Sulfonamide-Supported Aluminum Catalysts for the Ring-Opening Polymerization of rac-Lactide

The synthesis, structures, and ring-opening polymerization (ROP)capability of a wide range of sulfonamide-supported aluminum alkyl and alkoxide complexes are reported. The synthesis of the new protio-ligands PhCH2N(CH2CH2NHSO2R)(2) (R = Tol (15, (H2N2NPh)-N-Ts) or Me (16, (H2N2NPh)-N-Ms)) is described. These and the previously reported 1,2-C6H10(NHSO2R)(2) (R = Tol (11, H2CyN2Ts) or Mes (12, H2CyN2SO2Mes)) and RCH2N(CH(2)CH(2)NHSO(2)Tol)(2) (R = MeOCH2 (13, (H2N2NOMe)-N-Ts) or 2- NC5H4 (14, (H2N2Npy)-N-Ts)) reacted with AlEt3 to form Al(CyN2Ts)Et(THF) (17), Al(CyN2SO2Mes)Et(THF) (18), and Al((N2NR)-N-Ts)Et (R = Ph (19), OMe (20), or py (21)), respectively. Subsequent reaction of these ethyl complexes with R'OH (R' = Pr-i or Bn) resulted in protonolysis of the sulfonamide supporting ligands to yield a mixture of products including Al(OR')(3). In contrast, reaction of Al(OR')Et-2 (R' = Pr-i, Bn, CH2CH2NH2, or CH2CH2NMe2) with various protio-ligands formed the sulfonamide-supported alkoxides Al((N2Npy)-N-Ts)(OR') (R' = Pr-i (22) or Bn (23)), Al((N2NPh)-N-Ms)(OR') (R' = Pr-i (26) or Bn (27)), Al((N2NR)-N-Ts)(OCH2CH2NH2) (R = Ph (29), OMe (30), or py (31)), Al(CyN2Ts)(OCH2CH2NMe2) (32), and Al((N2NPh)-N-Ts)(OCH2CH2NMe2) (33). Unexpectedly, reaction of Al(OiPr)Et-2 with (H2N2NOMe)-N-Ts led to O-demethylation of the sulfonamide ligand. Reaction of AlMe2Cl with (H2N2NPh)-N-Ts gave [Al(NTs2NPh)Cl](2) (28). X-ray diffraction studies revealed four- or five-coordinate Cs-symmetric structures for 17-21, a five-coordinate C2-symmetric sulfonamide-bridged dimer for 28, and a five-coordinate Cs-symmetric monomer for 30 stabilized by intramolecular hydrogen bonding between the sulfonyl oxygens and the amine protons. Compounds 19, 21, 22-27, and 29-33 are all catalysts for the ROP of rac-lactide, with the alkoxide compounds 22-27 and 32 giving well-defined molecular weights and molecular weight distributions. These compounds were also active in the melt at 130 degrees C, giving atactic poly(rac-lactide) with moderate to narrow PDIs and extremely good control of Mn and high activity in the case of 23.