Proton-Coupled O−O Activation on a Redox Platform Bearing a Hydrogen-Bonding Scaffold

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Abstract
Porphyrin architectures bearing a hydrogen-bonding scaffold have been synthesized. The H-bond pendant allows proton-coupled electron transfer (PCET) to be utilized as a vehicle for effecting catalytic O−O bond activation chemistry. Suzuki cross-coupling reactions provide a modular synthetic strategy for the attachment of porphyrins to a rigid xanthene or dibenzofuran pillar bearing the H-bond pendant. The resulting HPX (hanging porphyrin xanthene) and HPD (hanging porphyrin dibenzofuran) systems permit both the orientation and acid−base properties of the hanging H-bonding group to be controlled. Comparative reactivity studies for the catalase-like disproportionation of hydrogen peroxide and the epoxidation of olefins by the HPX and HPD platforms with acid and ester hanging groups reveal that the introduction of a proton-transfer network, properly oriented to a redox-active platform, can orchestrate catalytic O−O bond activation. For the catalase and epoxidation reaction types, a marked reactivity enhancement is observed for the xanthene-bridged platform appended with a pendant carboxylic acid group, establishing that this approach can yield superior catalysts to analogues that do not control both proton and electron inventories.