A test for the coexistence of reactive intermediates with different molecular composition in chiral Lewis acid-catalysed reactions:: the case of Ti-TADDOLate-catalysed Diels-Alder reactions

The Diels-Alder reactions between cyclopentadiene 2 and (E)-3-butenoyl-1,3-oxazolidin-2-one 1 catalysed by several TADDOL-TiCl2 complexes have been studied with different [dienophile]/[catalyst] ratios and different concentrations of reagents and catalyst. The enantioselectivity of some of the reactions depends on these factors, which indicates the participation of intermediate complexes with different catalyst and dienophile compositions (1:1, 1:2 and 2:1). The best results are obtained under conditions that favour the formation of an equimolecular intermediate, whereas the conditions favouring a the formation of intermediates containing two molecules of dienophile and one of catalyst give rise to lower enantiomeric excesses (e.e.s). In one case the asymmetric induction was not dependent on the above factors, meaning that the effect described strongly depends on the structure of the chiral ligand. The results described show that this kind of mechanistic study complements those carried out oil the influence of the enantiomeric composition of the chiral ligand on the enantioselectivity. (C) 2001 Elsevier Science Ltd. All rights reserved.