In a Quest for Selectivity Paired with Activity: A Ruthenium Olefin Metathesis Catalyst Bearing an Unsymmetrical Phenanthrene‐Based N‐Heterocyclic Carbene

Abstract

Robust, selective, and stable in the presence of ethylene, ruthenium olefin metathesis pre‐catalyst, {[3‐benzyl‐1‐(10‐phenyl‐9‐phenanthryl)]‐2‐imidazolidinylidene}dichloro(o‐isopropoxyphenylmethylene)ruthenium(II), Ru‐3, bearing an unsymetrical N‐heterocyclic carbene (uNHC) ligand, has been synthesized. The initiation rate of Ru‐3 was examined by ring‐closing metathesis and cross‐metathesis reactions with a broad spectrum of olefins, showing an unprecendented selectivity. It was also tested in industrially relevant ethenolysis reactions of olefinic substrates from renewable feedstock with very good yields and selectivities.

Specialized Olefin Metathesis Catalysts Featuring Unsymmetrical N-Heterocyclic Carbene Ligands Bearing N-(Fluoren-9-yl) Arm

“…Interestingly, the Mes-derived fluorenyl catalysts (Ru9, Ru11) performed with lower conversion, however, exhibiting only slightly reduced selectivity. These results (TON up to 7140) obtained for Ru10 and Ru12 outperform not only earlier Ru uNHC complexes [16] but also more recent complexes bearing uNHC with a thiophene wing, as well the recently disclosed Ru complex bearing highly engineered bulky phenanthrene-based uNHC, which under the same conditions provided TON of 4550 [19] and 1370, respectively [48]. It shall be at the same time noted, however, that our fluorenyl uNHC catalysts that need 100 ppm to maintain acceptable conversion are less powerful than CAAC complexes, such as Ru13, which was reported to work in ethenolysis with grade 3 ethylene under similar conditions at 25 ppm resulting in TONs up to 28,000 [63].…”

“…The introduction of fluorene moiety in conjunction with either the Mes or Dipp group was found to have a profound influence on stability and activity of the resulting catalysts, especially in reactions of demanding substrates, such as cross-metathesis of oleic substrate with ethylene and self-CM of α-olefins. For ethenolysis, the Dipp substituted N-fluorenyl uNHC complexes (independent of their initiating alkylidene) gave results better than some other recently disclosed uNHC systems [48], but worse than modern engineered CAAC and bis(CAAC) systems [62,63]. Importantly, in the self-CM of α-olefins, the Mes substituted N-fluorenyl uNHC complexes performed very well, making it possible to decrease catalysts loading of Ru9 to 1 ppm and obtain the desired primary metathesis product 27 with high efficiency (TON = 260,000) and selectivity.…”

“…With the basic profiling of new uNHC complexes completed, we examined them in a more diverse set of RCM reactions (Table 2). We attempted these tests not because we believed the fluorenyl uNHC catalysts will find applications in natural products synthesis or in medicinal chemistry-the fields of applications where catalysts bearing symmetrical NHC are traditionally much more suitable [50,51]-but to be consistent with our previous studies using similar sets of more advanced substrates [26,48,52]. All reactions were performed in 0.1 M solution in toluene at 60 • C in the presence of 1000 ppm (0.1 mol %) of catalysts to compare the new system with well-established general purpose Umicore M2 (Ru2) and Hoveyda-Grubbs (Ru5) catalysts.…”

“…Typically, complexes containing unsymmetrical NHC ligands display increased stability in a solution, also at elevated temperatures [22,32,48]. We were curious if fluorene moiety, combining structural elements introduced by Mauduit [32] and us [22], would also have a positive influence on catalysts' resistance towards decomposition.…”

“…Decreasing the temperature to 50 °C ( Figure 7b) slightly reduced the reaction rate-after 3 h 79% of the substrate was consumed-but at the same time significantly improved selectivity (from 87% Therefore, we opted to check how the new uNHC complexes work in this case. To do so, we selected ethyl oleate as a model substrate (oleic acid esters are commonly used for such purpose) under industrially-friendly conditions [19,48] (i.e., in an ordinary steel autoclave outside a glovebox using ethylene grade 3). Some preliminary studies have shown that the lowest Ru loading where our catalysts are still giving practically useful conversions is equivalent to 100 ppm.…”