A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant".

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the "oxidant" as well as "amino" group donor. It activates the high-spin Fe(II) (<i>S</i>_t = 2) catalyst [Fe(acac)₂(H₂O)₂] (<b>1</b>) to generate a high-spin (<i>S</i>_t = 5/2) intermediate (<b>Int I</b>), which decays to a second intermediate (<b>Int II</b>) with <i>S</i>_t = 2. The analysis of spectroscopic and computational data leads to the formulation of <b>Int I</b> as [Fe(III)(acac)₂-<i>N</i>-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, <b>Int II</b> is formed by N-O bond homolysis. However, it does <i>not</i> generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (<i>J</i> = -524 cm^-1) to an iminyl radical, [Fe(III)(acac)₂-NH·], giving <i>S</i>_t = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-<i>N</i>-acyloxy intermediate (<b>Int I</b>), which undergoes N-O bond cleavage to generate the active iron-nitrogen intermediate (<b>Int II</b>), is unprecedented. Relative to Fe(IV)(O) centers, <b>Int II</b> features a weak elongated Fe-N bond which, together with the unpaired electron density along the Fe-N bond vector, helps to rationalize its propensity for <i>N</i>-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis.