Ruthenium-Catalyzed Polycyclization Reactions

Ruthenium carbene Cl₂(PCy₃)₂Ru=CHCH=CPh₂ mediates the efficient and selective conversion of acyclic dienynes to fused bicyclic [n.m.0] dienes containing five-, six- and seven-membered rings. Studies with various X-substituted acetylenes (X = H, alkyl, Ph, CO₂Me, SnBu₃, SiMe₃, halogen) suggest that the dienyne metathesis is not only sensitive to these substituents but also to the catalysts employed. Among the various metal alkylidenes examined, only the ruthenium catalyst I exhibited metathesis activity for a range of substrates. In no case, however, were acetylenes with heteroatomic substituents metathesized; these substrates either underwent simple diene RCM or failed to react with the catalyst in a productive fashion. These observations further expand the scope of catalytic RCM for the construction of complex organic compounds. Ruthenium alkylidene Cl₂(PCy₃)₂Ru=CHPh has been utilized in the tandem ring-opening/ring-closing metathesis of cycloolefins. This reaction produces a bicyclic molecules conatining nonconjugated dienes. Reactivity of the precursors is dependent upon strain, and thus ring size, of the cycloolefins. Competing oligomerization is observed in substrates having low ring strain; this process is inhibited by increasing dilution of the reaction or by adding alkyl substitution to the acyclic olefins. The application of ruthenium alkylidene Cl₂(PCy₃)₂Ru=CHPh to the catalysis of polycyclization reactions is detailed. Several acyclic precursors have been synthesized and reacted with the ruthenium alkylidene. These precursors vary in topology and contain either acetylenic or cyclic olefin metathesis relays or both. The cyclization reactions proceed in moderate to good yield to produce polycyclic polyenes when the precursors are subjected to catalytic amounts of the ruthenium complex. Precursors bearing n relay units generate polycycles containing (n + 1) rings.