Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems†

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. S^Pr-Al exhibited a higher catalytic activity than O^Pr-Al (3.2 times as high for CL polymerization; [CL] : [S^Pr-Al] : [BnOH] = 100 : 0.5 : 2; [S^Pr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and US^Cl-Al exhibited a higher catalytic activity than U^Cl-Al (4.6 times as high for CL polymerization; [CL] : [US^Cl-Al] : [BnOH] = 100 : 0.5 : 2; [US^Cl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [S^Pr-AlOMe₂]₂ decomposed into the four-membered ring S^Pr-AlOMe₂. However, [O^Pr-AlOMe₂]₂ did not decompose because of its strong bridging Al–O bond. The overall activation energy required for CL polymerization was lower when using [S^Pr-AlOMe₂]₂ (18.1 kcal mol⁻¹) as a catalyst than when using [O^Pr-AlOMe₂]₂ (23.9 kcal mol⁻¹). This is because the TS2a transition state of S^Pr-AlOMe₂ had a more open coordination geometry with a small N–Al–S angle (72.91°) than did TS3c of [O^Pr-AlOMe₂]₂, the crowded highest-energy transition state of [O^Pr-AlOMe₂]₂ with a larger N–Al–O angle (99.63°).