Competitive Intramolecular Amination as a Clock for Iron-Catalyzed Nitrene Transfer

Reaction of the complex [(Tp^Ph,Me)Fe^II(NCMe)₃]BF₄, where Tp^Ph,Me = hydrotris(3-phenyl,5-methyl-1-pyrazolyl)borate, with the iodonium heteroylide PhI═NTs (1.5 equiv) is proposed to result in the insertion of N-tosylnitrene into one C-H bond at the ortho ring position of a 3-pyrazole phenyl substituent; subsequent deprotonation of the nascent aniline and one-electron oxidation of iron forms TsNH₂ (0.5 equiv) as a coproduct. The covalent ligand modification and oxidation results in an intense purple-brown anilinato-iron(III) LMCT chromophore. This intramolecular reaction is utilized as a consistent clock to determine relative rates of competitive intermolecular nitrene transfer to added substrates, specifically to para-substituted styrenes and thioanisoles. Prior addition of substrate to the reaction of PhI═NTs with the iron(II) complex attenuates the CT absorbance of the equilibrium solution. Fitting of the concentration-dependent absorption data gives the ratio of intra- versus intermolecular nitrene transfer. Because the former is independent of substrate, ratios for various substrates are directly comparable, and this approach enables acquisition of data for a single substrate under nearly stoichiometric, as opposed to competitive catalytic, conditions. Hammett analyses of such data are consistent with an electrophilic intermediate consistent with known or suspected imidoiron(IV) complexes. Because this intermediate was not observed directly, plausible geometric and electronic structures were modeled and assessed using density functional theory.