Synthesis and characterization of new Na+ complexes of N-benzyl cyclic peptoids and their role in the ring opening polymerization of l-lactide

Abstract

Cyclic peptoids are biocompatible/biodegradable cyclooligomers constituted by N-substituted glycines showing high binding constants with the first group alkali metals (K_a ∼ 10⁶ for Na⁺, Li⁺ and K⁺) in organic solvents. Three new metallated species bearing N-benzyl groups as side chains (a cyclic pentamer, [1·2Na]²⁺, and two cyclooctamer [3·Na]⁺ and [3·2Na]²⁺ peptoids) were synthesized together with the known N-perbenzylated sodiated cyclohexamer [2·Na]⁺ and [2·2Na]²⁺. Na⁺ complexes were investigated by means of spectroscopic (NMR), computational (DFT) and X-ray crystallographic studies. The first application of these complexes as catalysts in the ring opening polymerization (ROP) of L-lactide (L-LA) was examined. Our studies suggest that, for the intrinsic properties of the catalytic site (revealed by steric topographic maps) the homooligomeric pentamer complex [1·2Na]²⁺ is the most active member of the cyclooligomeric family.