A Rationale of the Baeyer-Villiger Oxidation of Cyclohexanone to ε-Caprolactone with Hydrogen Peroxide: Unprecedented Evidence for a Radical Mechanism Controlling Reactivity

• Published in: Chemistry : a European journal Vol. 16; no. 43; pp. 12962 - 12969
• Main Authors: Cavani, Fabrizio, Raabova, Katerina, Bigi, Franca, Quarantelli, Carla
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 15-11-2010; WILEY‐VCH Verlag
• Subjects: Caproates - chemistry; Catalysis; Cyclohexanones - chemistry; Free Radical Scavengers - chemistry; heterogeneous catalysis; Hydrogen Peroxide; lactones; Lactones - chemistry; Molecular Structure; oxidation; Oxidation-Reduction; radical reactions; Silicates - chemistry; Stereoisomerism; Thermodynamics; Titanium - chemistry; titanium silicalite; Water - chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201001777

We demonstrate, for the first time, in the Baeyer–Villiger oxidation of cyclohexanone with aqueous hydrogen peroxide under conditions aimed at obtaining ε‐caprolactone, that a thermally activated radical reaction leads to the concurrent formation of adipic acid, even when a stoichiometric amount of the oxidant is used. In fact, ε‐caprolactone is the primary reaction product, but it is more reactive than cyclohexanone, and quickly undergoes consecutive transformations. When titanium silicalite‐1 (TS‐1) is used as a catalyst, the high concentration of hydroxy radicals within its pores accelerates the reaction rates, and the consecutive formation of adipic acid (and of lighter diacids as well) becomes largely kinetically preferred. The proper choice of the solvent, which also may act as a radical scavenger, both without catalyst and with TS‐1, is a powerful tool for controlling the rates of the various reactions involved.
Oxidation control: In the oxidation of cyclohexanone with hydrogen peroxide under conditions aimed at obtaining ε‐caprolactone, a thermally activated radical reaction leads to the concurrent formation of adipic acid (see figure). The relevant reaction rates are modified when Ti‐silicalite is used as catalyst. The proper choice of solvent, which may also act as a radical scavenger, also allows control over the reaction rates.