Cascade Aerobic Selective Oxidation over Contiguous Dual-Catalyst Beds in Continuous Flow

Abstract

Cascade reactions represent an atom-economical and energy-efficient technology by which to reduce the number of manipulations required for chemical manufacturing. Biocatalytic cascades are ubiquitous in nature; however, controlling the sequence of interactions between reactant, intermediate(s), and active sites remains a challenge for chemocatalysis. Here, we demonstrate an approach to achieve efficient cascades using chemical catalysts through flow chemistry. Close-coupling of Pd/SBA-15 and Pt/SBA-15 heterogeneous catalysts in a dual bed configuration under continuous flow operation affords a high single pass yield of 84% (a 20-fold enhancement over batch operation) and high stability for >14000 turnovers in the cascade oxidation of cinnamyl alcohol to cinnamic acid, despite both catalysts being individually inactive for this reaction. Judicious ordering of Pd (first bed) and Pt (second bed) catalysts is critical to promote cascade oxidation with respect to undesired hydrogenation and hydrogenolysis, the latter favored over the reverse-bed sequence or a single mixed PdPt reactor bed. The intrinsic catalytic performance of each bed is preserved in the optimal dual-bed configuration, enabling quantitative prediction of final product yields for reactants/intermediates whose individual oxidation behavior is established. Continuous processing using contiguous reactor beds enables plug-and-play design of cascades employing "simple" catalysts.