La(III)-Catalyzed Depolymerization of Poly(L-Lactic Acid) Yielding Chiral Lactates

Abstract

Biomass-based polymers can be a valuable resource of chiral compounds through depolymerization. [La(acac)3]-catalyzed (1 mol%) depolymerization of poly(L-lactic acid) (PLLA) (Mw = 1.58 © 105) pellets in MeOH at 90 °C for 3 h produced methyl L-lactate in >99% yield (96% ee). When powdered PLLA is used, this reaction can be achieved at 40 °C for 6 h in >99% (95% ee). PLLA is depolymerized in EtOH by La(NO3)3¢6H2O or [Fe(acac)3] (1 mol%) at 135 °C for 4 h to give ethyl L-lactate in 86% (96% ee) and 87% yield (96% ee), respectively. The depolymerization by [La(acac)3] (5 mol%) in HNEt2 at 150 °C for 3 h gives N,N-diethyllactamide in 85% but the enantiomeric excess decreases to 27% ee.