Continuous Production of Biorenewable, Polymer-Grade Lactone Monomers through Sn-β-Catalyzed Baeyer-Villiger Oxidation with H2O2

The Baeyer-Villiger oxidation is a key transformation for sustainable chemical synthesis, especially when H2O2 and solid materials are employed as oxidant and catalyst, respectively. 4-substituted cycloketones, which are readily available from renewables, present excellent platforms for Baeyer-Villiger upgrading. Such substrates exhibit substantially higher levels of activity and produce lactones at higher levels of lactone selectivity at all values of substrate conversion, relative to non-substituted cyclohexanone. For 4-isopropyl cyclohexanone, which is readily available from -pinene, continuous upgrading was evaluated in a plug-flow reactor. Excellent selectivity (85% at 65% conversion), stability, and productivity were observed over 56h, with over 1000 turnovers (mol product per mol Sn) being achieved with no loss of activity. A maximum space-time yield that was almost twice that for non-substituted cyclohexanone was also obtained for this substrate [1173 vs. 607g(product)kg(catalyst)(-1)cm(-3)h(-1)]. The lactone produced is also shown to be of suitable quality for ring opening polymerization. In addition to demonstrating the viability of the Sn-/H2O2 system to produce renewable lactone monomers suitable for polymer applications, the substituted alkyl cyclohexanones studied also help to elucidate steric, electronic, and thermodynamic elements of this transformation in greater detail than previously achieved.