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

Abstract Bioinspired chiral iron(II) complex [(( R )-(−)-N4Py*)Fe II (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 [Fe IV (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.