Decoding Key Transient Inter-Catalyst Interactions in a Reductive Metallaphotoredox-Catalyzed Allylation Reaction

Abstract

Metallaphotoredox chemistry has recently witnessed a surge in interest within the field of synthetic organic chemistry through the use of abundant first-row transition metals combined with suitable photocatalysts. The intricate details arising from the combination of two (or more) catalytic components during the reaction and especially the inter-catalyst interactions remain poorly understood. As a representative example of a catalytic process featuring such intricacies, we here present a meticulous study of the mechanism of a cobalt-organophotoredox catalyzed allylation of aldehydes. Importantly, the commonly proposed elementary steps in reductive metallaphotoredox chemistry are more complex than previously assumed. After initial reductive quenching, a transient charge-transfer complex forms that interacts with both the transition-metal catalyst and the catalytic base. Surprisingly, the former interaction leads to deactivation due to induced charge recombination, while the latter promotes deprotonation of the electron donor, which is the crucial step to initiate productive catalysis but is often neglected. Due to the low efficiency of this latter process, the overall catalytic reaction is photon-limited and the cobalt catalyst remains in a dual resting state, awaiting photoinduced reduction. These new insights are of general importance to the synthetic community, as metallaphotoredox chemistry has become a powerful tool used in the formation of elusive compounds through carbon−carbon bond formations. Understanding the underlying aspects that determine the efficiency of such reactions provides a conceptually stronger reactivity paradigm to empower future approaches to synthetic challenges that rely on dual metallaphotoredox catalysis.

Photocatalytic Metal Hydride Hydrogen Atom Transfer Mediated Allene Functionalization by Cobalt and Titanium Dual Catalysis

“…Fluorimetry revealed that HE, Cp 2 TiCl 2 , and Co‐1 could quench the fluorescence of 4CzIPN (Figure 6). The Stern–Volmer plot was found to deviate from linearity since 4CzIPN possesses both prompt fluorescence (PF) and thermally activated delayed fluorescence (TADF) [64–66] . Therefore, it is necessary from TCSPC data to determine their individual quenching rate constants in the two decay pathways.…”

Investigating Oxidative Addition Mechanisms of Allylic Electrophiles with Low-Valent Ni/Co Catalysts Using Electroanalytical and Data Science Techniques

“…Oxidative addition activates the allyl electrophiles to provide access to the nucleophilic Co­(II)/Ni­(II) π-allyl intermediate (after a subsequent redox event). This intermediate then is proposed to engage in additions to various electrophiles. ,,,, Understanding the intimate details of these steps will impact the further development of this emerging reaction class.…”

Kinetics of a Ni/Ir-Photocatalyzed Coupling of ArBr with RBr: Intermediacy of ArNiII(L)Br and Rate/Selectivity Factors

“…Indeed, to the best of our knowledge, to date, there has only been one study of the overall kinetics of dual Ni/photocatalysis: a report by Seeberger in 2020 on the acetoxylation of an aryl iodide, in which a homogeneous Ir­(ppy) 3 photocatalyst was compared with heterogeneous graphitic carbon nitride photocatalyst, using a calibrated in situ Fourier transform-infrared (FT-IR) method . A recent study by Kleij and co-workers of an organophotocatalyst/Co-catalyzed decarboxylative allylation also incorporated some bulk in situ FT-IR and ultraviolet–visible (UV-vis) kinetic measurements and modeling …”