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”

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) ( S t = 2) catalyst [Fe(acac) 2 (H 2 O) 2 ] ( 1 ) to generate a high-spin ( S t = 5/2) intermediate ( Int I ), which decays to a second intermediate ( Int II ) with S t = 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 S t = 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.

The advent of electrophilic hydroxylamine-derived reagents for the direct preparation of unprotected amines

“…119 Recently, a collaboration between our group and the groups of Neese and DeBeer revealed important mechanistic features of this reaction. 120 Inspired by the structural similarities between the Pc ligand and the structure of hemoproteins, the Arnold group developed an asymmetric variant of the aminohydroxylation reaction of vinyl arenes (Scheme 4B). 121 Opting for a haemoprotein, a very thermostable Rhodotermus marinus cytochrome C enzyme was selected as a starting point for directed evolution.…”

“… 119 Recently, a collaboration between our group and the groups of Neese and DeBeer revealed important mechanistic features of this reaction. 120 …”

Recent Advances in Nonprecious Metal Catalysis

“…The mechanism of the aminofunctionalization of olefins, however, remained poorly understood. A recent contribution by Chatterjee, Neese, and DeBeer reports detailed mechanistic work using a wide range of analytical and spectroscopic techniques supported by computations on the aminofunctionalization of styrenyl olefins . The reaction is proposed to operate via the formation of an Fe­(III) acyloxy species that undergoes N–O bond homolysis to provide an Fe­(III) iminyl radical species.…”

“…A recent contribution by Chatterjee, Neese, and DeBeer reports detailed mechanistic work using a wide range of analytical and spectroscopic techniques supported by computations on the aminofunctionalization of styrenyl olefins. 19 The reaction is proposed to operate via the formation of an Fe(III) acyloxy species that undergoes N−O bond homolysis to provide an Fe(III) iminyl radical species. The iron iminyl species shows a particularly long Fe−N bond, which facilitates nitrogen transfer to the olefinic partner.…”

Unprotected amine transfer performed by non-heme iron(ii) complexes

“…The first mechanistic studies, dealing with the aminoetherification of alkenes, catalyzed by a fully oxygenated iron(II) complex, just came out this year. 13 This seminal work is paving the way to a future boom of mechanistic studies for all kind of amination reactions (aziridination, aliphatic or aromatic amination) potentially catalyzed by a large variety of iron complexes. In this work, we are expanding the repertoire for this reaction by studying the reactivity of a previously described family of nitrogen based iron complexes 14,15 (Complexes 1 and 2, Scheme 1) with the unprotected amine donor PivONH2 (Scheme 1) for nitrene transfer reactions.…”