Catalytic and stoichiometric C[sbnd]H oxidation of benzylalcohols and hydrocarbons mediated by nonheme oxoiron(IV) complex with chiral tetrapyridyl ligand.

Bioinspired chiral iron(II) complex [((R)-(−)-N4Py*)FeII(CH 3 CN)]2+ (1) (N4Py* = N , N -bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) has been shown to efficiently catalyze the benzylic C H oxidation of ethylbenzene with tert -butyl hydroperoxide (TBHP), H 2 O 2 , and meta -chloroperoxybenzoic acid (mCPBA) resulting in enantiomerically enriched 1-phenylethanol up to 12.5% ee and the corresponding acetophenone, where the [FeIV(N4Py*)(O)]2+ (2) intermediate has been detected by UV/Vis spectrometry. The stoichiometric oxidation of benzyl alcohol and various hydrocarbon derivatives including the asymmetric hydroxylation of ethylbenzene with 2 has also been investigated. Detailed kinetic, and mechanistic studies (kinetic isotop effect (KIE) of 31 and 38, and Hammett correlation with ρ = −0.32 and −0.98 for PhCH 2 OH and PhCH 3 , respectively, and the linear correlation between the normalized bimolecular reaction rates and bond dissociation energies (BDE CH)) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the substrate and the Fe(IV)-oxo species. The stoichiometric 2 -mediated hydroxylation of ethylbenzene affords 1-phenylethanol in up to 33% ee , suggesting clear evidence for the involvement of the oxoiron(IV) species in the enantioselective step. The moderate enantioselectivity may be explained by the epimerization of the long-lived substrate radical before the rebound step (non-rebound mechanism, where k ep > k reb). The kinetic resolution of the resulting chiral alcohol due to its metal-based overoxidation process into acetophenone in the catalytic metal-based ethylbenzene oxidation can be excluded. The reactivity of oxoiron(IV) complex, [FeIV(asN4Py)(O)]2+ with chiral pentadentate ligand, has been investigated in the oxidation of various hydrocarbons and alcohols. Unlabelled Image • Catalytic ethylbenzene oxidation with chiral Fe(II)-polypyridyl complex • Evidence for the formation of oxoiron(IV) intermediate (oxidant) • Oxoiron(IV)-mediated C H activation of benzyl alcohols and hydrocarbons • Reaction kinetics with UV–Vis spectroscopy • Oxoiron(IV)-mediated enantioselective oxidation [ABSTRACT FROM AUTHOR]