Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides

The model precatalyst <i>sp³</i>- and <i>sp²</i>-N dinitrogen-coordinated zinc-heteroimidazole has been used as an efficient catalyst for the ring-opening polymerization of cyclic esters. Subsequent to our exceptional active 5,6,7-trihydroquinolin-8-amine-zinc catalysts for the ring-opening polymerization (ROP) of <i>ε</i>-caprolactone, various pyridine-fused cycloalkanones (ring size from five to eight) are developed for the correspondent fused amine-pyridine derivatives and their zinc-heteroimidazole chloride complexes <b>Zn1</b>-<b>Zn8</b> (<b>L</b>ZnCl₂) bearing <i>N</i>-diphenylphosphinoethyl pendants. Activated with two equivalents of LiN(SiMe₃)₂, the title zinc complexes efficiently promote the ROP of <i>L</i>-lactide (<i>L</i>-LA) in situ; among them, <b>Zn4</b>/2Li(NSiMe₃)₂ catalyzed 500 equivalent <i>L</i>-LA at 80 °C with 92% conversion in 5 min (TOF: 5520 h^-1). Under the same conditions, the catalytic efficiency for the ROP of <i>rac</i>-LA by <b>Zn1</b>-<b>Zn8</b>/2Li(NSiMe₃)₂ was slightly lower than that for <i>L</i>-LA (highest TOF: 4440 h^-1). In both cases, cyclooctyl-fused pyridyl-zinc complexes exhibited higher activity than others, while the cycloheptyl-fused zinc complexes showed the lowest activity. The microstructure analysis of the polymers showed they possessed a linear structure capped with CH₃O as major and cyclic structure as minor. In this work, all the ligands and zinc complexes were well characterized by ¹H/¹³C/³¹P NMR, FT-IR spectroscopy as well as elemental analysis.