REDUCTION OF KETONES WITH LIALH4 COMPLEXES OF HA',ALPHA'-TETRAARYL-1,3-DIOXOLANE-4,5-DIMETHANOLS (TADDOLS) - A COMBINATION OF ENANTIOSELECTIVE REDUCTION AND CLATHRATE FORMATION WITH A DISCUSSION OF LAH REAGENTS BEARING C-2-SYMMETRICAL LIGANDS

A complex prepared from one equivalent each of LiAlH4, EtOH and a TADD OL ha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol) reduces aryl alk yl ketones to sec. alcohols with enantiomer ratios (er) up to 96 : 4. The chiral LAH derivative is used in two-fold excess in THF solution a nd at dry ice temperatures. The ability of TADDOLs to form clathrates diastereoselectively can be exploited to increase the er of the initia lly formed alcohols by a simple modification of the work-up procedure and hence, products of very high enantiopurity (er 99 : 1) can be isol ated. When (R,R)-TADDOLs (from (R,R)-tartrate) are applied in the reac tion, the 1-aryl-alkanols formed preferentially have (S) configuration , as for the products obtained with the corresponding (P)-BINOL and (P )-BIPHENOL derivatives. A common mechanistic model is discussed.