Binolate complexes of lithium, zinc, aluminium, and titanium; preparations, structures, and studies of lactide polymerization

Abstract

The reaction of racemic 5,5′,6,6′- tetramethyl-3,3′-di-tert-butyl-1,1′-biphenyl-2,2′-diol (biphenolate-H₂) with 4 mol equiv. of ⁿBuLi yields [(μ₃,μ₃-biphenolate)₂Li₄(ⁿBuLi)₄] (1) in high yield. 1 further reacts with 4 mol equiv. of 2,4-dimethyl-3-pentanol in the presence of tetrahydrofuran (THF) or cyclohexene oxide (CyHO) to give the lithium aggregate [(μ,μ-biphenolate)Li₂(μ₃-OCH(ⁱPr)₂)₂Li₂(L)₂] (2-THF, L = THF; 2-CyHO, L = CyHO). Treatment of biphenolate-H₂ with 3 mol equiv. of ZnEt₂, followed by addition of 2 mol equiv. of 2,4-dimethyl-3-pentanol provides the zinc complex [(μ,μ-biphenolate)Zn(μ-OCH(ⁱPr)₂)₂Zn₂Et₂] (3). Aluminium alkoxide incorporating biphenolate ligand can also be obtained via a similar synthetic route. The compound [(μ-biphenolate)AlMe(μ-OCH(ⁱPr)₂)AlMe₂] (4) is prepared from the reaction of biphenolate-H₂ with 2 mol equiv. of AlMe₃ in the presence of 1 mol equiv. of 2,4-dimethyl-3-pentanol. The titanium (4+) binolate complex [(biphenolate)Ti₂Cl₆] (5) is synthesized from the reaction of biphenolate-H₂ and 2 mol equiv. of TiCl₄. In addition, 2-THF, 3, and 4 have been examined for rac-lactide polymerization, and the comparative studies of polymerization are also presented.