Mimicking the Reactivity of LPMOs with a Mononuclear Cu Complex

Tuning Reactivity in Cu/TEMPO Catalyzed Alcohol Oxidation Reactions

A dinuclear copper(I) complex Cu2L22 (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)2(bis-μ-oxo) and Cu(II)2(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 O2 activation mechanisms. The stronger donation of the secondary amine group stabilizes a mononuclear CuIL3 core, which nevertheless follows a dinuclear O2 activation mechanism as in Cu2L22. Notably, the CuIL3/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 CuI 2L42 complex involving a non-innocent phenolate group, in contrast, exhibits depleted catalytic activity, because of the instability of the Cu(III)2(bis-μ-oxo) core against intramolecular H-atom abstraction to form an alkoxo bridged dicopper(II) complex.

Leveraging ligand-based proton and electron transfer for aerobic reactivity and catalysis

While O2 is an abundant, benign, and thermodynamically potent oxidant, it is also kinetically inert. This frequently limits its use in synthetic transformations. Correspondingly, direct aerobic reactivity with O2 often requires comparatively harsh or forcing conditions to overcome this kinetic barrier. Forcing conditions limit product selectivity and can lead to over oxidation. Alternatively, O2 can be activated by a catalyst to facilitate oxidative reactivity, and there are a variety of sophisticated examples where transition metal catalysts facilitate aerobic reactivity. Many efforts have focused on using metal-ligand cooperativity to facilitate the movement of protons and electrons for O2 activation. This approach is inspired by enzyme active sites, which frequently use the secondary sphere to facilitate both the activation of O2 and the oxidation of substrates. However, there has only recently been a focus on harnessing metal-ligand cooperativity for aerobic reactivity and, especially, catalysis. This perspective will discuss recent efforts to channel metal-ligand cooperativity for the activation of O2, the generation and stabilization of reactive metal-oxygen intermediates, and oxidative reactivity and catalysis. While significant progress has been made in this area, there are still challenges to overcome and opportunities for the development of efficient catalysts which leverage this biomimetic strategy.

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).