Selective, catalytic aerobic oxidation of alcohols using CuBr2 and bifunctional triazine-based ligands containing both a bipyridine and a TEMPO group

Abstract

Three novel, bifunctional triazine-based ligands, namely , and , containing both a TEMPO and a bipyridine moiety have been synthesized. These bpy/TEMPO-based molecules have been used as catalyst precursors for the copper-catalyzed aerobic oxidation of alcohols to aldehydes and ketones, in the presence of tert-BuOK as co-catalyst. The complexes obtained in situ from ligands and with copper(II) bromide in a 2:1 acetonitrile/water mixture, selectively catalyze the aerobic oxidation of primary benzylic, allylic and aliphatic alcohols and secondary benzylic alcohols. The rate of oxidation achieved using compound is slightly lower than that of compound . Surprisingly, the [copper/] system is not an efficient catalyst. The distinct catalytic behaviour of the three complexes is most likely due to the different position of the anchoring point of the bipyridine moiety on the triazine core, thereby inducing dissimilar steric effects. The effect of the substitution position of the bipyridine unit is reflected by the Vis-NIR spectra of the corresponding copper(II) complexes, which show similar LMCT and d-d transitions for and , while these absorption bands are significantly red-shifted in the case of the [Cu(II)/] complex. These differences are indicative of different coordination environments around the Cu(II) centres in those compounds.Single-crystal X-ray diffraction studies reveal that [Cu(2)(4-bpyT)(2)Br(4)](CH(3)CN)(7) () and [Cu(2)(5-bpyT)(2)Br(4)](CH(3)CN)(2) () are comparable dinuclear compounds with pentacoordinated copper ions, in a distorted square-pyramidal geometry in and in a distorted trigonal-bipyramidal geometry in . These two coordination geometries are also reflected by their slightly different Vis-NIR results. Cu(6-bpyT)Br(2) () is mononuclear, with the Cu(II) ion in a distorted tetrahedral geometry, suggesting a relationship with its catalytic inactivity.

Readily Accessible Oxazolidine Nitroxyl Radicals: Bifunctional Cocatalysts for Simplified Copper Based Aerobic Oxidation

“…While L1, and indeed all nitroxyl radicals in this study, were moderately unstable, [21] they could be prepared, isolated, and used over the course of a week without special handling. It was decided to explore copper-based oxidation reactions analogous to those reported by Sheldon, [7a] owing to their extensive investigation, [7,8,13] thereby providing an appropriate benchmark. Specifically, the oxidation of benzyl alcohol to benzaldehyde, using nitroxyl radical L1, in acetonitrile/water, with 5 mol% CuBr 2 , 5 mol% 2,2'-bipy, 5 mol% KOH, [22] and oxygen in air as the stoichiometric oxidant was attempted.…”

“…[11] We envisaged the assembly of catalysts with coordinating groups adjacent to the nitroxyl radical centre. [12] In addition to introducing chirality such structures would display bifunctionality, [13] potentially increasing catalytic activity. To deliver such structures it was decided to study 2,2,4,4-tetramethyloxazolidine-N-oxyl (TOXYL 1) [14] as a scaffold.…”

Immobilization of Cu Complex into Zr-Based MOF with Bipyridine Units for Heterogeneous Selective Oxidation

“…The peaks attributed to Ir in the spectrum are due to the sputtered Ir for the measurement.In order to elucidate the local coordination environment of the Cu atom in the framework of Zr-MOF-bpy, diffuse reflectance UV-vis spectroscopy and X-ray absorption fine structure (XAFS) spectroscopy were performed.Figure 4shows diffuse reflectance UV-vis spectra of Zr-MOF-bpy and Zr-MOF-bpy-CuBr2. The UV-vis spectrum of Zr-MOF-bpy-CuBr2 shows two typical absorption bands in the region of 470-530 nm corresponding to metal-to-ligand charge transfer of the Cu complex and in the region of 650-780 nm corresponding to d-d transition of Cu,42 although the spectrum of Zr-MOF-bpy exhibits absorption bands below 380 nm. It was also confirmed that the absorption property is similar to that of the corresponding homogeneous complex ((bpy)CuBr2).…”

Room temperature aerobic oxidation of alcohols using CuBr2 with TEMPO and a tetradentate polymer based pyridyl-imine ligand

“…Also, there has been significant interest in using the free radical TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxy) or analogues as efficient co-catalysts in the aerobic oxidation of alcohols [26][27][28][29][30][31][32][33]. Sheldon and Reedijk et al have reported the efficient use of copper-bipyridine and copper-pyrazole/TEMPO systems in oxidation reactions under air [34][35][36][37]. Subbaryant et al have reported a TEMPO/Copper-salen system that could efficiently oxidize a wide range of benzylic and heterocyclic alcohols (e.g.…”