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

A variety of lithiated calix[n]arenes, for which n = 6 or 8, have been isolated, structurally characterized, and evaluated as catalysts for the ring-opening polymerization (ROP) of the cyclic esters epsilon-caprolactone (epsilon-CL), delta-valerolactone (delta-VL), and raclactide (r-LA). In particular, interaction of p-tert-butylcalix[6]-areneH(6) ((LH6)-H-6) with LiOtBu in THF led to the isolation of [Li-14((LH)-H-6)(2)(CO3)(2)(THF)(6)(OH2)(6)].14THF (1.14THF), the core of which has a chain of five Li2O2 diamonds. Similar use of p-tert-butylcalix[8]areneH(8) ((LH8)-H-8) afforded [Li-10(L-8)(OH)(2)(THF)(8)]. 7THF (2.7THF), where the core is composed of a six-rung Li-O ladder. Use of debutylated calix[8]areneH(8) (deBuL(8)H(8)) led to an elongated dimer [Li-18( deBuL(8))(2)(OtBu)(2)(THF) (14)].4THF (3. 4THF) in which the calix[8] arenes possess a wavelike conformation forming bridges to link three separate LixOy clusters (where x and y = 6, ignoring the THF donor oxygens). Interaction of (LH8)-H-8 with LiOH.H2O afforded [Li-4((LH4)-H-8)(OH2)(4)(THF)(6)].5.5THF (4.5.5THF), 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 Me3Al to the solution generated from (LH8)-H-8 and LiOtBu led to the isolation of [(AlMe2)(2)Li-20((LH2)-H-8)(2)(OH2)(4)(O2-)(4)(OH)(2)(NCMe)(12)].10MeCN (5.10MeCN) 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 epsilon-CL, delta-VL, and r-LA. For the ROP of epsilon-CL, delta-VL, and r-LA, systems 1-4 exhibited moderate activity at 130 degrees C over 8 h. In the case of ROP using the mixed-metal (Li/Al) system 5, better conversions and high molecular weight polymers were achieved. In the case of the ROP of omega-pentadecalactone (omega-PDL), the systems proved to be inactive under the conditions employed herein.