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-H 2 ) with 1.2 molar equiv of AlMe 3 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-O i Pr)] 2 ( 2 ). Compound 2 can also be obtained by the treatment of MCIMP-H 2 with a stoichiometric amount of Al(O i Pr) 3 in refluxing toluene. However, the reaction of 1 with another 1 equiv of AlMe 3 in CH 2 Cl 2 furnishes [Me 2 Al(μ-MCIMP)AlMe 2 (THF)] ( 3 ), in which the MCIMP 2 - 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-O i Pr) 2 AlMe 2 ] ( 4 ). Compound 4 can be directly obtained from the reaction of 1 with AlMe 3 in the presence of 2-propanol. However, in the presence of a stoichiometric amount of H 2 O, 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.