Asymmetric C-C Bond Formation with Titanium Carbohydrate Complexes

A novel class of titanium complexes, cyclopentadienyl-dialkoxy-chlorotitanates, is described. Two chiral representatives 4 and 8, the former with two diacetone-glucose ligands, the latter with the bidentate (2R,3R)-2:3-isopropylidene-l,l,4,4,-tetraphenyl-threitol ligand, give highly enantioselective reagents, if the chloride is substituted by a transferable group. Allyl-groups and ester-enolates are thus added to various aldehydes, affording homoallyl-alcohols, acetate and propionate aldols, as well as P-hydroxy-a-aminoacids of high optical purity, generally 90 - 98% ee. Reagents derived from 4 attack the re-side, those derived from 8 the si-side of aldehydes. With several a-chiral substrates almost complete diastereocontrol is achieved in allylations of the threitol-derived reagents. The complexes 4 and 8 have been analysed by X-ray-diffraction, 'H- and 13C-NMR. Molecular modelling has been used to analyse the conformation of 4, and to estimate the strain of dioxa-titanacycles of various ring size.