Synthesis of ansa-Titanocenes via a Double-Skattebøl Rearrangement

Novel synthetic approaches to chiral ethylene-bridged ansa-titanocenes possessing stereogenic centers on the bridging carbon chain are described. A key step in the preparation of these compounds is the double-Skattebøl rearrangement of bis(vinyldibromocyclopropane) intermediates derived from 1,4-disulfones. The influence of tether substitution on the diastereo-selection in ansa-titanocene formation has been examined through stereospecific 2,3-dimethyl substitution on the ethylene bridge of β-methyl-substituted bis(cyclopentadienyl) ligands. The ethylene-bridged bis(cyclopentadienes) were converted to their dilithium salts 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 configuration of β-methyl cyclopentadienide ligands whereas the racemic configuration of tether-methyl substituents was found to have little effect on the racemic to meso ratio relative to ethylenebis-[η⁶-1-(3-methylcyclopentadienyl)]titanium dichloride. The spectral and physical characteristics of these compounds are discussed.