SYNTHESIS OF ANSA-TITANOCENES VIA A DOUBLE-SKATTEBOL REARRANGEMENT

Novel synthetic approaches to chiral ethylene-bridged ansa-titanocenes possessing stereogenic centers on the bridging carbon chain are descr ibed. A key step in the preparation of these compounds is the double-S kattebol rearrangement of bis(vinyldibromocyclopropane) intermediates derived from 1,4-disulfones. The influence of tether substitution on t he diastereo-selection in ansa-titanocene formation has been examined through stereospecific 2,3-dimethyl substitution on the ethylene bridg e of beta-methyl-substituted bis(cyclopentadienyl) ligands. The ethyle ne-bridged bis(cyclopentadienes) were converted to their dilithium sal ts and treated with TiCl3.3THF to afford mixtures of meso and racemic ansa-titanocenes. Several isomers were isolated, and their structures were determined by X-ray diffraction. The meso configuration of tether -methyl substituents was found to promote the formation of a racemic c onfiguration of beta-methyl cyclopentadienide ligands whereas the race mic configuration of tether-methyl substituents was found to have litt le effect on the racemic to meso ratio relative to enebis-[eta5-1-(3-m ethylcyclopentadienyl)]titanium dichloride. The spectral and physical characteristics of these compounds are discussed.