New chiral C3-symmetric triols as ligands for vanadium and titanium complexes

Several new chiral triols 1a-c with C-3 symmetry have been prepared in enantiopure form via a route involving the iterative catalytic asymmetric alkylation of aldehydes by organozinc reagents followed by diastereoselective hydroboration. Reactions of these chiral triols with titanates Ti(OR)(4) and vanadates V(O)(OR)3 are reported. The structure of the binuclear vanadium triolato(3-) complex [V(O)C{(OCH-Et)(3)CH}](2) (12a) has been determined by X-ray analysis. A series of model reactions was conducted to assess the catalytic properties of the chirally modified titanium and vanadium secondary alkoxides. The reactions chosen were the addition of diethylzinc and of trimethylsilyl cyanide to benzaldehyde and the oxidation of geraniol and of ethyl phenyl sulfide with tert-butyl hydroperoxide. Only a moderate asymmetric induction was found in some model reactions catalyzed by titanium complexes 11a,b. On the other hand, a very efficient chemoselective oxidation of sulfides to sulfoxides with tert-butyl hydroperoxide was possible in the presence of vanadium complex 12a.