Selective Alcohol Oxidation by a Copper TEMPO Catalyst: Mechanistic Insights by Simultaneously Coupled Operando EPR/UV-Vis/ATR-IR Spectroscopy

The first coupled operando EPR/UV-Vis/ATR-IR spectroscopy setup for mechanistic studies of gas-liquid phase reactions is presented and exemplarily applied to the well-known copper/TEMPO-catalyzed (TEMPO = (2,2,6,6-tetra-methylpiperidin-1-yl)oxyl) oxidation of benzyl alcohol. In contrast to previous proposals, no direct redox reaction between TEMPO and Cu-I/Cu-II has been detected. Instead, the role of TEMPO is postulated to be the stabilization of a (bpy)(NMI)Cu-II-O-2(-)-TEMPO (bpy = 2,2'-bipyridine, NMI = N-methylimidazole) intermediate formed by electron transfer from Cu-I to molecular O-2.