Abstract
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) (St = 2) catalyst [Fe(acac)2(H2O)2] (1) to generate a high-spin (St = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with St = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)2-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N–O bond homolysis. However, it does not 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 (J = −524 cm–1) to an iminyl radical, [Fe(III)(acac)2-NH·], giving St = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N–O bond cleavage to generate the active iron–nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II 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 N-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.
Highlights
AND BACKGROUNDAmines are found ubiquitously throughout the natural world as key functional groups in amino acids and nucleotide bases, and are fundamental components of pharmaceuticals, agrochemicals, dyes and polymers.[1−6] Installation of “amino-functionality” remains one of the major challenges in organic synthesis
Of the experimental results to the theoretical calculations are pursued in the subsequent section for electronic and structural elucidation of the reaction components. We first apply this approach to the precursor complex 1, to establish a benchmark for the correlations before extending this analysis to the geometric and electronic structures of the reaction intermediates (Int I and iron−nitrogen intermediate (Int II)), which are discussed in the part of the manuscript. 3.1.1
From the correlation of the experimental observables with calculated parameters for 1, it becomes evident that the precursor 1 is an St = 2, high spin Fe(II) species, with a distorted Oh geometry, coordinated by two monoanionic acac ligands in a bidentate fashion, and two coordinated water molecules either in cis disposition (1-cis model) or trans disposition (1-trans model) and an equilibrium between the 1-cis and 1-trans isomer exist in solution. 3.2
Summary
Amines are found ubiquitously throughout the natural world as key functional groups in amino acids and nucleotide bases, and are fundamental components of pharmaceuticals, agrochemicals, dyes and polymers.[1−6] Installation of “amino-functionality” remains one of the major challenges in organic synthesis. This study provides clear evidence for the involvement of two novel iron− nitrogen intermediates having interesting electronic and bonding properties, which play a pivotal role in controlling the catalytic N-transfer activity (Scheme 2b). These intermediates are formed in a stepwise manner upon reaction of an Fe(II) catalyst with the hydroxylamine derived N−O reagent. The results shed light into the mechanism of N−O bond cleavage to generate active iron− nitrogen intermediates and, as such, have broad implications for the field of synthetic N-transfer reactions that are important in countless areas of chemistry
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