Tuning Reactivity in Cu/TEMPO Catalyzed Alcohol Oxidation Reactions

TL;DR

This paper explores new copper complexes that efficiently catalyze alcohol oxidation reactions, with one complex working well without an external base.

Contribution

The study introduces new ligand systems that tune the electronic properties and reactivity of copper complexes in oxidation reactions.

Findings

CuL3 performs alcohol oxidation efficiently without an external base.

Cu2L42 shows reduced catalytic activity due to self-oxidation.

Intermediate species like Cu(III)2(bis-μ-oxo) were identified and characterized.

Abstract

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.

Novel copper(I) complexes bearing a secondary amine moiety (CuL3) or an additional non‐innocent functionality with intact imine group (Cu2L42
) were synthesized and tested towards oxidation reactions. Several intermediates, such as Cu(III)2(bis‐μ‐oxo) and Cu(II)2(bis‐μ‐hydroxo), species were trapped and spectroscopically characterized. Notably, CuL3 displayed good catalytic activity without the use of an external base whereas the reactivity of Cu2L42
decreased due to a favored self‐oxidation process.

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