No abstract

Chiral metallosalen complexes have been recognized as one of the most promising catalysts for various asymmetric reactions. The studies on asymmetric epoxidation catalyzed by chiral (salen)manganese complexes will be reviewed at first. The enantioselectivities have reached to the level as high as 90—99% ee for the epoxidation of conjugated cis-di- and trisubstituted olefins by developing the second generation of chiral salen ligands. Although it has been proven by Freichtinger and Plattner that epoxidation proceeds through an oxo species, the mechanism of asymmetric induction is still surrounded by controversy. However, our recent studies have clarified that the conformation of the salen ligand in oxo(salen)manganese(V) species plays a very important role in determining the direction of the olefin’s access to the oxo species. This study led to the new finding that an achiral (salen)metal complex can be used as a chiral catalyst by regulating the conformation of achiral ligand. The appropriately modified (salen)manganese(III) complexes can also be applicable for asymmetric oxidation of enol ether derivatives, kinetic resolution of racemic allenes, enantioselective benzylic C–H oxidation, asymmetric desymmetrization of meso-heterocycles, and asymmetric oxidation of alkyl aryl sulfides. In the asymmetric aziridination of styrene derivatives by using PhI=NTs as a terminal oxidant, high enantioselectivity of up to 94% has been achieved by the further modification of the chiral (salen)manganese(III) complex. The chiral (salen)cobalt(III) complex is a good catalyst for asymmetric cyclopropanation of styrene derivatives and asymmetric S-ylide formation from allyl aryl sulfides, in which the resulting S-ylides undergo [2,3]-Wittig rearrangement in situ to produce chiral 2-(phenylthio)pentanoate derivatives.

No abstract

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

No abstract

No abstract

No abstract

No abstract