Bis(((S)-binaphthoxy)(isopropoxy)titanium) oxide as a mu-oxo-type chiral Lewis acid: application to catalytic asymmetric allylation of aldehydes.
A new, chiral bis-Ti(IV) oxide of type 1 was successfully designed and can be utilized for strong activation of aldehyde carbonyls, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin. The chiral bis-Ti(IV) catalyst (S,S)-1 can be readily prepared either by treatment of bis(triisopropoxy)titanium oxide with (S)-binaphthol or by reaction of ((S)-binaphthoxy)isopropoxytitanium chloride with silver(I) oxide. Reaction of hydrocinnamaldehyde with allyltributyltin (1.1 equiv) under the influence of in situ generated chiral bis-Ti(IV) oxide (S,S)-1 (10 mol %) in CH2Cl2 at 0 degrees C for 4 h afforded 1-phenyl-5-hexen-3-ol in 84% yield with 99% ee. The present asymmetric allylation using nonracemic bis-Ti(IV) oxide 1 with partially resolved (S)-binaphthol exhibits a positive nonlinear effect in correlating the enantiopurity of allylation product with the ee of (S)-binaphthol. This asymmetric approach provides a very useful way for obtaining high reactivity and selectivity by the simple introduction of the M-O-M unit in the design of chiral Lewis acid catalysts.
Hanawa
Hideo
H
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan.
Hashimoto
Takuya
T
Maruoka
Keiji
K