To appreciate the reason of the regiochemistry, we must compare this with the [2+2+1] reaction of diene-yne and its competing but disfavored [4+2+1] reaction (Figure ). reaction. This agrees with experiments of But why CP capped diene favors [4+2+1] reaction? We attribute this to the strain release of the methylenecyclopropyl (MCP) part in the CP-capped diene. MCP is a strained molecule because the bond angle of its alkene is 60°. This ring strain can be reflected by the hydrogenation reactions a and b, showing CP-capped diene is about 12 kcal/mol more exothermic than a common diene (Scheme 3). In Figure , the CP-capped diene part still has a strained MCP moiety. In the [4+2+1] transition state and product, this strain is released partially for the former and completely for the latter because of the change from MCP to CP. Due to this strain release, the reductive elimination transition state in the [4+2+1] pathway is easier for CP-capped diene compared to that using common diene by 7 kcal/mol (20.0 vs. 27.0 kcal/mol). In the [2+2+1] pathway, both diene and CPdiene have very similar reductive elimination activation barriers because no strain release in the reductive elimination transition states has been encountered for the CP capped diene compared to the common diene (25.9 vs 24.3 kcal/mol). Therefore, we conclude that ring strain release is the reason for the [4+2+1] reaction of CP-capped diene-ynes/diene-enes and CO.