Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Abstract

Using dimeric dialkylgallium and dialkylindium alkoxide catalysts for the polymerization of rac-lactide (rac-LA), we have shown for the first time that the formation of homochiral dimeric species [Me₂MOR]₂ (M = Ga, In), induced by chiral recognition of monomeric Me₂MOR units in the presence of Lewis base, leads to an increase of the heteroselectivity of the ring opening polymerization (ROP) of rac-LA, and therefore provides a new tool for controlling the stereoselectivity of the polymerization of heterocyclic monomers. To explain the origin of the heteroselectivity of the [Me₂Ga(μ-OCH(Me)CO₂Me)]₂/Lewis base system in the ROP of rac-LA, structure of (S,S)-[Me₂Ga(μ-OCH(Me)CO₂Me)]₂ ((S,S)-1) and rac-[Me₂Ga(μ-OCH(Me)CO₂Me)]₂ (1) in the absence and presence of tertiary amines and pyridines was investigated. Studies were further extended by analysis of the structure/activity data for both (S,S)-[Me₂In(μ-OCH(Me)CO₂Me)]₂ ((S,S)-2) and rac-[Me₂In(μ-OCH(Me)CO₂Me)]₂ (2). Contrary to gallium complex 1, which exists in a solution as equimolar mixture of homo- and heterochiral diastereomers, an excess of homochiral (R*,R*)-2 species was observed in the case of 2. For both the Ga and In complexes, the interaction of amines with the metal center increased the tendency for the formation of homochiral species with retention of the dimeric structure in the solution. This tendency was additionally demonstrated by the structure of model dialkylgallium (3) and indium (4) complexes with monoanionic ligands possessing chiral centers in the α-position to the alkoxide oxygen and pyridine functionalities. The polymerization of rac-LA with gallium and indium catalysts (S,S)-1 and (S,S)-2 resulted in the formation of heterotactically enriched polylactide (PLA) (P_r = 0.50–0.85) and (P_r = 0.54–0.72), respectively. The heteroselectivity of the investigated systems was in line with the excess of the homochiral catalytic species. The higher activity of homochiral species activated by amines resulted in a positive non-linear effect between an excess of homochiral (R*,R*)-1 or (R*,R*)-2 catalysts and the heterotacticity of the obtained PLA. The observed dependence of stereoselectivity of rac-LA polymerization on the excess of homochiral species was similar to the asymmetric amplification in enantioselective organic catalysis; however, it is exceptional in polymerization processes.