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)(2)M((OPr)-Pr-i)]Li(THF) (2a, M = Ti; 4a, M = Zr) and [(Ph-binapht-O)(2)M((OPr)-Pr-i)]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 O'Pr 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 (Delta, aS, aS)/(Lambda, 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)(2)Zr(THF)(2) (3a) and (Ph-binapht-O)(2)Zr(THF)(2) (3b) were both isolated and X-ray characterized as stereomers in a heterochiral configuration (Delta, aR, aS)/(Lambda, 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 PIA, as deduced from SEC, kinetic, and MALDI-TOF data. The Zr-(OPr)-Pr-i 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 (P-r=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 pi donation of the four Zr-O-ArO 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.