Synthesis of 2D and 3D Borylated Scaffolds via Selective Activation of Multifunctional Organoboron Reagents

Abstract

Organoboron compounds play a central role in the chemical sciences. The classical approach to access this class of molecules entails incorporation of the boron motif in an advanced intermediate and has been implemented in a variety of different transforms, from the venerable Brown hydroboration to innovative platforms like C─H borylation. Alternatively, a core scaffold already containing a boron moiety can be subjected to further functionalization. In order to avoid side reactions at the boron center, protecting groups are generally employed. This has the drawback of adding additional steps to the synthetic sequence. The resulting inefficiency can be overcome by relying on methodologies capable of selectively manipulating multifunctional units that contain and retain an unprotected boron handle. This thesis describes the development of transforms of this latter kind. By relying on either photochemistry or transition metal catalysis, the expedient conversion of multifunctional boron reagents into complex 2D and 3D organoboron compounds could be achieved. Chapter 2 details the development of a method that generates α-boryl radicals from α-iodo boronic acid pinacol esters without the need for catalysts. Interception of the generated open shell species with styrenes afforded E-allylic boronic esters. The mild conditions of the protocol, which requires only a simple Lewis base additive and visible light, showed broad functional group tolerance and allowed its strategic alignment with energy transfer catalysis to access the Z-stereoisomers of the products. Chapter 3 describes the expansion of this chemoselective activation platform to a trifunctional reagent, featuring an additional nucleophilic handle (Si, Ge or B based). C─I bond homolysis yielded an α-bimetalloid radical of increased electrophilicity. Thanks to this, a wider array of SOMOphiles could be engaged, producing valuable 3D borylated scaffolds. These, in turn, could unlock a vast portion of chemical space via the chemoselective manipulation of the two installed nucleophilic moieties. Finally, the mechanism was interrogated via in depth experimental and computational investigations. Chapter 4 describes the development of a transition metal catalysed method for the synthesis of complex 2D organoboron scaffolds. A regioselective Suzuki-Miyaura cross coupling/cyclization reaction between readily available vicinal diborylated alkenes and 2- halophenols allowed the synthesis of 3-substituted benz(oxa/aza)borines, heterocycles that have been receiving increasing attention from medicinal chemists. The modular nature of the protocol provided rapid access to a Xeruborbactam derivative and its functional group tolerance allowed incorporation of multiple synthetic handles. These were exploited in downstream chemoselective manipulations and to install biomolecular probes.