Silica Immobilized Second Generation Hoveyda‐Grubbs: A Convenient, Recyclable and Storageable Heterogeneous Solid Catalyst

Abstract

Abstract:The second generation Hoveyda-Grubbs metathesis catalyst was successfully immobilized on silica in pellet and powder form following a practical and fast synthesis procedure. The activity of the solid system is truly heterogeneous, efficient in various metathesis reaction types and stable for at least 4000 TON. Ru contamination of the products was very low (ppb levels). The successful use of the robust system has been demonstrated in a continuous reactor set-up.Keywords: heterogeneous catalysis; immobilization; metathesis; N-heterocyclic carbenes; ring-opening polymerization; ruthenium Olefin metathesis has since its discovery left a mark on organic synthesis, polymer chemistry and catalyst development, making metathesis a key reaction of the last decade. The discovery of Grubbs ruthenium catalyst 1 inspired the development of a new family of metathesis catalysts (2-6, Figure 1) with high activity and remarkable tolerance towards functional groups, air and moisture. [1,2] Taking into account Ru cost and high end-product purity, straightforward catalyst separation, recyclability and minimal complex leaching become decisive criteria for widespread applications of metathesis in organic synthesis.Various immobilization strategies for metathesis catalysts have been recently reviewed.[3] Anchoring of stable ruthenium alkylidenes 3 and 4a via phosphine or halogen exchange, via the N-heterocyclic carbene ligand, or via alkylidene exchange (boomerang-type catalysts) has been performed most frequently on insoluble organic polymers and silica supports.[4] Whereas immobilization of homogeneous catalysts combines advantages of homogeneous and heterogeneous catalysis, [5] most reports on supported metathesis catalysts describe a loss of activity as a result of immobilization. Moreover, many catalysts barely survive a few recycles, while only a handful of systems guarantee fully recyclable activity. Unfortunately, despite ingenious organic syntheses, their preparation often consists of multiple steps or requires additional reagents and modified Ru complexes, consequently making them less accessible for general use. A practical, inexpensive immobilization strategy for recyclable and reusable catalysts, compatible with organic and combinatorial chemistry and with large-scale potential remains thus challenging.In our search for immobilizing metathesis catalysts, we were attracted by the recovery experiments of Hoveyda et al.[6a] and later Fogg et al.[6b] They recycled catalyst 4a and related aryl oxide catalysts simply by silica gel column chromatography. While a chromatographic recovery is time-, solvent-and materialconsuming, sometimes with metal losses of 5% Ru per cycle and more, [3c] we envisioned that the silicabound catalyst could potentially be used as a heterogeneous catalyst by playing with solvent polarity.

Valorisation of vegetable oils via metathesis reactions on solid catalysts: Cross-metathesis of methyl oleate with 1-hexene

“…All the HG/SiO 2 samples exhibited only the amorphous halo of SiO 2 support. The absence of XRD peaks due to HG crystalline structure suggests that the HG complex is highly dispersed on the silica support, as it was reported previously [5]. Fig.4shows the FTIR spectra of HG complex, SiO 2 , HG(6.0%)/SiO 2 and HG(11.6%)/SiO 2 obtained in the 500-800 cm -1 region.…”

SBA-15 Immobilized Ruthenium Carbenes as Catalysts for Ring Closing Metathesis and Ring Opening Metathesis Polymerization

“…The catalytic activity of heterogeneous catalysts prepared in this way was proved in ring closing metathesis (RCM) of diethyl diallylmalonate [19][20][21] and N,N 0 -diallyl-p-toluenesulfonamide [22]. Recently, immobilization of Hoveyda-Grubbs catalyst on silica by direct reaction of chloro ligands with silica OH groups was described [23]. The recyclable catalyst applicable in flow reactor was reported.…”

“…As supports for immobilization both organic polymers and silica material of various types have been used [6,7]. Generally, three strategies have been developed for immobilization of Ru carbenes to the support: (i) through alkylidene ligand [8][9][10][11][12][13][14][15][16], (ii) via phosphine or N-heterocyclic carbene (NHC) ligand [17,18], and (iii) through exchange of halogen ligands [19][20][21][22][23]. As Ru-O bond dissociates and alkylidene ligand is exchanged in the initiation step [24], the propagating species of catalysts prepared in way (i) are in fact not bound to the surface of the support.…”

Tailored Ru‐NHC Heterogeneous Catalysts for Alkene Metathesis

“…One possible solution would be the development of an efficient heterogeneous catalysts that is highly active (TON and TOF), stable (minimum recycling and leaching) and tolerant to functional groups. Despite numerous efforts in this area (involving permanent grafting of Ru-NHC complexes on various supports [7][8][9][10] or other immobilization strategies [11] ), heterogeneous catalysts has not fulfilled the aforementioned requirements. Recently, tailored made organic-inorganic materials have proved to be an alternative and advantageous route towards highly active and well-defined heterogeneous catalysts.…”