Hybrid amino/iminopyridines with auxiliary phosphines enabling active zinc catalysts for the ring-opening polymerization of lactide

Abstract

The series of 2-(diphenylethyl)amino-6-iminopyridines, 2-((Ph₂PCH₂CH₂)NCMe)-6-(ArNCMe)C₅H₃N (Ar = 2,6-ⁱPr₂C₆H₃ (L1), 2,6-Et₂C₆H₃ (L2), 2,6-Me₂C₆H₃ (L3), 2,4,6-Me₃C₆H₂ (L4), and 2,6-(CHPh₂)₂-4-MeC₆H₂ (L5)), and the analogue 2-(n-butyl)amino-6-iminopyridine L6 were synthesized and used to form their dichlorozinc complexes, LZnCl₂ (Zn1–Zn6). All organic compounds and zinc complexes were well characterized by ¹H/¹³C/³¹P NMR spectroscopy and elemental analysis, along with single-crystal X-ray diffraction of Zn1 and Zn4, indicating bistrigonal geometry around zinc. By treatment with two equivalents of LiN(SiMe₃)₂, Zn1–Zn6 exhibited high activities toward the ring-opening polymerization (ROP) of L-lactide (L-LA), indicating the positive influence of the substituents as well as the auxiliary phosphine. Notably, excellent activity was achieved even at high temperature; for example, Zn1/2LiN(SiMe₃)₂ catalyzed 2000 equiv. L-LA with 86% conversion in 10 minutes at 100 °C, with a TOF of 10 320 h⁻¹. The observed activities with respect to substituents were isopropyl > ethyl > methyl; thus, higher activities were achieved for more alkyl substituents. The auxiliary phosphine enhanced the activities of the zinc complexes; Zn1–Zn5 showed higher activities than Zn6 (with butyl), which formed the PLA with a lower molecular weight. The coordination–insertion and ring-expansion mechanisms have been proposed for the ROP of L-lactide.