Use of heterometallic alkali metal–magnesium aryloxides in ring-opening polymerization of cyclic esters

Abstract

In this work, alkali metal–magnesium aryloxides [Mg₂Li₂(MesalO)₆] (1), [Mg₂Na₂(MesalO)₆(THF)_x] for x = 2 or 4 (2), and [Mg₂K₂(MesalO)₆(THF)₄] (3) derived from the reaction of MgⁿBu₂ and ⁿBuLi, metallic Na or K with methyl salicylate (MesalOH) were used as molecular platforms for the synthesis of new heterometallic compounds [Mg₂Li₂(EtsalO)₆] (4), [MgK(EtsalO)₃]_n (5), [Mg₆Na₄Al(MesalO)₁₃(OH)₆(MesalOH)(THF)_0.5(H₂O)_0.5] (6), and [Mg₄Na₂(MesalO)₆(SalO)₂(THF)₄] (7) (EtsalOH = ethyl salicylate and SalOH₂ = salicylic acid) by the reaction with EtOH, exposure to atmospheric moisture or addition of stoichiometric quantities of water. Compounds 4 and 5 were synthesized by transesterification of 1 and 3. Cluster 6 was formed haphazardly by exposing a THF solution of 2 derived using MgⁿBu₂ stabilized with 1 wt% AlEt₃ to atmospheric moisture. Compound 7 was synthesized by partial hydrolysis of 2. Homometallic magnesium aryloxide [Mg₄(MesalO)₄(OMe)₄(HOMe)₄] (8) was obtained by reaction of MgⁿBu₂ and MesalOH in a methanol solution. The catalytic activity of 1–3 and 6–8 was investigated in the ring-opening polymerization (ROP) of L-lactide (L-LA) or benzaldehyde Tishchenko reaction.