Continuous production of bio-renewable, polymer-grade lactone monomers through Sn-β catalysed 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. We demonstrate that 4-substituted cycloketones, 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, readily available from β-pinene, continuous upgrading was also evaluated in a plug flow reactor. Excellent selectivity (< 90 % at 65 % conversion), stability and productivity were observed over 56 h, with over 1000 turnovers (mol. product / mol. Sn) being achieved with no loss of activity. A maximum space-time yield almost twice that obtained for non-substituted cyclohexanone was also obtained for this substrate (1173 vs. 607 g (product) kg-1 (cat) cm-3 (reactor) h-1). The lactone produced is also shown to be of suitable quality for ring opening polymerisation. In addition to demonstrating the viability of the Sn-β/H2O2 system to produce renewable lactone monomers suitable for polymer applications, the substituted alkyl cyclohexanones studied in this work also allowed us to probe the steric, electronic and thermodynamic elements of this transformation in greater detail than previously achieved.