An investigation of steric influence on the reactivity of FeV(O)(OH) tautomers in stereospecific C–H hydroxylation

Abstract

Two new tetradentate N4 ligands (L^N4), L^N4 = ^Me2,Me2PyzTACN (1-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-4,7-dimethyl-1,4,7-triazacyclononane) and ^Me2,MeImTACN (1-((1-methyl-1H-imidazol-1-yl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane) have been synthesized and their corresponding Fe(II) complexes [Fe^II(^Me2,Me2PyzTACN)(CF₃SO₃)₂], 1^Pz, and [Fe^II(^Me2,MeImTACN)(CF₃SO₃)₂], 1^Im, have been prepared and characterized. Complexes 1^Pz and 1^Im catalyse the hydroxylation of C–H bonds of alkanes with excellent efficiencies, using hydrogen peroxide as oxidant. The high H/D kinetic isotope effect values for C–H hydroxylation, large normalized tertiary/secondary C–H (C3/C2) bond selectivities in adamantane oxidation, and high degrees of stereoretention in the oxidation of cis-1,2-dimethylcyclohexane are indicative of metal-based oxidation processes. The complexes also catalyse the oxidation of cyclooctene to form its corresponding epoxide and syn-diol. For 1^Pz the epoxide is the main product, while for the analogous complex 1^Im the syn-diol predominates. The active oxidant is proposed to be an [(L^N4)Fe^V(O)(OH)]²⁺ species (2^Pz, L^N4 = ^Me2,Me2PyzTACN and 2^Im, L^N4 = ^Me2,MeImTACN) which may exist in two tautomeric forms related by a proton shift between the oxo and hydroxo ligands. Isotope labelling experiments show that the oxygen atom in the hydroxylated products originates from both water and hydrogen peroxide, and labelling experiments involving oxygen atom transfer to sterically bulky substrates provide indirect information on the steric influence exerted by the two ligands in the relative reactivities of the two hypervalent iron tautomers. Based on these labelling studies, the steric influence exerted by each of the ligands towards the relative reactivity of the oxo ligands of the corresponding pair of Fe(V)(O)(OH) tautomers can be derived. Furthermore, this steric influence can be gauged relative to related complexes/ligands.