On the Mechanisms of Enantioselective Reactions Using α,α,α′,α′ ‐Tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol(TADDOL)‐Derived Titanates: Differences between C₂‐ and C₁‐symmetrical TADDOLs – facts, implications and generalizations

Abstract

The titanates derived from C(,C(,LY ',ol'-tetraaryl-l,3-dioxolane-4,5-dimethanols (TADDOLs, prepared from tartrate) act as catalysts for enantioselective additions of dialkylzinc compounds to aldehydes. For the standard reaction chosen for this investigation of the mechanism, the addition of diethylzinc to benzaldehyde, there is very little change of selectivity with different aryl substituents on the TADDOLate ligands (Tables 2 4 , examples). With 0.02 to 0.2 equiv. of the chiral titanates, selectivities above 90% are observed only in the presence of excess tetraisopropyl titanate! According to NMR measurements (Fig. 2), the chiral bicyclic titanate and the achiral titanate do not react to give new species under these conditions. From experiments with different stoichiometries of the components, and with different achiral or chiral OR groups on the Ti-atom of the seven-membered ring titanate, it is concluded i ) that a single chiral titanate is involved in the product-forming step, ii) that the bulky TADDOLate ligand renders the Ti-center catalytically more active than that of (i-PrO)4Ti, due to fast dynamics of ligand exchange on the sterically hindered Ti-center (Table 5, Fig.3), and iii) that the role of excess (i-PrO),Ti is to remove -by ligand exchange -the product alkoxides (R*O) from the catalytically active Ti-center (Scheme 4, Table 6 ) . Three new crystal structures of TADDOL derivatives (two clathrates with secondary amines, and a dimethyl ether) have been determined by X-ray diffraction (Figs. 5-7), and are compared with those previously reported. The distances between the C(aryl)*O oxygen atoms in the C2-and C1-symmetrical structures vary from 2.58 to 2.94 A, depending upon the conformation of their dioxolane rings and the presence or absence of an intramolecular H-bond (Fig. 8). A single-crystal X-ray structure of a spiro-titanate, with two TADDOLate ligands on the Ti-atom, is described (Fig. 9 ) ; it contains six different seven-membered titanate-ring conformations in the asymmetric unit (Fig. lo), which suggests a highly flexible solution structure. The structures of Ti TADDOLate complexes are compared with those of C,-symmetrical Ru, Rh, and Pd disphosphine chelates ( Table 7). A common topological model is presented for all nucleophilic additions to aldehydes involving Ti TADDOLates (Si attack with (R,R)-derivatives, relative topicity unlike ; Fig. 11 ). Possible structures of complexes containing bidentate substrates for Ti TADDOLate-mediated ene reactions and cycloadditions are proposed (Fig. 12). A simple six-membered ring chair-type arrangement of the atoms involved can be used to describe the result of TADDOLate-mediated nucleophilic additions to aldehydes and ketones, with Ti, Zr, Mg, or A1 bearing the chiral ligand (Scheme 6). A proposal is also made for the geometry of the intermediate responsible for enantioselective hydrogenation of N-(acety1amino)cinnamate catalyzed by Rh complexes containing C2-symmetrical dipbosphines (Fig. 13).

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

“…It has been suggested repeatedly that the chirality of the phosphane backbone is transmitted to the substrate through the face ± edge [48] or quasi-equatorial array [49] of phenyl groups. Certain quadrants around the metal center become more sterically hindered than others, forcing the prochiral substrate into a certain conformation leading to chiral induction.…”

Structure Investigation of TiIV‐BODOLates Involved in the Catalytic Asymmetric Reduction of Ketones Using Catecholborane

“…[11] Moreover, the BINOL and the TADDOL systems gave quite different complexes depending on the Ti:diol ratios. [5,6] The fact that resonances of only one type of BODOL unit was observed implied that the complex had a high symmetry. The two non-equivalent isopropyl group signals indicated that we were examining a single complex, and not an average signal from multiple complexes.…”

“…In this case the structure was shown to be a monomeric Ti-diolate of composition [(TADDOLate)Ti(OiPr) 2 ] (Figure 1c). [6] It should be noted that the structure and composition of the complexes formed are strongly dependent on the diol to [Ti(OiPr) 4 ] ratio used on mixing the components. In this report we will focus on the 1:1 complexes, which do not contain halogens or semihydrated alcoholates.…”

Recent development of asymmetric syntheses based on the Meerwein‐Ponndorf‐Verley reduction

“…Seebach et al 20 tested the enantioselectivity of the intermolecular MPV of the TADDOL ligand. The TADDOLzirconate complex 6 (Scheme 6) (equimolar amount) dissolved in i-PrOH (0.06 M) catalyzed an enantioselective MPV reduction of acetophenone.…”