Development of new cycloaddition reactions of conjugated allenes catalyzed by transition metal complexes is described. In the first place, the synthesis and structures of vinylallene-rhodium complexes were investigated. (Vinylallene) rhodium complexes of three kinds of coordination modes, that is, η2-coordination, η4-coordination, and planar σ2-coordination, were selectively synthesized by facile ligand substitution of the Wilkinson's complex with vinylallenes of different substitution patterns. The endo-exo isomerization observed with the η4-complex suggested a considerable contribution from the metallacyclic form. This structural study led us to develop a new carbonylative [4+1] cycloaddition reaction of vinylallenes, which was further extended to a highly enantioselective synthesis of 2- (alkylidene) cyclopentenone. Next, the intermolecular directed [4+2] cycloaddition of a vinylallene with an ordinary 1, 3-diene was achieved by the use of a palladium (0) catalyst. An excellent specificity observed between the vinylallene geometries and the cycloadduct stereostructures was explained by assuming a mechanism involving a (π-allyl) palladium intermediate. The iridium-catalyzed carbonylative [5+1] cycloaddition reaction was also developed, wherein a six-membered cyclohexenone skeleton was assembled from allenylcyclopropane and carbon monoxide.
