Mechanism of asymmetric dialkylzinc addition to aldehydes catalyzed by titanium(IV) complexes of N-sulfonylated β-amino alcohols

A series of dimeric complexes [TiL*X-2](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 H-1 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)(2)TiL*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)(2)TiL*Ti(O-i-Pr)(3)Me (10), which reacted stoichiometrically with benzaldehyde to afford the complex (i-PrO)(2)TiL*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.