Search

A series of dimeric complexes [TiL*X2]2 (H2L* = (1R,2S)-2-(4-methylbenzenesulfonylamino)-1,3-diphenyl-1-propanol (5); X = O-i-Pr (4), NMe2 (6), or O-t-Bu (7)) were prepared, and the asymmetric Et2Zn additions to benzaldehyde catalyzed by the complex 4, 6, or 7 with the addition of excess Ti(O-i-Pr)4 give excellent enantioselectivities up to 96.3% ee. The ¹H NMR study shows that the catalytic systems of these complexes involve a common active intermediate. The reaction of 4 with 2 molar equiv of Ti(O-i-Pr)4 afforded another dimeric complex, 9, with the structure (i-PrO)2TiL*Ti(O-i-Pr)4. Complex 9 is demonstrated to provide a suitable environment, achieving an enantioselectivity of 95.6% ee. Complex 9 in solution is shown as a mixture of complexes 4, 9, and Ti(O-i-Pr)4, and the equilibrium among these three complexes is solvent and temperature dependent. The reaction of complex 4 or 9 with MeTi(O-i-Pr)3 furnished the complex (i-PrO)2TiL*Ti(O-i-Pr)3Me (10), which reacted stoichiometrically with benzaldehyde to afford the complex (i-PrO)2TiL*Ti(O-i-Pr)3(OCHMePh) (11), which dissociates to give a mixture of the complexes 4, 11, and Ti(O-i-Pr)4. A complete scope of the mechanism is clearly deduced from stepwise reactions starting from complex 4 to 9, 10, 11 and then back to 4. This is an example of a mechanism with all intermediates confirmed structurally or spectroscopically except complex 12, which proceeds directly to complex 11. This mechanism is also suggested to apply for the reactions catalyzed by the titanium complexes of BINOLs and diols. One of the roles of excess Ti(O-i-Pr)4 is to facilitate removal of the product from the metal center. However, more importantly, the two major roles of Ti(O-i-Pr)4 are to exchange an alkyl group with the dialkylzinc reagent and to regenerate complex 9 from complex 11 for next cycles of reactions.

A series of N-sulfonylated β-amino alcohols (R,S)-3, (1R,2S)-PhCH(OH)CH(NHSO2R)CH2Ph, was prepared from reaction of various alkyl- or aryl-sulfonyl chlorides with (1R,2S)-2-amino-1,3-diphenylpropanol. A ¹H NMR study of these ligands shows a correlation of electron-withdrawing abilities of ligands with electronic properties of R substituents on the sulfonyl group. The asymmetric diethylzinc additions to benzaldehyde catalyzed by titanium(IV) complexes of (R,S)-3 were carried out, and yields and enantioselectivities of the desired (R)-1-phenylpropanol increase with increasing electron-donating abilities of R substituents. A dimeric titanium(IV) complex 4 was prepared, and the structure of 4 shows an interesting feature of inequivalent titanium metal centers with one sulfonamide of the chiral ligand bonded to a Ti metal center in a η²-fashion and another sulfonamide bonded to the second Ti metal center in a η¹-fashion. The complex 4 is not an effective catalyst for the asymmetric diethylzinc addition reaction. With further addition of Ti(O-i-Pr)4, the resulting systems become effective with results nearly identical to the in situ-formed catalytic systems. Another interesting feature is that the ¹H spectrum of 4 shows the presence of at least four isomeric species in CD2Cl2 at 0 °C, and possible structures of these species are discussed.

Enantioselective addition of diethylzinc to aldehydes catalyzed by Ti(IV) complexes of diol derivatives of d-mannitol was carried out. Good enantioselectivities were obtained with ee values up to 86% of R-configuration. A ¹H NMR study shows that the catalytic system in solution contains unreacted Ti(O-i-Pr)4, a dimeric Ti(IV) complex [Ti(O-i-Pr)2(μ-diolate)]2 (4) containing two bridging diolate ligands, and a dimeric Ti(IV) complex [(diolate)(O-i-Pr)Ti(μ-O-i-Pr)2Ti(O-i-Pr)3] (5) containing one diolate ligand attached to only one Ti(IV) metal center. The complex 5 with the more electropositive Ti(IV) metal center is considered the most active catalyst in solution. The structure of 4 was determined by X-ray analysis, and the FAB-mass spectroscopic study confirms the existence of complex 5. The role of excess Ti(O-i-Pr)4 in the catalytic system is that Ti(O-i-Pr)4 further reacts with the complex 4 to give the most active catalyst 5 in solution.

Facile syntheses of TiCl2(TADDOLate)L2 (TADDOL = α,α,α‘,α‘-tetraphenyl-1,3-dioxolane-4,5-dimethanol; L = THF (1), AcOEt (2)) were developed from reactions of TiCl4 with the TADDOL ligand in coordinating solvent L. The labile solvent ligands L can be replaced by a neutral bidentate ligand such as 1,2-bis(diphenylphosphino)ethane (dppe) or 3,3-dimethyl-2,4-pentanedione (diketone) to give TiCl2(TADDOLate)(L2) (L2 = dppe (3), diketone (4)). The molecular structure of complex 3 shows a structure with two chlorides in trans positions, and the dppe ligand is trans to the strong alkoxide ligands. The most significant features of the structure are the long Ti−P(phosphine) bond distances observed, indicating considerable steric hindrance arising from the TADDOLate ligand in the complex. In a reaction of the TADDOL analogue α,α,α‘,α‘-tetramethyl-1,3-dioxolane-4,5-dimethanol with TiCl4 in Et2O, the unexpected complex 6 derived from rearrangement of the chiral diol ligand was obtained. This rearrangement is apparently mediated by TiCl4 with the conversion of the 2,3-ketal-1,4-diol into the isomeric 3,4-ketal-1,2-diol. The asymmetric ethylations of benzaldehyde catalyzed by the complex 1, 2, or 3/Ti(O-i-Pr)4 system are efficient, and the enantioselectivities are good, with values of enantiomeric excess up to 89%.

Immobilized dirhodium(II) catalysts having mixed chiral ligands enhance reactivity (AH = azetidinone) and influence stereoselectivity in cyclopropanation and carbon−hydrogen insertion reactions.

Chiral dirhodium(II) tetrakis[methyl 2-oxypyrrolidine-5(S)-carboxylate] has been immobilized on polystyrene−poly(ethylene glycol) and Merrifield resins. Intra- and intermolecular cyclopropanation reactions with these catalysts demonstrate their high selectivity, comparable to or better than their homogeneous counterpart, and recovery and reuse for up to nine sequential applications have been achieved without loss of selectivity. Catalyst stability is related to the percentage of available sites to which the ligand is affixed, but the percentage of ligand sites on the polymer that are bound to dirhodium(II) does not appear to influence stability or selectivity.