Synthesis of lithium and aluminum complexes supported by [OC(But)CHP(Ph2)=NBut]− ligand and catalysis of [R2Al{OC(But)-CHP(Ph2)=NBut}] (R = Me, Et) and [Me2Al{1-{OC(Ph)CH}-3-R1-5-MeC3HN2}] (R1 = Me, But) in the ring-opening polymerization of ɛ-caprolactone

Abstract

A series of lithium and aluminum complexes bearing [OC(Bu t )CHP(Ph 2 )=NBu t ] -ligand were synthesized and characterized. Reaction of Bu t C(O)CH 2 Br with Ph 2 PNHBu t afforded [Ph 2 P(NHBu t )CH 2 C(O)Bu t ] + Br -(1). Treatment of 1 with excess of NaH in THF generated ligand precursor Ph 2 P(CH 2 C(O)Bu t )=NBu t (2). Reaction of 2 with AlR 3 (R = Me, Et) gave N,O-chelate aluminum complexes [R 2 Al{OC(Bu t )CHP(Ph 2 )=NBu t }] (3, R = Me; 4, R = Et). Lithiation of 2 with an equiv. of LiBu n formed lithium complex [Li{OC(Bu t )CHP(Ph 2 )=NBu t }] (5). Reaction of the lithium complex with an equiv. of AlCl 3 yielded [Cl 2 Al{OC(Bu t ) CHP(Ph 2 )=NBu t }] (6). Complex 6 was also obtained by reaction of 3 with an equiv. of AlCl 3 . Compounds 2-6 were characterized by NMR spectroscopy, elemental analysis and single crystal X-ray diffraction techniques (for 2, 3 and 6). Catalysis of complexes 3 and 4 as well as [Me 2 Al{1-{OC(Ph)CH}-3-R 1 -5-MeC 3 HN 2 }] (R 1 = Me, Bu t ) toward the ring-opening polymerization of ε-caprolactone was investigated. N,O-chelate ligands, complex, lithium, aluminum, synthesis, catalysis Citation: Yang D, Cai C F, Wang Z X. Synthesis of lithium and aluminum complexes supported by [OC(Bu t )CHP(Ph 2 )=NBu t ] -ligand and catalysis of [R 2 Al{OC(Bu t ) CHP(Ph 2 )=NBu t }] (R = Me, Et) and [Me 2 Al{1-{OC(Ph)CH} -3-R 1 -5-MeC 3 HN 2 }] (R 1 = Me, Bu t ) in the ring-opening polymerization of ε-caprolactone. Synthetic aliphatic polyesters such as poly(ε-caprolactone) (PCL) and polylactide (PLA) are important materials in biomedical and pharmaceutical applications due to their biodegradable, biocompatible and permeable properties [1-5]. These polymers are usually synthesized by the ring-opening polymerization (ROP) of cyclic esters such as lactones and lactides initiated by metal complexes. The metal-initiated-ROP can enable control of polymer molecular weight and polymer architecture and can yield macromolecular samples with narrow molecular weight distributions [3-8]. Among the catalysts/initiators studied, aluminum complexes have attracted much attention due to their *Corresponding author (email: zxwang@ustc.edu.cn) high activity and selectivity as well as low toxicity of aluminum [9-13]. On the other hand, properties of complexes strongly rely on supporting ligands besides the metal itself. The ligands can tune the properties of the metal center and minimize the aggregation processes and side reactions during the process of polymerization. N,O-Chelate ligands are one of the most often used ligands in aluminum-based catalytic systems. A series of N,O-chelate aluminum complexes such as ketiminate, salicylaldimine, SALAN and SALEN aluminum complexes exhibited excellent catalytic behavior in the ring-opening polymerization of cyclic esters [14-19]. Herein we report synthesis and characterization of lithium and aluminum complexes bearing [OC(Bu t )CHP(Ph 2 )= NBu t ] -ligand and catalysis of [R 2 Al{OC(Bu t )CHP(Ph 2 )=