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, preventing 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 to probe the accessibility and reactivity of Cu(III)-NR/Cu(II)N(•)R species. In this paper, we report a breakthrough in this field that was achieved by trapping a transient copper-tosylnitrene species, 3-Sc, in the presence of scandium triflate. The sufficient stability of 3-Sc at -90 °C enabled its characterization with optical, resonance Raman, NMR, and X-ray absorption near-edge spectroscopies, which helped to establish its electronic structure as Cu(II)N(•)Ts (Ts = tosyl group) and not Cu(III)NTs. 3-Sc can initiate tosylamination of cyclohexane, thereby suggesting Cu(II)N(•)Ts cores as viable reactants in oxidation catalysis.

Figure 1

Top: Absorption Spectra of 1–BF₄ (solid line), 2 (dash–dotted line), and 3–Sc (dashed line) in CH₂Cl₂ at −90 °C. Inset: rRaman spectra of 3–Sc–¹⁴N (black line), 3–Sc–¹⁵N (red line) and 3–Sc–decay product at RT (gray line) upon exciting at 514 nm. Bands originating from the solvent are marked by asterisk. Bottom: Normalized XANES spectra for 1–BF₄ (dotted line), 2 (solid line), and 3–Sc (dashed line). The inset depicts an expansion of the pre–edge region for 2 and 3-Sc.

Figure 2

Changes in the absorption spectra associated with the reaction of 3–Sc (0.34 mM) with ferrocene (30 equiv.) at −90 °C. Inset: Absorption spectra of 2 (dotted line) (0.36 mM) and its reaction product (solid line) with ferrocene (30 equiv.) in presence of 1 equivalent of Sc(OTf)₃ at −90 °C. The yield of the ferrocenium was determined on the basis of the known extinction coefficient (ε) of 620 nm band = 505 M⁻¹cm⁻¹ in CH₂Cl₂ at −90 °C.

Figure 3

Plot of log k' for 3–Sc at −90 °C against BDE of different C–H substrates. k’ is determined by dividing the second–order rate constant k₂ by number of equivalent H–atoms in the substrate. Inset: Plot of pseudo–first order rate constants (k_obs) against different concentrations of DHA and d₄–DHA. The point at zero substrate concentration corresponds to the pseudo–first order rate of the self–decay of 3–Sc at −90 °C.

Scheme 1

Formation and reactivity of 2 and 3–Sc. Color code: Sc: purple; N: blue; S: yellow; O: red; Cu: orange and C: grey.