Coordination Modes and Catalytic Carbonylative [4 + 1] Cycloaddition of Vinylallenes

(Vinylallene)rhodium complexes with three kinds of coordination modes, that is, η 2 -coordination of the terminal π-bond of the allenyl group, η 4 -coordination of the conjugated diene skeleton, and planar σ 2 -coordination, were synthesized by ligand substitution of RhCl(PPh 3 ) 3 with vinylallenes of specific substitution patterns. Their structures were determined by X-ray crystallography. Coordination preferences can be explained in terms of spatial interactions between the vinylallene substituents and the phosphine ligands as well as effective delocalization of the π-electrons of the endo- and exocyclic double bonds. The structural studies were extended to the development of the rhodium-catalyzed carbonylative [4 + 1] cycloaddition reaction of vinylallenes, affording five-membered carbocycles. With a nonsymmetrical vinylallene, face-selective η 4 -coordination from the less hindered side of the allenyl group was observed. The achievement of [4 + 1] carbonylation can be attributed to (i) the substantial facility of substituted vinylallenes for η 4 -coordination and (ii) the significant contribution of a metallacyclo-3-pentene resonance form to the intermediate complex. A platinum(0) complex also catalyzed the [4 + 1] cycloaddition of vinylallenes as efficiently as rhodium.