Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents

Abstract

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co­(II) reagents that, unlike the previously examined Mn­(II) and Fe­(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co­(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co­(II) sites (S = 3/2) feature a minimal [N3N] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co­(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative L4Co catalyst (−COCMe3 arm) is operating faster than the L8Co analogue (−COCF3 arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C b atom of styrene, followed by product-determining ring-closure), the L4Co catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [L4Co]­NTs and [L8Co]­NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for L4Co (due to the polarizability of the tert-Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

Coordinatively Unsaturated Metallates of Cobalt(II), Nickel(II), and Zinc(II) Guarded by a Rigid and Narrow Void

“…The attendant π–π interactions between MeCN and the arene rings may enable observable coordination at ambient temperatures, as related tris-amidylamine cobaltates bearing alkyl substituents have been reported to retain four-coordinate geometries in acetonitrile solution . Examples of trigonal monopyramidal cobalt­(II) complexes that do coordinate acetonitrile include those bearing weaker trans donors , and cationic complexes, both of which can be reasoned to exhibit enhanced Lewis acidity. Note that variable-temperature UV–visible spectroscopy measurements for acetonitrile solutions of [NiL OMe ] – show changes potentially indicative of solvent coordination at lower temperatures (Figure S20), whereas ambient-temperature and low-temperature spectra of [ NiL OCH2O ] – in acetonitrile are essentially unchanged (Figure S21), again suggesting that only the nonmacrocyclized ligand permits acetonitrile coordination.…”

A Comparative Study of Cationic Copper(I) Reagents Supported by Bipodal Tetramethylguanidinyl-Containing Ligands as Nitrene-Transfer Catalysts

“…After completion of the reaction, the products were isolated by column chromatography (silica gel) and quantified by 1 H NMR (in CDCl 3 or CD 3 CN) versus an internal standard (4′-methoxyacetophenone). All aziridines, ,, any allylic/benzylic amination products ,, and insertion products (Table ) are known compounds. They have been identified with the assistance of 1 H NMR, by comparison with spectroscopic features reported for authentic samples in the literature.…”

“…However, by comparison to the tripodal [(TMG 3 trphen)Cu I ] + reagent noted above, all of these M(II) reagents are by far inferior as catalysts in C−H amination reactions and more modestly yielding in C�C aziridination reactions. Incidentally, when the equatorial TMG residues of the [N 3 N] ligand scaffold are replaced by N-amido residues featuring alkyl, aryl, or acyl substituents, the resulting library of anionic tripodal M(II) reagents (M = Mn, Fe, Co) 39,40 is only effective in selective aziridinations of aromatic olefins, presumably due to the diminished electrophilicity of the active metal nitrene moiety.…”

“…Incidentally, when the equatorial TMG residues of the [N 3 N] ligand scaffold are replaced by N-amido residues featuring alkyl, aryl, or acyl substituents, the resulting library of anionic tripodal M­(II) reagents (M = Mn, Fe, Co) , is only effective in selective aziridinations of aromatic olefins, presumably due to the diminished electrophilicity of the active metal nitrene moiety.…”

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

“…The reactivity of porphyrinoid (porphyrin, corrole, and phthalocyanine), salen, and oxazoline ligands in aziridination has led to a growing interest among the scientific community in developing nitrogen-donor ligand-based aziridination catalysts with varying properties [48]. A significant number of Fe-and Co-based aziridination catalysts have been reported in the last decade, most of which are supported by nitrogen-donor ligands [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66]. In recent years, a number of Fe-based aziridination catalysts supported by oxygen-donor [67,68] and carbon-donor [69][70][71][72][73][74][75] ligands have been reported as well.…”