Aerobic Oxidation Reactions Using Metal-based Homogeneous Systems

Abstract

Molecular oxygen is the reagent of choice for many organic reactions, including the oxidation of a variety of substrates. The importance of carbonyl and carboxylic compounds has boosted research on alternative approaches based on more sustainable reagents. To this end, aerobic oxidative approaches have emerged as a promising option to other entries involving Friedel-–Crafts acylation, Claisen condensation, esterification or amidation reactions. In comparison with the use of heterogeneous catalysts and organocatalysts, homogeneous catalysts provide considerably higher substrate : catalyst ratios and selectivities. This chapter summarizes the most remarkable reports on the use of homogeneous catalysts for the molecular oxygen-mediated oxidation of alcohols, active methylene compounds and oxidative cleavage of unsaturated hydrocarbons, with a focus on palladium, copper and ruthenium catalysts. Special attention will be paid to aspects such as catalyst amounts, the need for co-catalysts, the pressure of oxygen or air required and reaction media.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

“…Chemoselective catalytic oxidation of alcohols − is a powerful strategy for the sustainable production of fine and commodity chemicals with minimal time, waste, and cost. − Air is a convenient and attractive terminal oxidant, but partially reduced oxygen can generate highly active species that participate in competitive, nonselective reactions. , We previously reported that the dimeric cationic palladium complex [( L 1 )­Pd­(μ-OAc)] 2 [OTf] 2 ( L 1 = neocuproine = 2,9-dimethyl-1,10-phenanthroline) is an effective catalyst precursor for the oxidation of vicinal polyols under mild conditions to afford α-hydroxyketones with high regio- and chemoselectivities. − This catalyst is also selective for the direct oxidation of carbohydrates to 3-ketoses, ,− a complex transformation that typically demands multiple steps of protection/deprotection. , However, under aerobic conditions, H atom abstraction and subsequent oxidation of the benzylic methyl C–H bonds on the neocuproine ligand generate inactive palladium carboxylate species (Figure A, species 1** ). ,, This competitive ligand oxidation limits catalyst lifetime and necessitates high palladium loadings to achieve high conversions.…”