Iterative tandem catalysis of racemic AB monomers

Racemic AB monomers encompassing a secondary hydroxy group and a methyl ester moiety were synthesized and converted to chiral polyesters by iterative tandem catalysis (ITC). The concurrent action of an enantioselective acylation catalyst (Novozym 435) and a racemization catalyst (Ru(Shvo)) results in the high conversion of the racemic monomers to enantio-enriched polymers. Several factors are important for attaining high ee's and high molecular weights. The enantioselectivities observed for the novel AB monomers by Novozym 435 are high enough at 70 °C (E ratio 200) for the monomers to be useful for ITC. ITC of methyl 6-hydroxyheptanoate showed that a catalyst loading of 1.4 mol % Ru(Shvo), 25 mg Novozym 435/mmol AB monomer, and 0.5 mmol DMP/mmol monomer employing a monomer concentration of 1 mol/L gave a monomer conversion of 94%, an ee of 91%, and an Mp of 6.0 kg/mol. Application of these conditions to the other AB monomers revealed the sensitivity of the system. Reduced enantioselectivities were observed when longer reaction times were required for attaining high conversions. These long reaction times were necessary due to the slow (or absent) racemization activity of the Ru(Shvo) catalyst as a result of catalyst deactivation. Since quantitative conversions are crucial to attain high molecular weight polymers in polycondensation reactions, we could significantly improve the system by switching to isopropyl esters of the AB monomers and/or by strict exclusion of oxygen during the ITC