Nitro-substituted Hoveyda-Grubbs ruthenium carbenes: enhancement of catalyst activity through electronic activation.
The design, synthesis, stability, and catalytic activity of nitro-substituted Hoveyda-Grubbs metathesis catalysts are described. The highly active and stable meta- and para-substituted complexes are attractive from a practical point of view. These catalysts operate in very mild conditions and can be successfully applied in various types of metathesis [ring-closing metathesis, cross-metathesis (CM), and enyne metathesis]. Although the presence of a NO(2) group leads to catalysts that are dramatically more active than both the second-generation Grubbs's catalyst and the phosphine-free Hoveyda's carbene, enhancement of reactivity is somewhat lower than that observed for a sterically activated Hoveyda-Grubbs catalyst. Attempts to combine two modes of activation, steric and electronic, result in severely decreasing a catalyst's stability. The present findings illustrate that different Ru catalysts turned out to be optimal for different applications. Whereas phosphine-free carbenes are catalysts of choice for CM of various electron-deficient substrates, they exhibit lower reactivity in the formation of tetrasubstituted double bonds. This demonstrates that no single catalyst outperforms all others in all possible applications.
Institutes of Organic Chemistry and Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland.
Anna
A
Michrowska
Robert
R
Bujok
Syuzanna
S
Harutyunyan
Volodymyr
V
Sashuk
Grigory
G
Dolgonos
Karol
K
Grela