The addition of an allylzinc compound to a vinylmetal compound takes place rapidly and in high yield to produce a 1,1-dimetallic organometallic species. This reaction has; apparent kinetic and thermodynamic problems, and in addition, the nature of the true final product has not been known. Density functional (B3LYP) studies on this reaction for a combination of various metals including lithium, magnesium, zinc, and tin provided the first information on these unresolved issues. The key stage of the reaction was found to be an endothermic Lewis acid-assisted metalla-Claisen rearrangement that also shows some character of metallo-ene reaction of the vinylmagnesium species. The kinetic barrier of the C-C bond-forming step is quite low and the thermodynamic driving ford of the reaction is provided by oligomerization of the initial product that occurs after the C-C bond formation; The high level of diastereoselectivity observed experimentally has been ascribed to the short length of the forming C-C bond in the transition structure, which is due to the late nature of the transition state of the endothermic metalla-Claisen rearrangement reaction.