Copper/TEMPO-Catalyzed Aerobic Alcohol Oxidation: Mechanistic Assessment of Different Catalyst Systems

Abstract

Combinations of homogeneous Cu salts and TEMPO have emerged as practical and efficient catalysts for the aerobic oxidation of alcohols. Several closely related catalyst systems have been reported, which differ in the identity of the solvent, the presence of 2,2′-bipyridine as a ligand, the identity of basic additives, and the oxidation state of the Cu source. These changes have a significant influence on the reaction rates, yields, and substrate scope. In this report, we probe the mechanistic basis for differences among four different Cu/TEMPO catalyst systems and elucidate the features that contribute to efficient oxidation of aliphatic alcohols.

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

“…In particular, previous work on the Cu/TEMPO system suggests that the reaction proceeds via a concerted 2e -oxidation, wherein a Cu-bound alcohol is simultaneously oxidized by TEMPO and Cu(II). 15 In contrast, preliminary mechanistic experiments with MCl 3 ( 1 -TEMPO) suggest that the reaction proceeds via an initial 1e -hydrogen atom transfer event, which apparently makes this system unique amongst TEMPO-containing oxidants. As a result, we wanted to solidify our proposed mechanism by exploring the reactivity of MCl 3 ( 1 -TEMPO) with a variety of mechanistic probes, including activated alkanes and radical clocks.…”

Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

“…11 Recent progress indicates that both Cu(I) and Cu(II) complexes with a N,N-or N,O-ligands in combination with TEMPO could be efficient catalysts or catalyst precursors for the aerobic oxidation of various alcohols, and of particular note is a highly efficient copper/2,2'-bipyridine/TEMPO system developed by the Stahl group. [12][13][14][15][16][17] However, the types of organic ligands exploited in the copper catalyst systems are still limited and the structures of copper catalysts involved in the catalytic process are rarely explored. [17][18][19][20] The construction of metal-organic cluster compounds through self-assembly of predesigned ligands with appropriate metal ions is a prime research topic [21][22][23][24][25] due to the advantages in catalysing useful chemical transformations, in contrast to simple monometallic complexes.…”

“…[12][13][14][15][16][17] However, the types of organic ligands exploited in the copper catalyst systems are still limited and the structures of copper catalysts involved in the catalytic process are rarely explored. [17][18][19][20] The construction of metal-organic cluster compounds through self-assembly of predesigned ligands with appropriate metal ions is a prime research topic [21][22][23][24][25] due to the advantages in catalysing useful chemical transformations, in contrast to simple monometallic complexes. [26][27][28][29][30] Schiff base type ligands are excellent ligand candidates for high-nuclearity metallosupramolecular assemblies, in particular in cases where additional O-donor atoms have been introduced to the backbone of a Schiff base ligand scaffold.…”

“…12-17 However, the types of organic ligands exploited in the copper catalyst systems are still limited and the structures of copper catalysts involved in the catalytic process are rarely explored. [17][18][19][20] The construction of metal-organic cluster compounds through self-assembly of predesigned ligands with appropriate metal ions is a prime research topic [21][22][23][24][25] Herein, we report the one-pot syntheses and crystal structures of two tetranuclear and dinuclear copper(II) complexes resulting from the in situ synthesis of chiral Schiff base ligands (Scheme 1), and their catalytic applications in the TEMPO-mediated aerobic oxidation of benzylic alcohols. The catalytic potential of two complexes is compared based on their catalytic performance in various alcohol oxidation reactions, suggesting that the copper(II) complex with a Cu 4 core is a more efficient catalyst than that one having a Cu 2 core.…”

Scalable thin-layer membrane reactor for heterogeneous and homogeneous catalytic gas–liquid reactions

“…[22][23][24] However, the practical usage of aerobic oxidation in large-scale synthesis raises safety concerns, namely the formation of explosive mixtures (flammable organic solvents in oxygen). A number of microreactor-based biphasic flow implementations of aerobic oxidation improve the safety profile by accurately controlling oxygen flow in the microchannels.…”