[Structure: see text] Syn-configured aldol products of mandelic acid esters and aldehydes were synthesized by the catalytic use of amines in the presence of titanium(IV) tert-butoxide. Used along with chiral N-methylephedrine, anti-configured alpha,beta-dihydroxyesters were isolated with a high degree of enantioselectivity for the first time.
In order to interpret the results of regio-as well as stereoselectivity of direct aldol additions studies of structures of complexes used are ongoing. Knowledge of the molecular structure of metal complexes is very important to understand the catalytic mechanism of metal-catalyzed organic reactions. In contrast to the great number of X-ray structure analyses in the titanium(IV) alkoxide series, only little is known about titanium complexes of a-hydroxy acids. Especially X-ray structure results of titanium complexes of mandelic acid are rare.1 That's why, such complexes have been further studied.The TiCl4-complexes were synthesized to act as catalysts in enantioselective direct aldol additions. At first, optically active (R)-mandelic acid methylester was dissolved with equimolar amounts of TiCl4 at 0˚C in toluene. A trinuclear TiCl4-complex I, C18H18Cl8O7Ti3, was obtained. After four days, dark-brown crystals were collected, washed with toluene and dried in a high vacuo at room temperature. Interestingly, under the same conditions racemic mandelic acid methylester reacts to a dinuclear complex II, C18H18Cl6O6Ti2. Here, already after two days red-brown crystals were formed. They were also washed and dried as just mentioned.X-ray data were collected using graphite-monochromated MoKa radiation (l = 0.71073 Å) at low temperatures on a Stoe 4-circle diffractometer for compound I and on a Stoe image plate diffractometer for compound II, respectively. Data reduction was carried out with the Stoe Program Package. 2 The structures were solved by direct methods using SHELXS97.3 All nonhydrogen atoms were refined anisotropicially with SHELXL97 3a using blocked-matrix (for I) or full-matrix (for II) least-squares on F 2 . The positions of the hydrogen atoms were calculated and included in the refinement applying the riding model. Crystal and experimental data are listed in Table 1. The final positional parameters for non-hydrogen atoms in I and II (without solvent atoms) are given in Table 2. ORTEP 4 plots of I and II are depicted in Figs. 2 and 3, respectively. For saving space in Table 2 and Fig. 2, only one of the five independent complexes I is given, because all are chemicially equal.During the complexation reactions, the tetrahedral titanium increases its coordination number (c.n.) considerably and forms rather different polyhedra. One pentagonal bipyramid (c.n. = 7) and two tetragonal pyramids (c.n. = 5) are observed in I, X-ray Structure Analysis Online The complexes of trinuclear TiCl3-O-TiCl3/TiCl2(O-(R)-mandelic acid methylester)2 I, C18H18Cl8O7Ti3, and dimeric TiCl3O-rac-mandelic acid methylester II, C18H18Cl6O6Ti2, were determined unambiguously by X-ray diffraction. Both crystal structures contain one solvent molecule toluene per complex. In structure I there are five independent complexes in the asymmetric unit cell. The formation of both the crystal structure as well as the coordination sphere of titanium is controlled sensitively by the chirality of mandelic acid methylesters.
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