3 years ago

Formation of δ-Lactones with anti-Baeyer–Villiger Regiochemistry: Investigations into the Mechanism of the Cerium-Catalyzed Aerobic Coupling of β-Oxoesters with Enol Acetates

Formation of δ-Lactones with anti-Baeyer–Villiger Regiochemistry: Investigations into the Mechanism of the Cerium-Catalyzed Aerobic Coupling of β-Oxoesters with Enol Acetates
Irina Geibel, Anna Dierks, Thomas Müller, Jens Christoffers
The cerium-catalyzed, aerobic coupling of β-oxoesters with enol acetates and dioxygen yields δ-lactones with a 1,4-diketone moiety. In contrast to the Baeyer–Villiger oxidation (BVO), where the higher substituted residue migrates; in the case of this oxidative C−C coupling reaction, the less substituted alkyl residue undergoes a 1,2-shift. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate and submitted to conformational analysis by computational methods. As a result, the inverse regiochemistry is explained by a primary stereoelectronic effect. A Hammett analysis using different donor- and acceptor-substituted enol esters provides support for the oxycarbenium ion being the crucial intermediate in the rate determining step of the conversion. An overall mechanism is suggested with a radical chain reaction for the formation of endoperoxides from β-oxoesters, enol acetates and dioxygen with a cerium(IV) species as initiating reagent. Cer-tified mechanism: Reaction kinetics (Hammett analysis) supported by computational methods indicate an endoperoxidic oxycarbenium ion to be the reaction intermediate for the cerium-catalyzed coupling of β-oxoesters, enol acetates and dioxygen.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201605468

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