The mechanistic diversity of the selective aerobic oxidation of alcohols catalyzed by systems derived from CuI and a diamine ligand

Abstract

In this piece of work, CuI and its catalysis of the aerobic oxidation of alcohols have been mechanistically investigated with N,N,N′,N′-tetramethylethylenediamine (TMEDA) as a ligand and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as a co-catalyst. The system demonstrates excellent catalytic activity for selective aerobic oxidation of a wide range of primary alcohols under ambient conditions both in quantitative yield and on a preparative scale. In the course of mechanistic investigation, two binuclear copper complexes were isolated with the cores of “Cu(I)(μ-I)₂Cu(I)” (1) and “Cu(II)(μ-OH)₂Cu(II)” (2), respectively, using CuI as the precursor and 1 can also be quantitatively converted into 2 under aerobic conditions. Complex 1 initiates the oxidation as much as CuI does whereas complex 2 exhibits only about 60% of the activity of CuI. However, the addition of iodide can boost its catalytic activity to the scale of both CuI and complex 1. In combination with the stepwise monitoring of CuI and complex 2 catalytic systems, respectively, by using cyclic voltammetry, UV-vis spectroscopy, and tandem ESI-MS, we are able to establish that complex 1 may be one of the intermediates in the catalytic cycle starting with CuI, but overall, a monocopper species is the active intermediate with oxidation states shuttling between Cu(II) and Cu(I). Complex 2, however, undergoes an independent catalytic path by maintaining its dinuclear integrity during the catalysis.