TADDOL-BASED TITANIUM CATALYSTS AND THEIR ADDUCTS - UNDERSTANDING ASYMMETRIC CATALYSIS OF DIELS-ALDER REACTIONS

The parent TADDOL auxiliary 1a and newly synthesized variants 1b-f have been converted to TADDOLTiCl(2) catalysts 2a-f, which are effective catalysts for asymmetric Diels-Alder reactions of cyclopentadiene and unsaturated N-acyloxazolidinones like 3. As MW determination and VT-NMR demonstrated catalysts 2a-f to be well-defined, a series of VT-NMR experiments were conducted on the adducts of these catalysts and (i-PrO)(2)TiCl2 with Lewis bases like 3, diethyl dimethylmalonate, diethyl oxalate, and diisopropyl oxalate. These experiments led to the following conclusions: (1) the Lewis bases examined are bidentate ligands for 2a-f and (i-PrO)(2)TiCl2; (2) of the five possible adducts between 3 and 2a-f, only three are observed; (3) the most abundant adduct of 3 and 2a-f has all four oxygen ligands in the equatorial plane of octahedral titanium and binds 3 remote to the TADDOL aryl residues; (4) the second most abundant adduct of 3 and 2a-f has only three of the four oxygen ligands in the equatorial plane and binds 3 proximal to one of the pseudoaxial TADDOL aryl residues; (5) the exchange of catalyst 2a-f bound 3 for free 3 is rapid relative to the rate of Diels-Alder addition.