Rapid and Controlled Polymerization of rac-Lactide Using N,N,O-Chelate Zinc Enolate Catalysts

Abstract

The synthesis and characterization of N,N,O-chelate zinc enolate complexes and the catalysis of the complexes for the ROP of rac-lactide are reported. The pyrazole-based ligand precursors o-(3,5-Me2C3HN2)C6H4N═C(Me)CH═C(OH)R1 (R1 = Me, 1; R1 = Ph, 2; R1 = t-Bu, 3; R1 = CF3, 4)were synthesized by reaction of 2-(3,5-dimethyl-1H-pyrazol-1-yl)benzenamine with 1,3-diketones, including pentane-2,4-dione,1-phenylbutane-1,3-dione, 5,5-dimethylhexane-2,4-dione, and 1,1,1-trifluoropentane-2,4-dione. Treatment of 1–4 with ZnEt2 generated the N,N,O-coordinated zinc complexes [Zn(Et){o-(OC(R1)═CHC(Me)═N)C6H4(3,5-Me2C3HN2)}] (R1 = Me, 5; R1 = Ph, 6; R1 = t-Bu, 7; R1 = CF3, 8). The iminophosphoranyl-moiety-containing ligand precursors o-(3,5-Me2C3HN2)C6H4N═P(Ph2)CH2C(O)R2 (R2 = Ph, 9; R2 = t-Bu, 10) were synthesized by reaction of 1-(2-azidophenyl)-3,5-dimethyl-1H-pyrazole with 1-phenyl-2-(diphenylphosphino)ethanone and 3,3-dimethyl-1-(diphenylphosphino)butan-2-one, respectively. Treatment of 9 and 10 with ZnEt2 afforded the zinc complexes [Zn(Et){o-(OC(R2)═CHP(Ph2)═N)C6H4(3,5-Me2C3HN2)}] (R2 = Ph, 11; R2 = t-Bu, 12). The ligand precursors and complexes were characterized by NMR spectroscopy and elemental analyses. Complexes 5 and 11 were also characterized by single-crystal X-ray diffraction techniques. In the presence of BnOH complexes 5–8 efficiently catalyzed the ring-opening polymerization of rac-lactide in a controlled fashion, whereas complexes 11 and 12 showed much lower catalytic activity under the same conditions.