Highly Selective and Immortal Magnesium Calixarene Complexes for the Ring-Opening Polymerization of rac-Lactide

The lithiation of 1,3-dipropoxy-p-tert-butylcalix[4] arene (LH2) followed by reaction with nBuMgBr in THF resulted in the formation of the hetero-bimetallic complex [Li(THF)Mg(nBu)L] (1). By contrast, the treatment of tripropoxy-p-tert-butylcalix[4] arene (L'H) with nBu(2)Mg afforded a mononuclear complex [L'Mg(nBu)] (2). Single-crystal XRD studies revealed that in both structures the calix[4] arene adopts a cone conformation, and a Li cation resides in the cavity for 1. Both 1 and 2 were active for the ring-opening polymerisation of rac-lactide. Compound 2 displayed not only exceptional activity (100 equivalents, 3 min, 92% conversion, BnOH, room temperature) but also high selectivity (probability to form an r dyad, P-r=0.85) and exhibited an immortal character in THF. Surprisingly, compound 2 also showed isotactic bias (Pr=0.30-0.36) and an immortal character if toluene was employed as the solvent; 2D J-resolved H-1 NMR spectroscopy was employed for the assignment of the stereoselectivity.