Hoveyda–Grubbs-Type Precatalysts with Unsymmetrical N-Heterocyclic Carbenes as Effective Catalysts in Olefin Metathesis

Abstract

In our search for more-selective olefin metathesis catalysts, a series of Hoveyda–Grubbs-type second-generation complexes bearing unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized and tested in model reactions. It was found that the N-benzyl substituent in NHC has a positive influence on the selectivity of the newly obtained catalysts in the self-metathesis reaction of α-olefins. As expected, a typical relationship between activity and selectivity with respect to the N-aryl substituent used was observed. Dipp-containing complexes exhibited higher stability at elevated temperature, while Mes-bearing complexes typically gave better yields than their Dipp analogues.

Azoliniums, Adducts, NHCs and Azomethine Ylides: Divergence in Wanzlick Equilibrium and Olefin Metathesis Catalyst Formation

“…Our own investigations showed Ru9 exhibited improved stability compared to commercially available Ru3 , while Hoveyda type analogues also demonstrated improved reactivity . As such, we became interested in complex Ru10 , an indenylidene complex bearing an N ‐fluorene‐substituted uNHC.…”

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

“…In contrast, under our reactions conditions (40 °C, CD 2 Cl 2 , C=0.1 m ), Ru‐2 has a very low rate of initiation. It is known that the N ‐alkyl‐substituted Hoveyda‐type complexes, require higher temperatures to initiate . This clearly shows that Ru‐3 shows deviation in reactivity from previously published N ‐benzyl‐substituted catalysts, exhibiting initiation rates equal to commercially available catalyst Ru‐1 .…”

“…Generally, the geometrical parameters describing the molecular skeleton of Ru‐3 well correlate with those of Ru‐2 (Figure , L10‐C ), which may be treated as the reference compound (Figure , Table ) . Based on angles between ruthenium ligands, geometry index ( τ 5 ) was calculated for Ru‐3 and Ru‐2 , to quantify the square‐pyramid distortion.…”

“…These proved useful as olefin‐metathesis precatalysts, not only, in standard conditions under an argon atmosphere, but also in commercial grade nondegassed solvents (Figure , L10‐B ) . Importantly, those complexes were successfully applied in the ethenolysis of ethyl oleate (Figure , L12‐B , C ) and afforded low isomerization in self‐metathesis of α‐olefins (Figure , L11‐C ) . They were also used in reactions with 1 ppm pre‐catalyst loading yielding 232 735 TON (Figure , L14‐B ) .…”

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

“…Enhanced stability, especially under harsh conditions, has proven to be beneficial for macrocyclization reactions performed at high concentration [27]. Based on the successful design by Mauduit (Ru7) [21] and on the catalyst obtained previously by us (Ru8) [22,25,31], we decided to combine the selected structural characteristics of these complexes into a new system bearing an N-(fluoren-9-yl) side-arm ( Figure 2). We speculated that formal addition of two benzene rings fused into a cyclopentene ring of Mauduit's Ru7 [21,32] The first representatives of this new class of complexes were Grubbs-type uNHC catalysts obtained by Fürstner [11], before the Hoveyda-type complexes were synthesized by Blechert [12] and soon after by Verpoort and Ledoux [13].…”