Lithium calix[4]arenes: structural studies and use in the ring opening polymerization of cyclic esters

We have structurally characterized a number of lithiated calix[4]arenes, where the bridge in the calix[4]arene is thia (–S–, L(S)H(4)), sulfinyl (–SO–, L(SO)H(4)), sulfonyl (–SO(2)–, L(SO2)H(4)), dimethyleneoxa (–CH(2)OCH(2)–, L(COC)H(4)) or methylene (–CH(2)–, LH(4)). In the case of L(4S)H(4), interaction with LiOtBu led to the isolation of the complex [Li(8)(L(4S))(2)(THF)(4)]·5THF (1·5THF), whilst similar interaction of L(4SO)H(4) led to the isolation of [Li(6)(L(4SO)H)(2)(THF)(2)]·5(THF) (2·5THF). Interestingly, the mixed sulfinyl/sulfonyl complexes [Li(8)(calix[4]arene(SO)(SO(2))(SO(1.68))(2))(2)(THF)(6)]·8(THF) (3·8THF) and [Li(5)Na(L(SO/3SO2)H)(2)(THF)(5)]·7.5(THF) (4·7.5(THF) have also been characterized. Interaction of LiOtBu with L(SO2)H(4) and L(COC)H(4) afforded [Li(5)L(4SO2)(OH)(THF)(4)]·2THF (5·2THF) and [Li(6)(L(COC))(2)(HOtBu)(2)]·0.78THF·1.22hexane (6·0.78THF·1.22hexane), respectively. In the case of LH(4), reaction with LiOtBu in THF afforded a monoclinic polymorph [LH(2)Li(2)(thf)(OH(2))(2)]·3THF (7·3THF) of a known triclinic form of the complex, whilst reaction of the de-butylated analogue of LH(4), namely de-BuLH(4), afforded a polymeric chain structure {[Li(5)(de-BuL)(OH)(NCMe)(3)]·2MeCN}(n) (8·2MeCN). For comparative catalytic studies, the complex [Li(6)(L(Pr))(2)(H(2)O)(2)]·hexane (9 hexane), where L(Pr2)H(2) = 1,3-di-n-propyloxycalix[4]areneH(2), was also prepared. The molecular crystal structures of 1–9 are reported, and their ability to act as catalysts for the ring opening (co-)/polymerization (ROP) of the cyclic esters ε-caprolactone, δ-valerolactone, and rac-lactide has been investigated. In most of the cases, complex 6 outperformed the other systems, allowing for higher conversions and/or greated polymer M(n).