Metallaphotoredox chemistry has recently witnessed a renaissance 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 specially 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 catalysed 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, as well as 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 a crucial step in order to promote productive catalysis, but is often neglected. Due to the low efficiency of this 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 photoredox chemistry has become a powerful tool used in the creation of elusive compounds through carbon-carbon bond formations. Understanding the underlying factors 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.
Decoding key transient inter-catalyst interactions in a metallaphotoredox-catalysed cross-electrophile coupling reaction