| Abstract
| - 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 theTADDOL ligand in coordinating solvent L. The labile solvent ligands L can be replaced bya 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)). Themolecular 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 featuresof 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 ofthe 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 benzaldehydecatalyzed by the complex 1, 2, or 3/Ti(O-i-Pr)4 system are efficient, and the enantioselectivitiesare good, with values of enantiomeric excess up to 89%.
- Facile syntheses of TiCl2(TADDOLate)(L2) were developed, and an unexpected rearrangement of the tetramethyl analogue of the TADDOL ligand mediated by TiCl4 was observed. Efficient asymmetric ethylations of PhCHO by ZnEt2 with the use of TiCl2(TADDOLate)L2/Ti(O-i-Pr)4 catalytic systems were conducted with good enantioselectivities.
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