Lithiated Calix[]arenes ( = 6 or 8): Synthesis, Structures, and Use in the Ring-Opening Polymerization of Cyclic Esters.

A variety of lithiated calix[]arenes, for which = 6 or 8, have been isolated, structurally characterized, and evaluated as catalysts for the ring-opening polymerization (ROP) of the cyclic esters ε-caprolactone (ε-CL), δ-valerolactone (δ-VL), and -lactide (-LA). In particular, interaction of --butylcalix[6]areneH (LH) with LiOBu in THF led to the isolation of [Li(LH)(CO)(THF)(OH)]·14THF (·14THF), the core of which has a chain of five LiO diamonds. Similar use of --butylcalix[8]areneH (LH) afforded [Li(L)(OH)(THF)]·7THF (·7THF), where the core is composed of a six-rung Li-O ladder. Use of debutylated calix[8]areneH (deBuLH) led to an elongated dimer [Li(deBuL)(OBu)(THF)]·4THF (·4THF) in which the calix[8]arenes possess a wavelike conformation forming bridges to link three separate LiO clusters (where and = 6, ignoring the THF donor oxygens). Interaction of LH with LiOH·HO afforded [Li(LH)(OH)(THF)]·5.5THF (·5.5THF), where intramolecular H-bond interactions involving Li, O, and H construct a cage in the core of the structure with six- and eight-membered rings. Lastly, addition of MeAl to the solution generated from LH and LiOBu led to the isolation of [(AlMe)Li(LH)(OH)(O)(OH)(NCMe)]·10MeCN (·10MeCN) in which Li, O, Al, and N centers build a polyhedral core. These complexes have been screened for their potential to act as precatalysts in the ring-opening polymerization (ROP) of ε-CL, δ-VL, and -LA. For the ROP of ε-CL, δ-VL, and -LA, systems - exhibited moderate activity at 130 °C over 8 h. In the case of ROP using the mixed-metal (Li/Al) system , better conversions and high molecular weight polymers were achieved. In the case of the ROP of ω-pentadecalactone (ω-PDL), the systems proved to be inactive under the conditions employed herein.