Reactions of 2,2‘-Methylenebis(4-chloro-6-isopropyl-3-methylphenol) with Trimethylaluminum: Highly Efficient Catalysts for the Ring-Opening Polymerization of Lactones

The reaction of 2,2‘-methylenebis(4-chloro-6-isopropyl-3-methylphenol) (MCIMP-H2) with
1.2 molar equiv of AlMe3 in THF yields [(MCIMP)AlMe(THF)] (1), which further reacts with
a stoichiometric amount of 2-propanol (IPA), affording the isopropoxy-bridged dimer
[(MCIMP)Al(μ-OiPr)]2 (2). Compound 2 can also be obtained by the treatment of MCIMP-H2
with a stoichiometric amount of Al(OiPr)3 in refluxing toluene. However, the reaction of 1 with
another 1 equiv of AlMe3 in CH2Cl2 furnishes [Me2Al(μ-MCIMP)AlMe2(THF)] (3), in which
the MCIMP2- ligand is acting as a bridging ligand. Further treatment of compound 3 with
2 molar equiv of 2-propanol results in the formation of the isopropoxy-bridged dinuclear
aluminum complex [(MCIMP)Al(μ-OiPr)2AlMe2] (4). Compound 4 can be directly obtained
from the reaction of 1 with AlMe3 in the presence of 2-propanol. However, in the presence
of a stoichiometric amount of H2O, hydrolysis of 1 in THF gives the trimeric aluminum
compound [(MCIMP)Al(μ-OH···THF)]3 (5), in which the hydroxy group acts as a chelating
group bridging to two aluminum atoms. Compounds 2 and 4 have shown excellent catalytic
activity toward the ring-opening polymerization of ε-caprolactone and δ-valerolactone.