Dual Photoredox Catalysis with Vanadium Complexes in Low Oxidation State: Diastereoselective Pinacol Coupling

Abstract

Dual photoredox catalysis is modifying the approach to sustainable metal catalysis based on metals in low oxidation state, as the use of stoichiometric metal as reductants are avoided. In this study, we showcase the potential use of vanadium in a dual photoredox process, allowing us to effectively control the formation of V(II) and its corresponding chemical reactions. A diastereoselective pinacol coupling with aromatic aldehydes is described. The process shows broad scope, and interestingly, unprotected salicylic aldehydes are compatible with the reaction protocol. Additionally, among all the ligands investigated we found that inexpensive bipyridines control the diastereoselection of the reaction favoring the d,l chiral compound (dr>20:1 for most of the aldehydes investigated). To promote the reaction and avoid an unproductive and not stereoselective radical coupling of the generated ketyl radical, the use of [DMQA]+, a stable organic dye, was found crucial. Furthermore, this dye possesses the remarkable capability of efficiently capturing light in the red segment of the visible spectrum, effectively raising efficiency of the protocol. The utilization of red light carries notable advantages in terms of penetration and safety, making it particularly significant. Moreover, careful photophysical investigations were able to give indications about the mechanism of the reaction.

Synthesis of alcohols: streamlined C1 to Cn hydroxyalkylation through photoredox catalysis

“…10 In recent years, the advent of photoredox catalysis has led to the application of novel photochemical approaches in addressing this thermodynamic challenge as shown by Knowles, Yoon, Rueping, Cozzi, and others. 11–16 However, these methodologies require synergistic Lewis acid/photoredox catalysis and/or the use of Hantzsch ester as a sacrificial electron and hydrogen atom donor. Interestingly, an electro-photocatalytic carbonyl reduction has been recently proposed by Wickens and coworkers (Fig.…”