Abstract
Hoveyda–Grubbs metathesis catalysts undergo a relatively fast decomposition in the presence of olefins. Using a computational density functional theory approach, we show that positively charged derivatives of N-heterocyclic carbenes have little impact on the degradation/deactivation rates of such catalysts with respect to neutral carbenes. On the other hand, the hypothetical anionic Hoveyda–Grubbs-like catalysts are predicted to less likely undergo degradation in the presence of the olefin, while being as active as standard, neutral Hoveyda–Grubbs catalysts.
Highlights
The phenomenon of N-heterocyclic carbenes (NHCs) started in 1991, when they were first synthesized by Arduengo.[1]
One of the most popular applications of NHCs is as ruthenium olefin metathesis catalysts and, in particular, the Hoveyda−Grubbs complex.[4]
There is not a single example of any of the anionic NHC derivatives complexing Ru to form a potential metathesis catalyst, such systems have been proposed in computational studies and predicted to be stable and good candidates for efficient metathesis catalysts.[18,19]
Summary
The phenomenon of N-heterocyclic carbenes (NHCs) started in 1991, when they were first synthesized by Arduengo.[1]. One of the recently suggested and introduced modifications of carbenes is the addition of a positive or negative charge to NHCs. Anionic NHCs (Figure 1a) were obtained both in the isolated form and in metal complexes,[7−16] and in the latter systems, they were shown to possess, in general, good stability because of their high affinity for transition metals bearing a formal positive charge.[17] Surprisingly, there is not a single example of any of the anionic NHC derivatives complexing Ru to form a potential metathesis catalyst, such systems have been proposed in computational studies and predicted to be stable and good candidates for efficient metathesis catalysts.[18,19] The cationic NHCs (Figure 1b) were reported,[20−26] and it was shown that the positive charge on the NHC affects the electron properties by reducing the σ-donation properties and slightly increasing the π-acidity. In the context of olefin metathesis, cationic carbenes are commonly used for more than 10 years to obtain water-soluble catalysts by introducing various ammoniumtagged moieties.[26,28−30] Interestingly, it has been shown both experimentally and theoretically that the impact of the additional positive charge on the reactivity of such systems is very low.[31,32]
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