Aryl aldehydes are key synthetic intermediates in the manufacturing of active pharmaceutical ingredients. They are generated on scale (>1000 kg) through the palladium-catalyzed formylation of aryl bromides using syngas (CO/H2). The best-in-class catalyst system for this reaction employs di-1-adamantyl-n-butylphosphine (cataCXium A), palladium(II) acetate, and tetramethylethylenediamine. Despite nearly 20 years since its initial report, a mechanistic understanding of this system remains incomplete. Here, we use automation, kinetic analysis, and DFT calculations to develop a mechanistic model for this best-in-class catalyst. We suggest that a combination of the migratory insertion step and dihydrogen activation step is likely involved in the turnover-limiting sequence. The reaction kinetics are responsive to the nature of the substrate, with electron-rich aryl bromides reacting faster and more selectively than their electron-poor counterparts due to the influence of electronics in the migratory insertion step. Our findings add additional insight into the proposed mechanism of palladium-catalyzed formylation of aryl bromides.
© 2024 The Authors. Published by American Chemical Society.