ENANTIOSELECTIVE ALLYLTITANATION OF ALDEHYDES WITH CYCLOPENTADIENYLDIALKOXYALLYLTITANIUM COMPLEXES

The preparation, analysis, and reactions of novel, highly stereoselective cyclopentadienyldialkoxyallyltitanium reagents, available in both enantiomeric forms, are described. Chiral monochlorotitanates are readily prepared from CpTiCl3 or Cp*TiCl3 and chiral 1,4-diols, which in turn are obtained from tartrate ester acetals by Grignard addition. The resulting stable seven-membered titanacycles have been analyzed by H-1, C-13, and Ti-49 NMR spectroscopy. The structures of two representatives, the complexes 15 and 20, are confirmed by X-ray diffraction. The ally] reagents are obtained from the chlorides by transmetalation with allyllithium, allylpotassium, or allyl Grignard compounds. For the ensuing reactions with aldehydes these reagents do not have to be isolated or purified. Correlation of X-ray data and Ti NMR line widths with selectivity suggests that asymmetric distortion of the titanium coordination geometry could be essential for enantioface discrimination, rather than direct interactions of reactants with the chiral ligand. By variation of the ligand substituents, allyltitanates derived from chloride 15 (with 2,2-dimethyl-alpha,alpha,alpha',alpha'-tetraphenyl-1,3-dioxolane-3,4-dimethanol as the ligand) emerged as the most selective reagents. Excellent regio-, diastereo-, and enantioselectivities (usually greater-than-or-equal-to 95% ee, greater-than-or-equal-to 95% de) are obtained for reactions with various achiral and chiral aldehydes. The NMR spectra of the allyl and the crotyl complexes (R,R)-9 and (R,R)-29 exhibit fast 1,3-shifts, favoring the (E) isomer with titanium eta-1-bound to the unsubstituted allyl terminus. This equilibration, and also the equilibrations of other aryl-, alkoxy-, and silyl-substituted allyltitanium complexes, restricts this method to the preparation of branched regioisomers with the anti configuration.