Photocatalyzed Transition-Metal-Free Oxidative Cross-Coupling Reactions of Tetraorganoborates

Abstract

Readily accessible tetraorganoborate salts undergo selective coupling reactions under blue light irradiation in the presence of catalytic amounts of transition-metal-free acridinium and rhodamine photocatalysts to furnish unsymmetrical biaryls, heterobiaryls and arylated olefins. This represents an interesting conceptual approach to forge C-C bonds between aryl, heteroaryl and alkenyl groups under smooth photochemical conditions. Computational studies were conducted to investigate the mechanism of the transformation.

Computational Insights into Intramolecular Cross-Coupling of Quaternary Borate Salts

“…(2) proceeds directly from A to C via σ-bond cleavage and was also proposed for aryl-vinyl coupling reactions. [6][7][8] Subsequently, the boron-carbon bond in C is broken homolytically, furnishing the final product and removing the radical character. As a side product, borinic acids BR 2 OH are formed.…”

“…7 The oxidation of the borate salts was perfomed electrochemically with RVC electrodes in acetonitrile and in a subsequent work by means of a photocatalyst. 8 Similar reactions occur with chemical oxidants. 9 For aryl-aryl couplings, we proposed 6 the mechanisms shown in Fig.…”

Forging C–C Bonds through Intramolecular Oxidative Coupling of Organoborates – An Overview

“…48 In 2021, Didier and co-workers illustrated a photocatalytic version of the electrocoupling reaction, using acridinium species as photocatalysts under blue-light irradiation. 49 A wide variety of substrates 27 were used to demonstrate the applicability of the strategy, including halogenated derivatives (Scheme 16 ). It is important to note that polyhalogenated substrates usually undergo formation of undesired products under Pd-catalyzed conditions, including homocoupling products and polymers.…”