Cationic Ruthenium-Catalyzed Bis-Homo-Diels-Alder Cycloaddition

Objective: The ruthenium-catalyzed Bis-Homo-Diels-Alder cycloaddition between 1,5- cyclooctadiene and alkynes was explored, and the use of commercially available cationic catalysts was investigated. It was noted that [CpRu(CH3CN)3]PF6 was effective at catalyzing this cycloaddition and yields of the desired tricyclo[4.2.2.02,5]dec-7-ene adduct ranging from 13 to 83% were achieved using this cationic catalyst. Several cycloadducts that were previously unobtainable with the use of the neutral (Cp*RuCl(COD) catalysts were also successfully made using [CpRu(CH3CN)3]PF6 albeit in low yields. Methods: Commercially available, and previously synthesized alkynes were combined with 1,5-cyclooctadiene and treated with a ruthenium catalyst within a glovebox. The reaction mixture was stirred for 72h at temperatures ranging from 25 to 70oC. The desired cycloadduct was then isolated using flash column chromatography and analyzed and characterized using NMR, IR and MS. Results: Several previously unattainable adducts were synthesized using the cationic [CpRu(CH3CN)3]PF6. When this catalyst was compared to the neutral Cp*RuCl(COD) greater yields were observed. Conclusion: The present study describes an improved method for the formation of the tricyclo[4.2.2.02,5]dec-7- ene framework using a commercially available cationic ruthenium catalyst. It was noted that the use of [CpRu(CH3CN)3]PF6 led to improved yields when compared to Cp*RuCl(COD).