CYCLOADDITIONS CATALYZED BY TITANIUM COMPLEXES

Article abstract of DOI:10.1016/0040-4020(84)85014-0

The Ziegler catalyst TiCl4-Et2AlCl and the arenetitanium(II) complex (η⁶-C6H6)Ti(II)(AlCl4)2 induce <6+2> cycloaddition reactions of cycloheptatriene with dienes and acetylenes.Addition to 1,3-butadiene affords 7-endo-vinyl-bicyclo<4.2.1>nona-2,4-diene (main product) and bicyclo<4.4.1>-undeca-2,4,8-triene, a product of <6+4> cycloaddition.Isoprene reacts similarly, yielding mainly 7-endo-isopropenyl-bicylo<4.2.1>nona-2,4-diene. 2,3-Dimethyl-1,3-butadiene gives 8,9-dimethylbicyclo<4.4.1>undeca-2,4,8-triene, a product of <6+4> cycloaddition, while <6+2> cross-adducts are minor products.The reaction of cycloheptatriene with norbornadiene gives mainly hexacyclo<6.5.1.0^2.7.0^3.12.0^6.10.0^9.13>tetradec-4-ene via <6+2> cycloaddition followed by intramolecular Diels-Alder reaction.As a by-product, pentacyclo<7.5.0.0^2.7.0^3.5.0^4.8>tetradeca-10,12-diene is formed by a <2+2+2> mechanism.Addition of cycloheptatriene to phenylacetylene and bis-(trimethylsilyl)acetylene furnishes substituted bicyclo<4.2.1>nona-2,4,7-trienes.Alkenes, E,E-2,4-hexadiene and 1,3-cyclooctadiene are unreactive.The <6+2> cycloaddition is made possible by coordination of cycloheptatriene to titanium, which changes the symmetry of the frontier orbitals in the triene.The reactivity of the trienophile is also enhanced by coordination to the catalyst.