and applicability. This applicability was further extended in 2011 when the Wegner group employed the boron-based bidentate Lewis acid (BDLA), 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene, to show that the boron-nitrogen coordination can be used to catalyze inverse electron-demand Diels-Alder (IEDDA) reactions of phthalazines. By integrating the BDLA catalyzed IEDDA reaction into several domino processes, a diverse substrate scope with highly complex structures can be accessed. One of these domino processes combined an IEDDA reaction with a photoinduced ring-opening (PIRO) reaction of phthalazines with cyclic enamines and provided medium-sized carbocycles for the first time. These are core structures of many biologically active compounds and functional materials. Due to entropic and enthalpic reasons they cannot easily be synthesized and the synthesis normally requires transition-metal catalysis. Electron-rich as well as electron-poor phthalazines and cyclic enamines lead to various substituted 9- and 11-membered carbocycles in this BDLA catalyzed IEDDA/PIRO reaction. In the case of organic materials this coordination also has a major impact on boron-nitrogen doped polycyclic aromatic hydrocarbons (PAHs), e.g., in organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs), or in solar cells. In 2017 the Wegner group published the synthesis of a stable boron-nitrogen doped biradical with a diazadiboraacene backbone. To access this novel diazadiborabenzo[b]triphenylenes and investigate their photophysical properties, a new modular synthetic strategy was developed. The investigation of the photophysical properties showed that even small changes of the substituent on the boron-atom lead to significant changes in the emission spectra.