Metal-Catalyzed [4 + 2 + 2] Cycloadditions: Cycloadducts of Substituted Norbornadienes and Their Opening with Zeise's Dimer

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).

Ruthenium-catalyzed homo Diels-Alder [2 + 2 + 2] cycloadditions of alkynyl phosphonates with bicyclo[2.2.1]hepta-2,5-diene

Ruthenium-catalyzed homo Diels-Alder [2 + 2 + 2] cycloadditions between alkynyl phosphonates and bicyclo[2.2.1]hepta-2,5-diene were studied. The observed reactivity was found to be dependent on the presence of the phosphonate moiety. The Ru-catalyzed cycloaddition was compatible with a variety of aromatic and aliphatic substituted alkynyl phosphonates, providing the corresponding phosphonate substituted deltacyclenes in low to good yields (up to 88%).

"Formal" ruthenium-catalyzed [4+2+2] cycloaddition of 1,6-diynes to 1,3-dienes: formation of cyclooctatrienes vs vinylcyclohexadienes

[reaction: see text] A new "formal" Ru-catalyzed [4+2+2] cycloaddition of 1,6-diynes to 1,3-dienes giving conjugated 1,3,5-cyclooctatrienes and vinylcyclohexadienes is described. This formal cycloaddition is really a tandem process, the Ru(II)-catalyzed formation of (Z)-tetraenes or vinyl-(Z)-trienes followed by a pure thermal conrotatory 8 pi- or disrotatory 6 pi-electrocyclization. The proposed mechanism allows the differences in product ratio to be explained in terms of steric and stereochemical considerations.

A homo Diels-Alder approach to bicyclo[4.2.1]nonanes

[reaction: see text] A new, high-yielding route to functionalized bicyclo[4.2.1]nonanes has been achieved utilizing homo Diels-Alder chemistry with subsequent Zeise's dimer-catalyzed opening of the cycloadduct.