Aluminium complexes derived from tridentate thioetherbis(phenolate) ligands and their activity in Diels–Alder catalysis

Abstract

Organoaluminium(III) thioetherbis(phenolate) complexes derived from 2,2′-thiobis(4,6-diterbuthylphenolate) (Stdiol) and 2,2′-thiobis(4,6-dimethylphenolate) (Smdiol) were prepared by reaction of AlMe₃ with the diol proligands LH₂ (SmdiolH₂, StdiolH₂). Monomeric complexes of general formulae [LAlR(L′)] (L = Stdiol, R = Me, L′ = THF (1); L = Stdiol, R = Me, L′ = Et₂O (2); L = Stdiol, R = ⁱBu, L′ = THF) (3); L = Smdiol, R = Me, L′ = THF (4); L = Smdiol, R = ⁱBu, L′ = THF (5)) and [LAlCl(THF)] (L = Stdiol (6); L = Smdiol (7)) were obtained when the reactions were performed in THF or Et₂O in hexane or toluene, species of formulae [(L₂Al)AlR₂] (L = Stdiol, R = Me (8); L = Stdiol, R = ⁱBu (9); L = Smdiol, R = Me (10)) were formed as evidenced by spectroscopic methods. Crystals suitable for X-ray diffraction studies were obtained for 2, 3, 4, 6, 8, 9 and 10. In these cases the sulfur atom coordinates to the aluminium center with Al–S bond lengths between 2.43–2.75 Å adopting boat–boat conformations. Compounds 1, 3, 4, and 8 were tested in the catalytic Diels–Alder cycloaddition reaction between methacrolein and cyclopentadiene showing excellent regioselectivities with good conversion yields.