Identification and Reactivity of Peroxyhemiacetal Intermediate during Aldehyde Deformylation by a Heme...

Aldehyde deformylation reaction by heme and nonheme enzymes in biology has been associated with physiologically important processes that include human steroid metabolism and biosynthesis of long-chain hydrocarbons in cyanobacteria. A ferric-peroxide species generated from molecular oxygen has been invoked as the nucleophilic oxidant in both heme and nonheme enzymatic active sites, resulting in the formation of an elusive peroxyhemiacetal intermediate. However, recent investigations using nonheme synthetic analogues have suggested that the deformylation proceeds most likely via aldehyde C-H atom abstraction (Zhu, W. J. Am. Chem. Soc. 2025, 147(18), 15006-15018). Despite significant attempts in modeling aldehyde deformylation in synthetic heme systems, both reactivity and direct evidence for a heme-peroxyhemiacetal intermediate involved in the function have remained elusive. In this study, we demonstrate for the first time that a synthetic heme-peroxo complex, derived from molecular oxygen, promotes aldehyde deformylation of different aldehyde substrates via the formation of a peroxyhemiacetal intermediate. Investigation of the reaction mechanism with 2-phenylpropionaldehyde reveals the formation of S = 1/2 ferric low-spin peroxyhemiacetal intermediate, definitively characterized using different spectroscopic techniques for the first time outside the protein scaffold. Kinetic studies indicate a bimolecular reaction in which a ferric-peroxide species attacks the aldehyde, forming the peroxyhemiacetal intermediate. Density functional theory (DFT) calculations further reveal that the weak O-O bond in the peroxyhemiacetal intermediate promotes facile homolysis of the O-O bond. The unequivocal observation of peroxyhemiacetal with synthetic iron-porphyrin complexes during aldehyde deformylation clearly establishes nucleophilic attack of the peroxide to be the key step of the mechanism and not aldehyde C-H bond abstraction.