REGIOSELECTIVE AND STEREOSELECTIVE SYNTHESIS OF THE 1,3-CYCLOHEXADIENE NUCLEUS BY [2+2+2]CYCLOADDITION REACTIONS CATALYZED BY TITANIUM ARYLOXIDE COMPOUNDS

A variety of titanium aryloxide reagents catalyze the cross coupling o f two alkyne units with 1 equiv of olefin to produce the 1,3-cyclohexa diene nucleus. Catalysts include isolated titanacyclopentadiene or tit anacyclopentane complexes. The reaction proceeds via attack of the ole fin upon a titanacyclopentadiene compound initially formed by coupling of two alkyne units. The reaction is limited to bulky alkyne substrat es that undergo slow catalytic cyclotrimerization via competing attack of a third alkyne upon the titanacyclopentadiene ring. The organic pr oducts isolated are typically the result of an isomerization within th e initially produced 1,3-cyclohexadiene nucleus. Mechanistic studies s how that these isomerization processes occur via sequential; metal-med iated 1,5-hydrogen shifts upon a single face of the six-membered ring, exclusively leading to a cis-stereochemistry within the final product s. In the reactions of the diynes R-C=C(CH2)(4)C=C-R (R = Et, SiMe3), coupling with ethylene and or-olefins produces a variety of substitute d hexalins. A combination of NMR spectroscopy, photochemistry, and mol ecular mechanics calculations has been applied to determine the stereo chemistry and ground state conformations adopted by the product 1,3-cy clohexadienes and hexalins. The primary and secondary photoproducts ob tained from some of these 1,3-cyclohexadiene compounds have been chara cterized.