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.

本研究以母体膦——外消旋(racemate, rac)-(二苯并呋喃(dibenzofuran, dbf))甲基苯基膦[(dbf)MePhP，化合物6]——与有机叠氮化物反应，外消旋合成了P-手性磷亚胺配体(dbf)MePhP=NAr（化合物7：Ar=2,6-二异丙基苯基(2,6-diisopropylphenyl, Dipp)；化合物8：Ar=2,4,6-三甲基苯基(2,4,6-trimethylphenyl, Mes)）。配体7与8经布朗斯特酸(Brønsted acid)质子化后，得到氨基𬭸硼酸盐类化合物：[(7)-H][BAr4]（化合物9：Ar=五氟苯基(C6F5)；化合物11：Ar=苯基(phenyl, Ph)）以及[(8)-H][BAr4]（化合物10：Ar=C6F5；化合物12：Ar=Ph）。质子化后的配体9与10可与二乙基锌(diethylzinc)以及乙基-(S)-乳酸甲酯基锌(ethyl-[methyl-(S)-lactate]zinc)发生烷烃消除反应，生成杂配型配合物[{(dbf)MePhP=NAr}ZnR][B(C6F5)4]（Ar=Dipp时：化合物13：R=乙基(ethyl, Et)；化合物15：R=(S)-乳酸甲酯(methyl-(S)-lactate)；Ar=Mes时：化合物14：R=乙基(ethyl, Et)；化合物16：R=methyl-(S)-lactate）。与之相反，四苯基硼酸盐衍生物11与二乙基锌的反应则生成苯基转移产物[(dbf)MePhP=NDipp]ZnPh2（化合物17）。研究表明，配合物15可催化外消旋(racemate, rac)-丙交酯(rac-lactide)的开环聚合反应。