Aluminum Complexes Based on Tridentate Amidoalkoxide NNO-Ligands: Synthesis, Structure, and Properties

A series of novel NNO-type ligands 12a-17a, LH₂, R¹N(CH₂CH₂NHTs)CHR²CR³R⁴OH (R¹ = Bn, R³ = R⁴ = H; R² = H (12a), R²= (S)-Ph (13a), R²= (S)-Bn (14a), R²= (S)-i-Bu (15a); R¹ = R²= (R)-Me, R³ = H, R⁴= (S)-Ph (16a); R¹ = Bn, R²= (S)-i-Bu, R³ = R⁴ = Ph (17a)) was obtained; the ligands LH₂ feature various degree of substitution, steric bulkiness, and chirality. Treatment of 11a (R¹ = Me, R² = R³ = R⁴ = H), 12a-17a, and 18a (R¹ = Me, R² = H, R³ = R⁴ = Ph) with AlMe₃ afforded the complexes 11b-18b, [LAlMe]_n (n = 1, 2). It was demonstrated that the structure of the ligand determines the monomeric (16b-18b, R³, R⁴ = Ph) or dimeric (11b-15b, R³ = R⁴ = H) structure of the complexes. The discussion of the stereochemical consequences caused by dimerization and chirality of the ligands is performed. The molecular structures of dimeric 11b, 12b, 15b, and monomeric 17b were studied by single-crystal X-ray diffraction. Complexes 11b, 12b, 14b, and 17b are active initiators in ring-opening polymerization of l-lactide in the absence of alcohol. Furthermore, 11b-13b, 17b, and 18b demonstrate an outstanding catalytic activity and selectivity in alcoholysis of l-lactide to give (S,S)-methyl lactyllactate.