Facile syntheses of TiCl2(TADDOLate)(L2), efficient asymmetric ethylation of PhCHO, and an unexpected rearrangement of the tetramethyl analogue of the TADDOL ligand

Article abstract of DOI:10.1021/om980570w

Facile syntheses of TiCl₂(TADDOLate)L₂ (TADDOL = α,α,α′,α′-tetraphenyl-l,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-J-Pr)₄ system are efficient, and the enantioselectivities are good, with values of enantiomeric excess up to 89%.