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 4-bpyT, 5-bpyT and 6-bpyT, 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 4-bpyT and 5-bpyT 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 4-bpyT is slightly lower than that of compound 5-bpyT. Surprisingly, the [copper/6-bpyT] 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 4-bpyT and 5-bpyT, while these absorption bands are significantly red-shifted in the case of the [Cu(II)/6-bpyT] 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 [Cu₂(4-bpyT)₂Br₄](CH₃CN)₇ (6) and [Cu₂(5-bpyT)₂Br₄](CH₃CN)₂ (7) are comparable dinuclear compounds with pentacoordinated copper ions, in a distorted square-pyramidal geometry in 6 and in a distorted trigonal-bipyramidal geometry in 7. These two coordination geometries are also reflected by their slightly different Vis-NIR results. Cu(6-bpyT)Br₂ (8) is mononuclear, with the Cu^II ion in a distorted tetrahedral geometry, suggesting a relationship with its catalytic inactivity.