Preparation and screening of twenty new ligands, all analogs of a, a, a :oc '-tetraaryl-1,3-dioxolane-4,5dimethanol (TADDOL), for the Ti-catalyzed asymmetric addition of methyltri(isopropoxy)titanium and diethylzinc to benzaldehyde are described. These ligands have the dioxolane ring of the TADDOL's replaced by cyclobutane, cyclopentane, cyclohexene, cyclohexane, bicyclo[2.2. llheptene and -heptane and hicyclo[2.2.2]octene and -octane moieties; several have H-atoms or alkyl groups in place of the aryl groups, and nine of them have C2 symmetry. X-Ray crystallography and molecular mechanics are used to analyze the structure of the ligands, and two structural features appear to correlate with selectivity: i) the torsion angle for the chelating 0-atom and the ortho-C-atom of the axial Ph group (a small, ca. 19", angle is optimum, Fig.8) and i i ) the 'degree of perpendicularity' of the axial Ph group (Fig.9). Competition experiments indicate that TADDOL l a catalyzes both the methyltitanium and diethylzinc additions 2 50 limes faster than the related dioxolane analogs 12a, 12c, and 12e (Scheme 7), indicating that both axial and equatorial aryl groups (see Footnote 6 ) are necessary for ligand-accelerated catalysis of these reactions. A refined mechanistic hypothesis is presented (Fig. 10) to explain the selectivities observed for these new ligands. Our analysis suggests that a combination of structural features appear necessary for good catalytic efliciency and high selectivity. These features, especially the rather subtle conformational effects, appear to be optimized (among the ligands tested) in the TADDOL's.Introduction. -The stereoselective addition of organometallics to the heterotopic faces of a carbonyl group has been the subject of intense study for over forty years, ever since Curtin [l], Cram [2], and Prelog [3] first began rationalizing the selective addition of nucleophiles to diastereotopic faces of aldehydes and ketones. Of particular interest has been the gradual development of theories regarding the factors that are responsible for facial selectivity. E.g., following Cram's original postulate, notable contributions to the theory were made over the years by Karahatsos [4], Felkin [5], Dunitz [6], Anh [7], and Heathcock [8] and coworkers, among others. ') 2, 3, 4,
')Visiting Professor (1993)(1994) from the