Kinetic and Product Study of the S‐oxidation vs HAT Chemoselectivity in Reactions Promoted by Nonheme Iron(IV)‐oxo Complex/NHPI Mediator System

Abstract

AbstractThe chemoselectivity between S‐oxidation and hydrogen atom transfer (HAT) from C−H bonds has been investigated in the oxidations of a series of aryl sulfides, alkyl aromatic compounds and benzylic alcohols promoted by the iron(IV)‐oxo complex [(N4Py)FeIV(O)]2+ (N4Py: N,N‐bis(2‐pyridylmethyl)‐N‐bis(2‐pyridyl)‐methylamine) either alone or in the presence of the N‐hydroxyphthalimide (NHPI) mediator via kinetic and product studies. Kinetic analyses indicate a generally higher reactivity of [(N4Py)FeIV(O)]2+ for S‐oxidation process while HAT is favored in the reactions promoted by phthalimide‐N‐oxyl radical (PINO) deriving from NHPI oxidation. Product analysis in intermolecular competitive oxidations confirms the kinetic results with sulfoxides obtained as major products in the oxidation promoted by [(N4Py)FeIV(O)]2+. Conversely, when NHPI is employed as a mediator, significant differences in terms of chemoselectivity are observed, and HAT‐derived products are obtained in higher yields which translate into an inversion of selectivity in the case of the substrates containing activated C−H bonds like diphenylmethane, triphenylmethane and benzylic alcohols. A similar change of chemoselectivity is also observed in the oxidation of aromatic substrates containing both a sulfur atom and α to OH benzylic C−H bonds, with the sulfoxide product more abundant in the absence of NHPI and carbonyl products prevailing with the [(N4Py)FeIV(O)]2+/NHPI system.