Selective Coupling of 1,2-Bis-Boronic Esters at the more Substituted Site through Visible-Light Activation of Electron Donor-Acceptor Complexes

1,2-Bis-boronic esters are useful synthetic intermediates particularly as the two boronic esters can be selectively functionalized. Usually, the less hindered primary boronic ester reacts, but herein, we report a coupling reaction that enables the reversal of this selectivity. This is achieved through the formation of a boronate complex with an electron-rich aryllithium which, in the presence of an electron-deficient aryl nitrile, leads to the formation of an electron donor-acceptor complex. Following visible-light photoinduced electron transfer, a primary radical is generated which isomerizes to the more stable secondary radical before radical-radical coupling with the arene radical-anion, giving beta-aryl primary boronic ester products. The reactions proceed under catalyst-free conditions. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to provide trans-substituted products, complementing the selectivity observed in the Suzuki-Miyaura reaction.

Automated summary

In this study, a coupling reaction is reported that enables the reversal of traditional selectivity for functionalization of primary boronic esters. This is achieved through the formation of a boronate complex with an electron-rich aryllithium, followed by reaction with an electron-deficient aryl nitrile to form an electron donor-acceptor complex. Visible-light photoinduced electron transfer generates a primary radical, which isomerizes to a more stable secondary radical and undergoes radical-radical coupling with the arene radical-anion, resulting in the formation of beta-aryl primary boronic ester products. Furthermore, this method allows for stereodivergent coupling, providing trans-substituted products.