Lewis Acid Trapping of an Elusive Copper–Tosylnitrene Intermediate Using Scandium Triflate

Abstract

High–valent copper nitrene intermediates have long been proposed to play a role in copper catalyzed aziridination and amination reactions. However, such intermediates have eluded detection for decades, which prevents the unambiguous assignments of mechanisms. Moreover, the electronic structure of the proposed copper–nitrene intermediates has also been controversially discussed in the literature. These mechanistic questions and controversy have provided tremendous motivation for probing the accessibility and reactivity of CuIII–NR/CuIIN•R species. In this paper we report a breakthrough in this field by trapping a transient copper–tosylnitrene species 3–Sc in presence of scandium triflate. Sufficient stability of 3–Sc at −90 °C enabled its characterization with optical, resonance Raman, nuclear magnetic resonance, and x–ray absorption near edge (XANES) spectroscopies, which helped to establish its electronic structure as CuIIN•Ts (Ts = tosyl group) and not CuIIINTs. 3–Sc can initiate tosyl–amination of cyclohexane, thereby suggesting CuIIN•Ts cores as viable reactants in oxidation catalysis.

An Open‐Shell Spin Singlet Copper‐Nitrene Intermediate Binding Redox‐innocent Metal Ions: Influence of the Lewis Acidity of the Metal Ions on Spectroscopic and Reactivity Properties

“…Similarly, the metal ions proved to have no influence on the reactivity of the 1-M complexes since near-identical rates were obtained for PPh 3 + species, which may be generated in solution in presence of excess PPh 3 owing to the strong binding of PPh 3 to M (Scheme 2). Alternatively, the metal-independent spectroscopic properties of the Lewis-acid bound copper-nitrene complexes may point to a different binding mode of M than originally proposed [21] in our earlier communication; a tautomeric structure B of 1-M (Scheme 1) can be visualized, where M is bound to an amidate nitrogen of the ancillary ligand instead of the previously proposed binding to the nitrene nitrogen (structure A in Scheme 1). Detailed spectroscopic, reactivity, and theoretical studies are ongoing in our laboratory in order to obtain further insights into the structure and reactivity of the Lewis-acid bound copper-nitrene intermediates.…”

“…As we previously reported, the reaction of [21] The methodology of using Sc(OTf) 3 Figure 1; Table 1). The formation of 1-M is confirmed on the basis of the characteristic absorption features at 530 and 750 nm, and that of 3 by EPR spectroscopy.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Very recently, our group reported the spectroscopic characterization of a Lewis acid adduct of a cationic copper-tosylnitrene species [(AN)Cu II -(NTs • )Sc(OTf) 3 ] + (1-Sc) [Ts = tosyl; AN = 3,3Ј-iminobis(N,N-dimethylpropylamine)], the electronic structure of which could be best described in terms of a Cu II ion antiferromagnetically coupled to a nitrene radical thereby stabilizing an open-shell singlet ground state. [21] In this study although the binding-site of the scandium ion was not unambiguously identified, it is believed that the effect of the Lewis acid in the stabilization of the copper-nitrene intermediate is related to its interaction with the otherwise transient [(AN)Cu II (NTs • )] + core, which lowers the electron density at the nitrene nitrogen thereby reducing the strong electron repulsion between the electron-rich nitrogen and central copper atoms. Notably, a similar interaction between a high-valent manganese-oxo core and Ca 2+ ion is proposed to play a vital role during biological oxygen evolution in photosystem II.…”

A High‐Valent Heterobimetallic [CuIII(μ‐O)2NiIII]2+ Core with Nucleophilic Oxo Groups

“…Figure 2A shows a comparison of the Cu K -edge spectrum of 3 with its Cu(I) precursor and the previously reported [(AN)Cu II (NHTs)] [ 4 , AN = 3,3′-iminobis( N , N - dimethyl- propylamine; Ts= tosyl] [10] complex. A progressive blue shift in the edge energy from 2 - BF 4 to 4 to 3 is in accord with a stepwise increase in the oxidation state of copper in the series.…”

Characterization of Porphyrin-Co(III)-‘Nitrene Radical’ Species Relevant in Catalytic Nitrene Transfer Reactions

“…Thus, well-studied complexes with redox active aminyl ligand radicals are limited to only a few well-characterized examples, 65 and related complexes bearing redox non-innocent nitrene ligands are even scarcer. 9,43,65–67 Given the importance of these radicaloid nitrene species in metal-catalyzed nitrene transfer and C–H functionalization reactions, 10,12,13,39–42,68 we gathered more experimental evidence for the formation of the previously reported mono-nitrene radical complexes upon reaction of cobalt(II) porphyrins with nitrene transfer reagents. 9,43 Furthermore, we here reveal the first example of a bis-nitrene species of 1 P1 upon reacting the stronger oxidizing nitrene transfer agent N -nosyl iminoiodane 4 Ns with 1 P1 .…”