Palladium-Catalyzed Oligocyclizations of 2-Bromoalk-1-ene-(n+1),(m plus n+1)-diynes - Influence of Tether Lengths and Substituents on the Outcome of the Reaction (Part I)

The outcomes and the mechanistic pathways of the palladium-catalyzed Heck-type cascade oligocyclizations of various 2-bromoalkenediynes were explored with respect to the lengths of the tethers between the multiple bonds and the nature of the substituent at the acetylenic terminus. Just like substrates containing two three-atom tethers, 2-bromotridec-1-ene-6,13-diynes 10a,b with one three- and one four-atom tether undergo two consecutive intramolecular 5- and 6-exo-dig carbopalladations with subsequent 6 pi-electrocyclization and beta-hydride elimination to form tricycles 35a,b with a central benzene ring in 67 and 61 % yield, respectively, independent of the fact that 10a contains an electron donor and lob an electron acceptor at the acetylene terminus. However, when 2-bromotetradec-1-ene-7,13-diynes 22, 29 on one side and 16, 27, 31 on the other are subjected to Heck reaction conditions, tricycles with a central benzene ring are formed only, when the substituent at the acetylene terminus is not a methoxycarbonyl group as in 22, 29. Thus, the bisannelated benzene derivatives 36 and 37 are formed from 22 and 29 in 79 and 18 % yield, respectively, whereas 16, 27 and 31 with their methoxycarbonyl substituents at the acetylenic end yield tetracyclic systems 38, 39 and 40, consisting of a central five-membered and two annelated six-membered as well as an additional annelated three-membered ring, predominantly (54, 19 and 18 % yield, respectively). The cascade reaction leading to the latter products must involve a 5-exo-trig carbopalladation rather than 6 pi-electrocyclization as the third step. Apparently, the nature and the substitution pattern of the tether in the substrates 16, 22, 31 linking the vinyl bromide moiety with the internal acetylene affect the yield of the tetracyclic product