Toward Stereoselective Lactide Polymerization Catalysts: Cationic Zinc Complexes Supported by a Chiral Phosphinimine Scaffold

The P-stereogenic phosphinimine ligands (dbf)MePhPNAr (7: Ar = Dipp; 8: Ar = Mes; dbf = dibenzofuran, Dipp = 2,6-diisopropylphenyl, Mes = 2,4,6-trimethylphenyl) were synthesized as racemates via reactions of the parent phosphines (rac)-(dbf)MePhP (6) with organoazides. The ligands 7 and 8 were protonated by Brønsted acids to afford the aminophosphonium borate salts [(7)-H][BAr4] (9: Ar = C6F5; 11: Ar = Ph) and [(8)-H][BAr4] (10: Ar = C6F5; 12: Ar = Ph). The protonated ligands 9 and 10 were active toward alkane elimination reactions with diethylzinc and ethyl-[methyl-(S)-lactate]zinc to give the heteroleptic complexes [{(dbf)MePhPNAr}ZnR][B(C6F5)4] (Ar = Dipp, 13: R = Et; 15: R = methyl-(S)-lactate; Ar = Mes, 14: R = Et; 16: R = methyl-(S)-lactate). By contrast, reaction of the tetraphenylborate derivative 11 with diethylzinc yielded a phenyl transfer product, [(dbf)MePhPNDipp]ZnPh2 (17). Complex 15 was found to catalyze the ring-opening polymerization of rac-lactide.