ON THE MECHANISM OF TI-TADDOLATE-CATALYZED ASYMMETRIC DIELS-ALDER REACTIONS

The mechanism of the diastereo- and enantioselective Diels-Alder react ion of cyclopentadiene with N-acyloxazolidinones catalyzed by titanium ,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-d (Ti-TADDOLate) com plexes has been studied. On the basis of a series of experimental reac tions catalyzed by a Ti-TADDOLate-N-cinnamoyloxazolidinone complex, wh ich has been characterized by X-ray crystallography, it is found that both stoichiometric and catalytic reactions of this complex with cyclo pentadiene lead to the same endo:exo ratio and enantiomeric excess of the Diels-Alder product as the Ti-TADDOLate-catalyzed reaction. The Di els-Alder reaction of different N-acyloxazolidinones with cyclopentadi ene in the presence of modified Ti-TADDOLate complexes has also been i nvestigated, and it is found that the endo:exo ratio is very dependent on chloride and tosylato ligands bound to the titanium atom. In the p resence of chloride ligands attached to the titanium atom, the endo di astereomer of the Diels-Alder adduct is the major product formed, wher eas a significant decrease in the amount of the endo Diels-Alder adduc t is found when the more bulky tosylato ligands are bound to the titan ium atom. The enantiomeric excess of the Diels-Alder product decreases also when the chloride ligands are exchanged with the tosylato ligand s. On the basis of the experimental results, the mechanism of the Ti-T ADDOLate-catalyzed Diels-Alder reaction of cyclopentadiene with N-acyl oxazolidinones is discussed. It is proposed that the intermediate whic h accounts for the stereochemical outcome of the reaction is the one w ith the four oxygen atoms of the TADDOL and the N-acyloxazolidinone li gands bound to the titanium atom in the equatorial plane and the two a xial positions occupied by the chloride or tosylato ligands.