Base-Free Aerobic Oxidation of Alcohols over Copper-Based Complex under Ambient Condition

Abstract

Cu-based complex formed from Cu­(OAc)2 and [2,2′]-bipyridinyl-5,5′-dicarboxylic acid diethyl ester (BPYDCDE) ligand was synthesized for the first time. It was found that the complex could catalyze aerobic oxidation of alcohols to aldehydes or ketones very efficiently without any external base at ambient temperature and pressure, and the yield of the desired product reached >99% in 2–5 h. Combination of experimental and theoretical studies showed that the ligand enhanced the electron population on the Cu center by a ligand-to-metal charge transfer (LMCT) effect, which made OAc– in the complex have the appropriate alkalinity and be a good leaving group, and the Cu center and the OAc– catalyze the reaction cooperatively. Moreover, the amount of OAc– in the complex was much less than that of the external base added in the catalytic systems reported, suggesting that the basic anion in the complex is more efficient for promoting the reaction than the external base added.

Biomimetic Cu/Nitroxyl Catalyst Systems for Selective Alcohol Oxidation

“…The activity of the different complexes are governed by the metal, the alkalinity of the counter ion as well as the structure of the ligand. Furthermore, they also found that the Cu II -complex could be recycled for up to five times without the loss of activity [65].…”

“…It was found that the nature of the external bases have an influence on the catalytic reactivity, selectivity and stability [28] which have led to the development of catalyst systems capable of operating in the absence of a base. In 2018, Han and co-workers reported a base-free catalyst system, using TEMPO·, [2,2′]-bipyridinyl-5,5′-dicarboxylic acid diethyl ester (BPYDCDE) as the organic ligand and Cu(OAc)2 as the metal precursor ( Figure 1) [65]. Unfortunately, our catalyst system could not be used for the oxidation of secondary alcohols, attributed to the bulky structure of TEMPO• and the steric pressure exerted during the TEMPO•-mediated C-H abstraction step [2,8,28,59].…”

“…It was found that the nature of the external bases have an influence on the catalytic reactivity, selectivity and stability [28] which have led to the development of catalyst systems capable of operating in the absence of a base. In 2018, Han and co-workers reported a base-free catalyst system, using TEMPO•, [2,2 ]-bipyridinyl-5,5 -dicarboxylic acid diethyl ester (BPYDCDE) as the organic ligand and Cu(OAc) 2 as the metal precursor ( Figure 1) [65].…”

“…In 2018, Han and co-workers reported a base-free catalyst system, using TEMPO·, [2,2′]-bipyridinyl-5,5′-dicarboxylic acid diethyl ester (BPYDCDE) as the organic ligand and Cu(OAc)2 as the metal precursor ( Figure 1) [65]. Their catalyst system, operating at ambient temperatures and pressures, could be used for the conversion of a variety of primary and secondary alcohols to the corresponding aldehydes and ketones in a short reaction time, without the addition of external base [65]. The coordination between the ligand and OAcenriches the electron population on the Cu-centre, increasing the alkalinity of the OAccounter ion to afford a better leaving group.…”

“…Independently, Liu and co-workers also reported a CuI/NMI/TEMPO· (ratio of 1:2:1) catalyst system, in the absence of an external base, for the oxidation of both primary and secondary (partially successful) alcohols to their corresponding carbonyl products, under ambient conditions and Figure 1. The Cu(BPYDCDE)(OAc) 2 complex for the base-free catalyst system for alcohol oxidation [65]. Reproduced with permission from Han and co-workers.…”