Metathesis species from catalyst decomposition can lead to unwanted events such as double bond isomerization. Several general solutions for residual ruthenium and other metals removal were established as seen in Figure 4. Application of ammonium-tagged complexes allows the removal of residual ruthenium by silica gel work-up, water extraction or simple precipitation.
Reactivation of a Ruthenium-Based Olefin Metathesis Catalyst.
In most cases, ruthenium levels in metathesis products were reduced from over to below 5 ppm. Another approach that surpasses ruthenium contamination is catalyst immobilization. To obtain more stable solid-supported catalysts, ammonium tagged complexes having bulkier NHC ligands i. Metal scavengers represent a more general method for metals removal. Their use does not require structural modifications of the catalyst, allowing application of one scavenger with a wide variety of commercially available catalysts. SnatchCat has multiple advantages: a quick, irreversible, and quantitative binding of ruthenium also efficient with palladium b effective at small stoichiometric excess with respect to the catalyst; c easy removal of both the metal — scavenger complex and unbound scavenger; d compatible with broad range of functional groups and solvents; e easy to handle - stable, non-toxic, non-volatile, odor-free.
Figure 4 Solution for efficient ruthenium removal. Continuous development in the field of olefin metathesis is reflected in an increasing number of IP protected catalysts designed for specific processes. For related literature on Metathesis please see the links below:. Apeiron Metathesis Catalyst Kit.
Selected Ruthenium Metathesis Catalysts. Metathesis Catalysts and Ligands. The Strem Chemiker — Vol. Log In Register. References: a T. Nicola, M. Brenner, K. Donsbach, P. Kreye, Org. Process Res. Farina, C.
Shu, X. Zeng, X. Wei, Z. Han, N. Yee, C. Senanayake, Org. Boehringer Ingelheim Int. Tracz, M. Matczak, K. Skowerski, Beilstein J. Table 3: AROCM of cis norbornene- endo -2,3-dicarboxylic anhydride 75 with catalysts 61 , 64 , 65 , 67 and Exchanging the chloride ligands for iodide ligands led to catalysts 80 and 82 with superior latent behavior that allowed for the latent ROMP of norbornene derivatives e.
ICTM - Olefin Metathesis
The unsaturated indenylidene catalysts 85 and 86 were found to be more active than their saturated homologues, giving full conversions within 6 h and 24 h, respectively, thus showing better performances than IndII-IMes and Hoveyda-type catalysts 87 — 89 , As for the latter ones, the introduction of unsaturated NHCs with an N -cycloalkyl moiety did not provide any beneficial effect, since they were less efficient also than their symmetrical IMes and SIMes counterparts.
Scheme Self metathesis of 13 with catalyst Table 4: Self metathesis of 1-octene Indeed, the one-step multicomponent synthesis of unsaturated unsymmetrical NHCs could provide a cost-effective alternative to the multistep synthesis of their saturated counterparts . The basic idea was that these catalysts might be able to regenerate themselves upon consumption of the monomer in the reaction media. All the complexes were able to achieve the cyclization, although the catalytic activity of the homologous series 98 — was found to be strongly dependent on the tether length between the alkene group and the metal center.
WO2012032131A1 - Novel olefin metathesis catalysts - Google Patents
Importantly, later on Grubbs and co-workers utilized this kind of catalysts, featuring a chelating N-to-Ru arm, for the preparation of cyclic polymers from cyclic monomers via a ring-expansion metathesis polymerization REMP process [38,39]. In comparison to its homogeneously soluble analogues and , complex required longer reaction times to give the same yields, but was reusable up to three times. The cis isomers — exhibited catalytic activity only at high temperatures, where they likely reassume the trans form which is characteristic for the Grubbs-type ruthenium carbene complexes.
Figure Complexes and bearing a carbohydrate-based NHC.
These complexes were characterized in solution by NMR techniques which revealed, at room temperature, the presence of rotameric species resulting from rotation about the Ru—C benzylidene bond. Given the chiral nature of the carbohydrate attached to the NHC, complexes and were tested in the AROCM of a variety of norbornene derivatives with styrene. While isolated yields were generally excellent, enantiomeric excesses were poor.
The effect of a dangling amine tether incorporated into the NHC ligand on the catalytic efficiency of ruthenium benzylidene complexes was examined by Fryzuk et al. Figure Complexes and bearing a hemilabile amino-tethered NHC. NMR studies showed that complex exists as a mixture of two rotational isomers in a ratio. The major isomer was characterized by X-ray crystallography, while the minor isomer was characterized only in solution and was identified as consistent with two possible structures syn- and anti - Moreover, no coordination of the tethered amine to the ruthenium center was detected in the species by NMR spectroscopy.
The catalyst efficiency is further reduced in the pyridine derivative , suggesting that the pendant amine is deleterious for catalyst performance. The catalytic behavior of — was investigated in standard metathesis reactions using commercial grade solvents in air and compared to that of commercially available IndII-SIMes.
In more detail, , , and exhibited similar behavior, in spite of the different nature of aryl substituents, while was found to be less efficient.
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Solvent tests on IndII-SIMes , and demonstrated that dichloromethane is a better solvent with respect to toluene, even if in toluene the initiation of catalyst is faster. The low activity of , and was rationalized by supposing the presence of an interaction between the metal and the heteroatoms of the benzyl substituents [15,43,44]. On the contrary, they appeared not suitable in the synthesis of tetrasubstituted olefins. The catalysts and were also tested in the ring-closing ene—yne metathesis reaction RCEYM of standard substrate Table 5: Metathesis reactions of standard substrates.
Scheme Diastereoselective ring-rearrangement metathesis dRRM of cyclopentene Analogous results were observed in the RCM of more crowded substrates. The similar behavior of and indicated that steric effects are more relevant than electronic effects. According to the experimental results, mesityl-bearing catalysts generally gave better yields than Dipp-containing analogues. Self metathesis of 1-octene 96 was conducted in the presence of , , and , in order to selectively obtain tetradecene Under these conditions all the catalysts tested showed very high activity in RCM transformations, with the newly developed systems requiring shorter reaction times to give quantitative conversion.
In the RCEYM of , complexes , and were performing less effectively than all the other ones, however, no clear relationship between heterocyclic substituents and activity can be found.
The synthesis of indenylidene and Hoveyda-type complexes bearing N -phenylpyrrole and N -phenylindole moieties on their NHCs was also attempted . These two systems were tested in standard RCM and CM reactions and complex with a perbrominated N -phenylpyrrole moiety revealed as more stable and active than its parent catalyst Computational studies suggested that the rarely occurring phenyl—ruthenium intramolecular interactions are responsible for lower stability and slower reaction initiation.
With the goal to develop chemoselective catalysts, ruthenium complexes containing unsymmetrical N -trifluoromethyl NHCs were introduced by Togni et al. The presence of one N -trifluoromethyl substituent was supposed to impart positive effects on the catalytic performance, influencing both electronic and steric properties of the NHC ligand. Indeed, as already underlined, in symmetrical NHC ruthenium complexes with fluorinated N -aryl groups previously reported by Grubbs, a Ru—F interaction was considered as responsible for the observed enhanced metathesis activity .
X-ray crystallographic analysis of complexes , and showed a Ru—F interaction in the solid state. Scheme Ethenolysis of ethyl oleate It is worth to underline that this transformation mediated by ruthenium initiators is less well investigated, presumably as a consequence of the high activity of ruthenium catalysts toward the competitive ROMP that is leading to low yields of terminal dienes.
Scheme Ethenolysis of cis -cyclooctene Due to its superior activity, complex was also investigated in the ethenolysis of more challenging substrates such as norbornene derivatives, which typically are among the most popular ROMP monomers because of their high ring strain. In order to explain the selectivity observed in the ethenolysis of cyclic olefins, steric and electronic descriptors of the NHC ligands obtained computationally were evaluated.
Substitution at the backbone positions of the NHC framework has represented a remarkable advancement in the design of ruthenium olefin metathesis catalysts, due to the significant effects exerted on complexes' stability, reactivity and selectivity .
The first example of C 1 -symmetric ruthenium catalyst bearing a backbone-substituted N -alkyl, N' -aryl NHC ligand was reported by Collins et al. Figure Possible syn and anti rotational isomers of catalyst Scheme ARCM of substrates , and Table 6: ARCM of prochiral trienes , , and promoted by catalysts and It should be underlined that the ambiguous halide influence constitutes a relevant difference between and In fact, for the latter, the employment of halide additives had always a beneficial effect on the enantioselectivity .
The product ring size dependence observed in the desymmetrization of , and with was explained considering that an NHC rotation is possible during the catalytic cycle and that , and should have different relative rates of cyclization. If the cyclization is slow, for instance in the case of seven-membered ring alkenes, an NHC rotation could occur during the catalytic cycle, thus determining a decrease of the enantiomeric excesses. Surprisingly, despite such rotation the reactivity profiles and the enantioselectivities observed for and in the desymmetrization of and were comparable.