Catalytic Aldehyde Decarbonylase Activity by a Thiolate-Bound Iron Porphyrin: Observation of a Peroxyhemiacetal Intermediate during Catalysis

Aldehyde decarbonylation is a key chemical step in nature that is involved in the biosynthesis of hormones and long-chain hydrocarbons where O₂ is the oxidant. A cytochrome P450 enzyme, aromatase, catalyzes this reaction, where a thiolate-bound ferric peroxide is proposed to be the oxidant. Despite several attempts, only substoichiometric yields have been reported by synthetic iron porphyrins in organic solvents using peroxide. Several synthetic nonheme complexes have been reported to catalyze decarbonylation in stoichiometric yields using peroxides as oxidants. Catalytic decarbonylation with molecular O₂ using either heme or nonheme complexes remains elusive. Iron picket-fence porphyrin is attached to thiol-terminated self-assembled monolayers. In situ resonance Raman spectroscopy reveals the accumulation of an Fe(III)-OOH intermediate with Fe-O and O-O vibrations at 579 cm^-1 and 798 cm^-1, respectively, during heterogeneous electrocatalytic O₂ reduction in a pH 7 buffer solution. In the presence of different aldehydes in solution, catalytic aldehyde decarbonylase reactivity is observed with TON and TOF of ∼ 83000 and 277 s^-1, respectively, for 2-methyl-2-phenylpropionaldehyde and ∼ 13000 and 45 s^-1, respectively, for undecanal using molecular O₂ in a pH 7 buffer solution. In situ SERRS-RDE in the presence of 2-phenylpropionaldehyde indicates the formation of a peroxyhemiacetal species with Fe-O, O-O, and C-O vibrations at 520 cm^-1, 833 cm^-1, and 1185 cm^-1, respectively, as an intermediate in catalytic aldehyde decarbonylase activity.