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

Abstract

Bioinspired chiral iron(II) complex [((R)-(−)-N4Py*)FeII(CH3CN)]2+ (1) (N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine) has been shown to efficiently catalyze the benzylic CH oxidation of ethylbenzene with tert-butyl hydroperoxide (TBHP), H2O2, 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 PhCH2OH and PhCH3, respectively, and the linear correlation between the normalized bimolecular reaction rates and bond dissociation energies (BDECH)) 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 kep > kreb). 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.