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 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 electrochemical cross‐coupling of quaternary borate salts

“…[7] The oxidation of the borate salts was performed 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 Figure 1 based on cyclic voltammetry, galvanostatic measurements, and density functional theory (DFT) calculations.…”

“…The alternative mechanism (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.…”

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 However, the rearrangement of borates occurs specifically between the carbons directly attached to the boron atom, thereby allowing for a high functional group tolerance (39a-d).…”

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

“…Due to the cation radical intermediate the reaction loses its stereofidelity, but preferably produces the thermodynamically more stable alkene. One year later the group published a photocatalytic conversion of vinyl triarylborate complexes to alkenes in the presence of K 2 HPO 4 and irradiation with blue light ( 31 j ) [39] . Under related conditions overoxidation of the products to stilbeneoxides was observed.…”