Tuning Reactivity in Cu/TEMPO Catalyzed Alcohol Oxidation Reactions.

A dinuclear copper(I) complex CuL2 (L2 = 3,3-dimethyl-1-(1-methyl-1H-benzo[d]imidazole-2-yl)-N-(propan-2-ylidene)butan-2-amine) containing benzimidazole and imino donors was previously reported by some of us as an efficient catalyst for the aerobic oxidation of alcohols to aldehydes in presence of TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) and an external base NMI (N-methyl imidazole). Cu(III)(bis-μ-oxo) and Cu(II)(bis-μ-hydroxo) cores were trapped as viable intermediates in the reaction, which provided deeper mechanistic insights. Here, we report two new ligand systems L3 (N-isopropyl-3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-amine) and L4 ((Z)-2,4-di-tert-butyl-6-(((3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-yl)imino)methyl)phenol), which are designed to perturb the overall electronics of the complexes and the resulting effects on their O activation mechanisms. The stronger donation of the secondary amine group stabilizes a mononuclear CuL3 core, which nevertheless follows a dinuclear O activation mechanism as in CuL2. Notably, the CuL3/TEMPO catalyst system performs the aerobic oxidation of alcohols to aldehydes with good yields and turnover numbers, even in the absence of NMI. The dinuclear Cu L4 complex involving a non-innocent phenolate group, in contrast, exhibits depleted catalytic activity, because of the instability of the Cu(III)(bis-μ-oxo) core against intramolecular H-atom abstraction to form an alkoxo bridged dicopper(II) complex.