Neutral and cationic aluminum complexes supported by acetamidate and thioacetamidate heteroscorpionate ligands as initiators for ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1021/om1010676

The synthesis, structures, and ring-opening polymerization (ROP) activity of heteroscorpionate aluminum alkyl and aryloxide complexes are reported. The reactions of the acetamide and thioacetamide heteroscorpionate protio ligands pbptamH (pbptamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide), pbpamH (pbpamH = N-phenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), sbpamH (sbpamH = N-sec-butyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide), and (S)-mbpamH ((S)-mbpamH = (S)-(-)-N-α-methylbenzyl-2,2-bis(3,5- dimethylpyrazol-1-yl)acetamide) with 1 equiv of AlR₃ (R = Me, Et, ⁱBu) proceed in very high yields to give the neutral heteroscorpionate dialkyl aluminum complexes [AlR₂{κ ²-pbptam}] (R = Me (1), Et (2)), [AlR₂{κ ²-pbptam}] (R = Me (3), Et (4), ⁱBu (5)), [AlR ₂{κ²-pbptam}] (R = Me (6), Et (7), ⁱBu (8)), and [AlR₂{κ²-(S)-mbpam}] (R = Me (9), Et (10)). In the solid state, complexes 1-10 adopt a tetrahedral structure with the heteroscorpionate ligands arranged in a κ² coordination mode; in the case of the thioacetamidate derivatives 1 and 2 a κ²NN coordination mode is observed, whereas the acetamidate derivatives 3-10 present a κ²NO coordination mode. The structures in solution of 1-10 were investigated by VT NMR spectroscopy, and fluxional exchange between coordinated and noncoordinated pyrazole rings was observed, producing interconversion between the different isomers. Compounds 7 and 10 were used as convenient starting materials for the synthesis of the aryloxide aluminum compounds [Al(OR)₂{κ²-sbpam}] (11) and [Al(OR) ₂{κ²-(S)-mbpam}] (12) (R = 2,6-Me₂C ₆H₃O) by reaction with the corresponding 2,6-dimethylphenol. The complexes [AlMe{κ³-pbptam}][MeB(C ₆F₅)₃] (13), [AlMe{κ³-pbptam}] [B(C₆F₅)₄] (14), [AlEt{κ³- pbptam}][B(C₆F₅)₄] (15), [AlEt{κ ³-sbpam}][B(C₆F₅)₄] (16), and [AlEt{κ³-(S)-mbpam}][B(C₆F₅) ₄] (17) are derived from the ionization of the neutral dialkyl aluminum complexes 1, 2, 7, and 10 with the alkyl abstracting reagent B(C ₆F₅)₃ or [Ph₃C][B(C ₆F₅)₄]. The NMR data were consistent with an overall C_s-symmetric structure for 13-15 and C₁-symmetric structure for 16 and 17, which indicates an effective κ³ coordination of the corresponding heteroscorpionate ligand to the cationic aluminum center. The structures of the complexes were determined by spectroscopic methods, and the X-ray crystal structures of 1, 2, and 7 were also established. Finally, exhaustive comparative catalytic studies of the aluminum complexes 1-10, 13, and 14 in the ring-opening polymerization of rac-lactide, l-lactide, and ε-caprolactone are also described.