Electronic Structure of a CuII–Alkoxide Complex Modeling Intermediates in Copper-Catalyzed Alcohol Oxidations

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• 作者：Ellen C. Hayes ; Thomas R. Porter ; Charles J. Barrows ; Werner Kaminsky ; James M. Mayer ; Stefan Stoll
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：March 30, 2016
• 年：2016
• 卷：138
• 期：12
• 页码：4132-4145
• 全文大小：748K
• ISSN：1520-5126

文摘

In the copper-catalyzed oxidation of alcohols to aldehydes, a Cu^II-alkoxide (Cu^II–OR) intermediate is believed to modulate the αC–H bond strength of the deprotonated substrate to facilitate the oxidation. As a structural model for these intermediates, we characterized the electronic structure of the stable compound Tp^tBuCu^II(OCH₂CF₃) (Tp^tBu = hydro-tris(3-tert-butyl-pyrazolyl)borate) and investigated the influence of the trifluoroethoxide ligand on the electronic structure of the complex. The compound exhibits an electron paramagnetic resonance (EPR) spectrum with an unusually large g_zz value of 2.44 and a small copper hyperfine coupling A_zz of 40 × 10^–4 cm^–1 (120 MHz). Single-crystal electron nuclear double resonance (ENDOR) spectra show that the unpaired spin population is highly localized on the copper ion (≈68%), with no more than 15% on the ethoxide oxygen. Electronic absorption and magnetic circular dichroism (MCD) spectra show weak ligand-field transitions between 5000 and 12 000 cm^–1 and an intense ethoxide-to-copper charge transfer (LMCT) transition at 24 000 cm^–1, resulting in the red color of this complex. Resonance Raman (rR) spectroscopy reveals a Cu–O stretch mode at 592 cm^–1. Quantum chemical calculations support the interpretation and assignment of the experimental data. Compared to known Cu^II-thiolate and Cu^II-alkylperoxo complexes from the literature, we found an increased σ interaction in the Cu^II–OR bond that results in the spectroscopic features. These insights lay the basis for further elucidating the mechanism of copper-catalyzed alcohol oxidations.