Stereoselective Synthesis of Biphenolate/Binaphtolate Titanate and Zirconate Alkoxide Species: Structural Characterization and Use in the Controlled ROP of Lactide

Well-defined biphenol/binaphtolate group 4 alkoxide salt
species
[(Ph-Biphen-O)2M­(OiPr)]­Li­(THF)
(2a, M = Ti; 4a, M = Zr) and [(Ph-binapht-O)2M­(OiPr)]­Li­(THF) (2b, M = Ti; 4b, M = Zr) were found to be readily accessible
in good yields via alcohol elimination routes and/or substitution
reactions from the corresponding pro-ligands Ph-Biphen-OH (1a) and rac-Ph-Binapht-OH (1b). As established
via X-ray crystallographic analysis, the molecular structures of the
Ti derivatives 2a and 2b consist of Li+ salts of anionic Ti–OiPr moieties in which the Ti center adopts a distorted tbp geometry
and is effectively chelated by two biphenolate/binaphtolate units.
Remarkably, the solution and solid state data for salt species 2a,b agree with the sole presence of one diastereomer
(with a (Δ, aS, aS)/(Λ, aR, aR) configuration), thus indicating
that formation of the Ti and Zr alkoxide complexes 2a,b/4a,b proceeds stereoselectively. In
contrast, the neutral biphenolate/binaphtolate Zr complexes (Ph-biphen-O)2Zr­(THF)2 (3a) and (Ph-binapht-O)2Zr­(THF)2 (3b) were both isolated and
X-ray characterized as stereomers in a heterochiral configuration
(Δ, aR, aS)/(Λ, aS, aR). The Ti and Zr alkoxide anionic
chelates were found to initiate the ROP of rac-lactide
in a controlled manner for production of narrowly disperse and ester-end
group PLA, as deduced from SEC, kinetic, and MALDI-TOF data. The Zr–OiPr derivatives 4a,b exhibit superior performance to their Ti counterparts (whether regarding
activity, polymerization control, or stereoselectivity) to produce
narrowly disperse and heterotactically enriched PLA (Pr = 0.67, PDI < 1.15). The significantly decreased
Lewis acidity of the Zr metal center in anions 4a,b (versus neutral analogues) due to the anionic charge and
a likely substantial electronic π donation of the four Zr–OArO oxygens to the Zr metal
center may rationalize the moderate polymerization activity. Control
experiments suggest that the nature of the countercation has little
influence on lactide ROP activity and stereocontrol.