Yttrium– and Aluminum–Bis(phenolate)pyridine Complexes: Catalysts and Model Compounds of the Intermediates for the Stereoselective Ring-Opening Polymerization of Racemic Lactide and β-Butyrolactone

Abstract

Yttrium and aluminum complexes of an original pyridine-bis(phenolate) ligand have been prepared. Reactions of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1) with 1 equiv of methyl (R)-3-hydroxybutyrate and methyl (S,S)-lactyllactate afforded respectively {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) and {ONOMe,Cumyl}Y((S,S)-OCH(CH3)CO2CH(CH3)CO2Me) (3), which are rare models of the active species in the ring-opening polymerization (ROP) of β-butyrolactone (BBL) and lactide (LA), respectively. 13C NMR data for 2 and 3 indicate that, in solution, the carbonyl groups coordinate onto the yttrium centers, forming mononuclear species with six- and five-membered cycles, respectively. The aluminum compounds {ONOMe,Cumyl}Al(iPr (S)-lactate) (5), {ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6), and {ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7) were prepared analogously from the parent methyl complex {ONOMe,Cumyl}AlMe (4). NMR data and the solid-state structure of 6 confirm the coordination of the carbonyl group. Yttrium compounds 1 and 2 are active initiators for the ROP of racemic LA and BBL. Their performances (activity, control of the molecular weights, tacticity) are much affected by the nature of the solvent or by the addition of just a few equivalents of pyridine. Optimal conditions are quite contrasted for the ROP of rac-LA and rac-BBL, highlighting fundamental differences between these two monomers. In the best cases, highly heterotactic PLAs (Pr up to 0.96) and syndiotactic-enriched PHB (Pr up to 0.86), with good control over the molecular weights, were obtained.