Sulfonamide-Supported Aluminum Catalysts for the Ring-Opening Polymerization of <i>rac</i>-Lactide

Abstract

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, H2N2TsNPh) or Me (16, H2N2MsNPh)) is described. These and the previously reported 1,2-C6H10(NHSO2R)2 (R = Tol (11, H2CyN2Ts) or Mes (12, H2CyN2SO2Mes)) and RCH2N(CH2CH2NHSO2Tol)2 (R = MeOCH2 (13, H2N2TsNOMe) or 2-NC5H4 (14, H2N2TsNpy)) reacted with AlEt3 to form Al(CyN2Ts)Et(THF) (17), Al(CyN2SO2Mes)Et(THF) (18), and Al(N2TsNR)Et (R = Ph (19), OMe (20), or py (21)), respectively. Subsequent reaction of these ethyl complexes with R′OH (R′ = iPr 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′)Et2 (R′ = iPr, Bn, CH2CH2NH2, or CH2CH2NMe2) with various protio-ligands formed the sulfonamide-supported alkoxides Al(N2TsNpy)(OR′) (R′ = iPr (22) or Bn (23)), Al(N2MsNPh)(OR′) (R′ = iPr (26) or Bn (27)), Al(N2TsNR)(OCH2CH2NH2) (R = Ph (29), OMe (30), or py (31)), Al(CyN2Ts)(OCH2CH2NMe2) (32), and Al(N2TsNPh)(OCH2CH2NMe2) (33). Unexpectedly, reaction of Al(OiPr)Et2 with H2N2TsNOMe led to O-demethylation of the sulfonamide ligand. Reaction of AlMe2Cl with H2N2TsNPh 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 °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