Mechanism of Alcohol Oxidation by Fe(V)(O) at Room Temperature

Selective oxidation of alcohol to its corresponding carbonyl compound is an important chemical process in biological as well as industrial reactions. The heme containing enzyme CytP450 has been known to selectively oxidize alcohols to their corresponding carbonyl compounds. The mechanism of this reaction, which involves high-valent Fe(IV)(O)-porphyrin(•+) intermediate with alcohol, has been well-studied extensively both with the native enzyme and with model complexes. In this paper, we report for the first time the mechanistic insight of alcohol oxidation with Fe(V)(O) complex of biuret TAML (bTAML), which is isoelectronic with Fe(IV)(O)-porphyrin(•+) intermediate form in CytP450. The oxidations displayed saturation kinetics, which allowed us to determine both the binding constants and first-order rate constants for the reaction. The K and k values observed for the oxidation of benzyl alcohol by Fe(V)(O) at room temperature (K = 300 M(-1), k = 0.35 s(-1)) is very similar to that obtained by CytP450 compound I at -50 °C (K = 214 M(-1), k = 0.48 s(-1)). Thermodynamic parameters determined from van't Hoff's plot (ΔH∼ -4 kcal/mol) suggest hydrogen bonding interaction between substrate and bTAML ligand framework of the Fe(V)(O) complex. Analysis of H/D KIE (kH/kD ∼ 19 at 303 K), Hammett correlation and linearity in Bell-Evans-Polyanski plot points to the C-H abstraction as the rate determination step. Finally, experiments using Fe(V)(O(18)) for benzyl alcohol oxidation and use of the "radical clock" cyclobutanol as a substrate shows the absence of a rebound mechanism as is observed for CytP450. Instead, an ET/PT process is proposed after C-H abstraction leading to formation of the aldehyde, similar to what has been proposed for the heme and nonheme model compounds.