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.
A 95% yield in the cross metathesis of acrylonitrile with a model olefin is achieved at 25 degrees C with only 3 mol % of the new air-stable ruthenium catalyst 1f shown in the enclosed structural diagram. Even more remarkable are the performances of its boosted version 1g incorporating an electron-withdrawing group. Both these new enhanced versions of Hoveyda catalyst are readily available from Grubbs second generation precatalyst upon reaction with a styrenyl ether the end group of which has been functionalized by an ester function. The latter acts as a weakly coordinating ligand allowing to reach a desirable balance between antinomic properties such as the catalyst's stability, a high activity for challenging substrates, and a high initiation rate.
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