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 [(MCIMPAl(mu -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(mu -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(mu -O(i)Pr)(2)AlMe(2)] (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 H(2)O, hydrolysis of 1 in THF gives the trimeric aluminum compound [(MCIMP)Al(mu -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 epsilon -caprolactone and delta -valerolactone.