Simple magnesium alkoxides: synthesis, molecular structure, and catalytic behaviour in the ring-opening polymerization of lactide and macrolactones and in the copolymerization of maleic anhydride and propylene oxide

Abstract

The synthesis of two chiral bulky alkoxide pro-ligands, 1-adamantyl-tert-butylphenylmethanol HOCAd^tBuPh and 1-adamantylmethylphenylmethanol HOCAdMePh, is reported and their coordination chemistry with magnesium(II) is described and compared with the coordination chemistry of the previously reported achiral bulky alkoxide pro-ligand HOC^tBu₂Ph. Treatment of n-butyl-sec-butylmagnesium with two equivalents of the racemic mixture of HOCAd^tBuPh led selectively to the formation of the mononuclear bis(alkoxide) complex Mg(OCAd^tBuPh)₂(THF)₂. ¹H NMR spectroscopy and X-ray crystallography suggested the selective formation of the C₂-symmetric homochiral diastereomer Mg(OC^RAd^tBuPh)₂(THF)₂/Mg(OC^SAd^tBuPh)₂(THF)₂. In contrast, the less sterically encumbered HOCAdMePh led to the formation of dinuclear products indicating only partial alkyl group substitution. The mononuclear Mg(OCAd^tBuPh)₂(THF)₂ complex was tested as a catalyst in different reactions for the synthesis of polyesters. In the ROP of lactide, Mg(OCAd^tBuPh)₂(THF)₂ demonstrated very high activity, higher than that shown by Mg(OC^tBu₂Ph)₂(THF)₂, although with moderate control degrees. Both Mg(OCAd^tBuPh)₂(THF)₂ and Mg(OC^tBu₂Ph)₂(THF)₂ were found to be very effective in the polymerization of macrolactones such as ω-pentadecalactone (PDL) and ω-6-hexadecenlactone (HDL) also under mild reaction conditions that are generally prohibitive for these substrates. The same catalysts demonstrated efficient ring-opening copolymerization (ROCOP) of propylene oxide (PO) and maleic anhydride (MA) to produce poly(propylene maleate).