Palladium‐Catalyzed Oligocyclizations of 2‐Bromoalka‐1,(n+m+1)‐dien‐(n+1)‐ynes – Influence of Tether Lengths and Substituents on the Outcome of the Reaction (Part II)

Abstract

AbstractThe cascade reaction modes and hence the outcomes of the palladium‐catalyzed oligocyclizations of various 2‐bromoalka‐1,(n+m+1)‐diene‐(n+1)‐yne substrates were found to be highly dependent on the tether lengths between the multiple bond fragments, and on the nature of the substituent at the non‐brominated vinylic terminus. Just like 2‐bromododeca‐1,11‐dien‐6‐ynes with their two three‐atom tethers, 2‐bromotrideca‐1,12‐dien‐7‐ynes 8 and 2‐bromotetradeca‐1,13‐dien‐8‐ynes 17, with combinations of four‐ and three‐ as well as five‐ and three‐atom tethers, under Heck‐type reaction conditions undergo, after the initial oxidative addition step, two consecutive n‐exo‐dig and m‐exo‐trig carbopalladations followed by β‐hydride elimination and ensuing 6π‐electrocyclization to furnish tricyclic bisannelated cyclohexadiene derivatives, such as 37 and 40, in moderate yields (30 and 29 %). On the other hand, when 2‐bromoalka‐1,(n+m+1)‐dienynes with a four‐, five‐ or even a six‐atom tether between the bromoene and the yne moiety, and a four‐atom tether between the yne and the terminal ene unit, such as 34, 35 and 36, are subjected to Heck‐type reaction conditions, they undergo a cascade oligocyclization involving a 5‐exo‐trig‐ and ensuing 3‐exo‐trig‐carbopalladation as the fourth and fifth steps before the terminating β‐hydride elimination, eventually leading to tetracyclic skeletons such as 43, 44 and 45 with a bridging three‐membered ring in yields of 74, 76 and 32 %, respectively.