NEW CYCLOADDITION REACTIONS OF CONJUGATED ALLENES CATALYZED BY TRANSITION-METAL COMPLEXES

Development of new cycloaddition reactions of conjugated allenes catal yzed by transition metal complexes is described. In the first place, t he synthesis and structures of vinylallene-rhodium complexes were inve stigated. (Vinylallene) rhodium complexes of three kinds of coordinati on modes, that is, eta(2)-coordination, eta(4)-coordination, and plana r sigma(2)-coordination, were selectively synthesized by facile ligand substitution of the Wilkinson's complex with vinylallenes of differen t substitution patterns. The endo-exo isomerization observed with the eta(4)-complex suggested a considerable contribution from the metallac yclic form. This structural study led us to develop a new carbonylativ e [4+1]cycloaddition reaction of vinylallenes, which was further exten ded to a highly enantioselective synthesis of 2-(alkylidene) cyclopent enone. Next, the intermolecular directed [4+2] cycloaddition of a viny lallene with an ordinary 1,3-diene was achieved by the use of a pallad ium(0) catalyst. An excellent specificity observed between the vinylal lene geometries and the cycloadduct stereostructures was explained by assuming a mechanism involving a (pi-allyl)palladium intermediate. The iridium-catalyzed carbonylative [5+1] cycloaddition reaction was also developed, wherein a six-membered cyclohexenone skeleton was assemble d from allenylcyclopropane and carbon monoxide.