Unlocking metal-ligand cooperative catalytic photochemical benzene carbonylation: a mechanistic approach

A key challenge in green synthesis is the catalytic transformation of renewable substrates at high atom and energy efficiency, with minimal energy input (DG z 0). Non-thermal pathways, i.e., electrochemical and photochemical, can be used to leverage renewable energy resources to drive chemical processes at well-defined energy input and efficiency. Within this context, photochemical benzene carbonylation to produce benzaldehyde is a particularly interesting, albeit challenging, process that combines unfavorable thermodynamics (DG° = 1.7 kcal mol⁻¹) and the breaking of strong C-H bonds (113.5 kcal mol⁻¹) with full atom efficiency and the use of renewable starting materials. Herein, we present a mechanistic study of photochemical benzene carbonylation catalyzed by a rhodium-based pincer complex that is capable of metal-ligand cooperation. The catalytic cycle, comprising both thermal and non-thermal steps, was probed by NMR spectroscopy, UV-visible spectroscopy and spectrophotochemistry, and density functional theory calculations. This investigation provided us with a detailed understanding of the reaction mechanism, allowing us to unlock the catalytic reactivity of the Rh-pincer complex, which represents the first example of a metal-ligand cooperative system for benzene carbonylation, exhibiting excellent selectivity.