TADDOLS UNDER CLOSER SCRUTINY - WHY BULKY SUBSTITUENTS MAKE IT ALL DIFFERENT

A systematic investigation of the rotational behavior of aryl substitu ents in alpha alpha alpha'alpha'-tetraaryl-1,3-dioxolane- 4,5-dimethan ols (TADDOLs) is presented. In the use as chiral ligands for enantiose lective metal-catalyzed reactions, a change from phenyl to bulkier sub stituents, e.g., 1-naphthyl, gives rise to an astounding alteration of the selectivity. The possible existence of preferred rotamers of TADD OLs has so far not been given due attention, which encouraged us to lo ok at the validity of the Knowles model, originally formulated for dia ryl substituted bisphosphines. H-1-NMR Investigations at various tempe ratures as well as X-ray powder diffraction were employed to study the rotation in the case of tetra(1-naphthyl) TADDOL 1. To support the in terpretation of the experimental results, molecular mechanics, semiemp irical, and ab initio calculations were performed. For comparison, the energy surface of tetraphenyl TADDOL 2 was calculated as well. Our re sults lead to the conclusion that for 1, only one major conformation i s present in both solution and solid state, which determines the stere ochemical outcome of the catalyzed reactions.