Diene cycloisomerization hydrometalation

The ruthenium-catalyzed cycloisomerization of a variety of <5-enallenes was also achieved, forming cyclic 1,3-dienes or 1,4-dienes depending on the substrates and reaction conditions [32] (Eq. 22). This intramolecular coupling of the C=C bond and allenes can be envisioned by the initial hydrometallation of the allene moiety followed by intramolecular olefin insertion and isomerization. 

Besides enyne metathesis [66] (see also the chapter Recent Advances in Alkenes Metathesis in this volume), which generally produces 1-vinylcyclo-alkenes, ruthenium-catalyzed enyne cycloisomerization can proceed by two major pathways via hydrometallation or a ruthenacycle intermediate. The RuClH(CO)(PPh3)3 complex catalyzed the cyclization of 1,5- and 1,6-enynes with an electron-withdrawing group on the alkene to give cyclized 1,3-dienes, dialkylidenecyclopentanes (for n=2), or alkylidenecyclopentenes (for n= 1) [69,70] (Eq. 51). Hydroruthenation of the alkyne can give two vinylruthenium complexes which can undergo intramolecular alkene insertion into the Ru-C bond. 

Scheme 16. Catalytic Cycloisomerization of 1,6-Dienes and Enynes via (a) Hydrometalation or (b) Formation of the Metallacycle...

Apart from the formation of vinylmetals and metallacycles and all the possible pathways deriving from them, jt-aUyl metal pathway (n -metal pathway Scheme 7.10) are quite rare in the cycloisomerization of enynes. On the other hand, cycloisomerizations of dienes are common to be performed either through the generation jt-allyl complexes or through hydrometallation of alkenes [26a]. 

The major drawbacks concerning the use of dienes in cycloisomerization reactions usually rely on the poor control of the regiochemical outcome, based on the lack of significant reactivity difference between the reacting alkenes. What is more, olefinic positional isomers are commonly obtained due to the facile transposition of alkenes via the reversible hydrometallation/p-hydride elimination, which is usually feasible during the standard cycloisomerization conditions (Scheme 7.34). To address these issues, considerable efforts have been directed... 

Based on the diene substrates and the reaction conditions, two distinct mechanisms can be followed to induce alkene activation and consequent cycloisomerization. The first mechanism invokes the hydrometallation of an alkene, followed by its subsequent carbometallation into the second one, terminated by a p-hydride elimination. The second mechanism implies the formation of a it-allylic complex either by its prior or later to the cycloisomerization formation (Scheme 7.35). Dienes possessing heteroatom functionality in the allylic position is able to provide it-allylic complex with a metal prior its cyclization. On the other hand, polyenes bearing 1,3-dienes in their system are commonly producing a 1,3-metal complex, which is readily cycloisomerize to give a ii-allyl metal complex after the formation of the new carbon—carbon bonds. 

---