Zinc 8-aminotrihydroquinolines appended with pendant N-diphenylphosphinoethyl arms as exceptionally active catalysts for the ROP of ε-CL

Abstract

Five types of N-(2-(diphenylphosphino)ethyl)-2-R-5,6,7,8-tetrahydroquinolin-8-amines (R = H L1, Me L2, ⁱPr L3, Cl L4, Ph L5) along with one example of an N-isopropyl-5,6,7,8-tetrahydroquinolin-8-amine (L6) have been prepared and employed as neutral ligand supports for zinc(II) chloride complexes, LZnCl₂ (Zn1–Zn6). All zinc compounds were characterized by multinuclear NMR spectroscopy (¹H/¹³C/³¹P) and elemental analysis, while the structures of Zn2 and Zn5 were further studied by single crystal X-ray diffraction: distorted tetrahedral geometries with the diphenylphosphine moieties non-coordinated were a feature. Pre-treatment of Zn1–Zn5 with two equivalents of either LiCH₂SiMe₃ or LiN(SiMe₃)₂ led to catalysts displaying good to exceptionally high activity for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) at 50 °C. For example, Zn4/2LiCH₂SiMe₃ displayed a TOF of 2.1 × 10⁴ h⁻¹ with 71% conversion seen within 10 minutes with the ε-CL : Zn molar ratio at 5000 : 1; the resulting polycaprolactone (PCL) comprised predominately cyclic structures along with lower levels of trimethylsilylmethyl-capped linear PCL. By comparison, Zn1–Zn5 on treatment with two equivalents of LiN(SiMe₃)₂ at the same high monomer to zinc ratio, led to catalysts exhibiting an order of magnitude higher activity. Zn1/2LiN(SiMe₃)₂ in the presence of BnOH exhibited a remarkable TOF of 1.35 × 10⁵ h⁻¹ with 90% monomer conversion within two minutes. In these cases, a distinct selectivity towards linear polymers capped with CH₃O end-groups (derived from the quenching solvent) was observed with cyclic polyesters only observable with larger amounts of ε-CL. As a general observation, the 2-R substituent on the chelating ligand showed noticeable effects on the efficiency and on the molecular weight of the PCLs, with 2-Cl-containing Zn4 in combination with LiCH₂SiMe₃ and BnOH affording the highest molecular weight PCL of the study (9.57 × 10⁴ g mol⁻¹). Based on the microstructural properties of the PCL, distinct mechanisms (viz., coordination–insertion and ring expansion) have been proposed to account for the mode of propagation mediated by Zn/2LiCH₂SiMe₃ and Zn/2LiN(SiMe₃)₂. In addition, a zinc silylamido L1^−HZn[N(SiMe₃)₂] (Zn7) species has been tentatively assigned (obtained via deprotonation of L1 with Zn[N(SiMe₃)₂]₂), that proved inactive for the ROP of ε-CL but displayed good activity for ROP of rac-LA.