Recent Advances in “Formal” Ruthenium‐Catalyzed [2+2+2] Cycloaddition Reactions of Diynes to Alkenes

“Formal” and standard Ru II ‐catalyzed [2+2+2] cycloaddition of 1,6‐diynes to alkenes gave bicyclic 1,3‐cyclohexadienes in relatively good yields. When terminal 1,6‐diynes 1 were used, two isomeric bicyclic 1,3‐cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6‐diynes 7 were used, the reaction with acyclic alkenes took place regio‐ and stereoselectively to afford bicyclic 1,3‐cyclohexadienes 8 . A cascade process that behaves as a “formal” Ru II ‐catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru‐catalyzed linear coupling of 1,6‐diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5‐trienes of type 3 , which after a thermal disrotatory 6e − π‐electrocyclization led to the final 1,3‐cyclohexadienes 4 and 8 . When disubstituted 1,6‐diyne 10 was used with electron‐deficient alkenes, new exo ‐methylene cyclohexadienes 12 arose from a competitive reaction pathway.