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- Ruthenium-based olefin metathesis catalysts with monodentate unsymmetrical NHC ligands
- Olefin metathesis
- ChemInform Abstract: A Novel Class of Ruthenium Catalysts for Olefin Metathesis.
A novel class of quinone boronic esters was synthesized by utilizing a highly regioselective benzannulation of Fischer carbene complexes with 1-alkynylboronates Equation Read full chapter. The simplest form of metathesis is cross-metathesis , involving the intermolecular reaction of two alkene units in the presence of a metathesis catalyst.
Cross-metathesis of two dissimilar monosubstituted alkenes, the most common situation in cross-metathesis, is represented by the general reaction equation in Scheme 7. Scheme 7. Hypothetical reaction products from cross-metathesis of dissimilar alkenes. In the general equation for alkene cross-metathesis depicted in Scheme 7 , the major competing process is homodimerization metathesis of the reactant alkenes i. The first examples of successful cross-metatheses involved the reaction of one alkene with a large excess of another alkene to circumvent this side-reaction.
The reactions depicted in Scheme 8 represent two extreme cases. Throughout this chapter metathesis reactions will not have the catalyst specified; unless otherwise noted all of the metathesis reactions are initiated by one of the carbene complex catalysts in Figure 1. As more examples of this reaction began to accumulate, a better understanding of the scope and limitations and general predictability of cross-metathesis reactions developed.
An observation of numerous cross-metathesis processes has led to a predictive model for success in cross-metathesis reactions. This model predicts that successful cross-metathesis will occur under the following scenarios: i the cross-metathesis of two type-one alkenes will require a large excess of one of the partners; ii selective cross-metathesis can be achieved using type-one alkenes with either type-two or type-three alkenes, since the type-one homodimers can engage in secondary cross-metathesis reactions—type-three alkenes became useful only with the advent of more reactive catalysts e.
The various types of alkenes can be summarized as: type-one—monosubstituted alkenes that are not hindered or do not contain strongly electron-withdrawing groups; type-two—hindered monosubstituted alkenes or unhindered electron-deficient alkenes; type-three—1,1-disubstituted alkenes, unhindered trisubstituted alkenes, vinyl sulfones, and vinyl phosphonates; and type-four—vinyl nitro compounds and sterically hindered alkenes. Cross-metathesis typically affords mixtures of E - and Z -alkenes. In general, the highest selectivity for E -alkenes is observed with the most reactive metathesis catalysts.
This has been attributed to secondary metathesis of the product alkenes and the greater reactivity of metathesis catalysts with Z -alkenes.
Scheme 9. Reprentative examples of cross-metathesis and metathesis homodimerization. In addition to cross-metathesis for making small molecules, the cross-metathesis of 1, n -dienes can lead to polymeric materials Scheme Scheme Russell N. Grimes, in Carboranes Third Edition , Formation of exo -polyhedral rings from suitably functionalized C -alkenyl- o -carboranes has been demonstrated. Cyclization of o -bromophenyl derivatives of o -carborane obtained via base-promoted reactions of o -bromoalkynes with decaborane afford benzocarboranes containing biphenyl and p -terphenyl units fused to the carborane cage, as in and [ ].
In these compounds, the exo-polyhedral ring system is rigidly coplanar with the carborane carbon atoms. Benzocarborane derivatives can also be obtained via nickel-mediated cycloaddition of 1,2-dehydro- o -carborane carboryne with alkenes or alkynes [ , , , , review ]. Reactions of with alkenes afford zirconacyclopentanes, i. Vinyl- o -carboranyl acetic acid undergoes cyclization to form on treatment with sodamide in liquid ammonia [ ].
The second-generation Hoveyda—Grubbs catalyst 0. The escape of ethylene side product was enabled by non-hermetically sealing of the milling jars. Metathesis products were isolated by catalyst removal by milling with aqueous l -Cys or disodium EDTA, followed by ethyl acetate or acetone wash and filtration. Reactions conducted with a solid auxiliary were first washed with water to remove the salt additive. Scheme 2.
Olefin cross-metathesis. Table 2. Olefin Cross-Metathesis a. Mechanochemical ruthenium-catalyzed olefin metathesis. J Am Chem Soc ;—9.
Copyright , American Chemical Society. Ring-closing metathesis RCM using ruthenium catalyst also afforded cyclopentene and dihydropyrroles in high yields Scheme 2. Synthesis of dinitro dihydropyrrole product is particularly notable, as an earlier synthesis gave only a 3.
Ruthenium-based olefin metathesis catalysts with monodentate unsymmetrical NHC ligands
Further studies showed that RCM of an ionic reactant BF 3 was not achieved by neat-milling or by LAG conditions, except when highly polar propylene carbonate was added. Ring olefin cross-metathesis. Ring Olefin Cross-Metathesis a. Naval Bajwa, Michael P. Jennings, in Strategies and Tactics in Organic Synthesis , Our initial approach toward the synthesis of aigialomycin D centered on a cross-metathesis CM reaction. As shown in Scheme 1 , the most evident disconnection would be the formation of the macrolide ester 4 via a Yamaguchi macrolactonization.
Thus, the acyclic precursor 7 could be formed by a chelation-controlled addition utilizing the MOM acetal as a directing group.
A key disconnection is made in going from 8 to 9 and 10 corresponding to the CM reaction using Grubbs' second generation catalyst. We envisioned that compound 10 could be prepared from the commercially available 5,7-dihydroxy-2,2-dimethyl-1,3-benzodioxanone. Scheme 1. First retrosynthetic analysis of aigialomycin D. We initially anticipated that the CM reaction between two olefinic moieties 9 and 10 using Grubbs' second generation catalyst would result in the formation of 8 , thereby constructing the desired trans -olefin Scheme 1.
The synthesis of styrene 10 started from commercially available 5,7-dihydroxy-2,2-dimethyl-4H-1,3-benzodioxinone Synthesis of the aromatic segment With the completion of olefin coupling partner 10 , we turned our attention to the synthesis of the aliphatic alkene 9.
We envisaged that the completion of the CM partner 9 could be accomplished from commercially available R -glycidol as shown in Scheme 3. Having substrates 9 and 10 in hand, we initiated our investigation into the CM reaction. Scheme 3. Attempted CM between 9 and 10 with At first glance, the CM between 9 and 10 appeared to be fairly straightforward, as the aliphatic olefin should be classified as a type I and the styrene portion as a type II alkene.
There are numerous reports in the literature of type I and II olefins selectively cross-metathesizing to afford a single product . Thus, we initially anticipated very little problems with the coupling. Upon further thought, we proposed that the Ru-catalyst initially inserted into the styrene olefin to provide compound 17 , even though one might assume that insertion into the aliphatic alkene would be favored. The first was that the ortho -carbonyl exerts an influence on the Ru catalyst with respect to chemoselective insertion within an unsymmetrical diene, when lone electron pairs on the carbonyl oxygen are free and available.
According to the authors, the mechanism of the reaction that occurs only in the presence of oxygen, involves a pericyclic reaction followed by an irreversible oxidation step, and, finally, a rearomatization. Figure 3: C—H insertion product 2. To avoid the C—H activation of aryl-substituted NHC ligands the corresponding ortho positions have to be substituted by different groups. Indeed, almost contemporaneously, Grubbs et al. Scheme 1: RCM of diethyl diallylmalonate 7.
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Complex 5a showed a higher initiation rate with respect to GII-SIMes , but eventually was found to be less efficient due to a decrease in its catalytic activity related to concomitant decomposition. Finally, 6a as well as the phosphine-free 6b showed to be very poor olefin metathesis catalysts. Possibly the presence of a Ru—F interaction is responsible for the positive impact on the reaction rates .
Scheme 2: RCM of diethyl allylmethallylmalonate 9. Scheme 3: RCM of diethyl dimethallylmalonate Scheme 4: CM of allylbenzene 13 with cis -1,4-diacetoxybutene Scheme 5: ROMP of 1,5-cyclooctadiene In the attempt to rationalize the catalytic performances of this family of N -fluorophenyl complexes the related [Rh CO 2 Cl NHC ] complexes were synthesized. Unfortunately the shifts of the CO stretching frequencies showed that no correlation between the catalytic performances of Ru-catalysts and electronic properties of the corresponding NHC ligand is found.
Figure 5: Grubbs 18a — 21a and Hoveyda-type 18b — 21b catalysts bearing uNHCs with a hexafluoroisopropylalkox This catalyst, bearing a mesityl and a helicene as the aryl groups, was preliminary examined in some model asymmetric metathesis transformations and showed promising levels of enantioselectivity.
Further studies on the development of this new concept for enantioinduction are still ongoing . Mol et al. However, no beneficial effect on the catalytic activity was observed. Indeed complex 22 revealed a very poor olefin metathesis catalyst, likely as a consequence of the excessive steric hindrance of the adamantyl moiety at the ruthenium center.
Complexes 24 and 25 were found to exist in solution as a single rotational isomer having the benzylidene moiety located under the mesityl group, and for complexes 24b and 25b this orientation was observed also in the solid state. Table 1: Examples of metathesis reactions performed with catalysts 24a and 24b.
Ledoux, Verpoort et al. The catalytic performances of these complexes and of complex 24a were evaluated for the RCM of diethyl diallylmalonate 7 and the ROMP of cis -1,5-cyclooctadiene Indeed, an increase in the size of the alkyl group resulted in a lower catalyst activity. Indeed, complex 24a bearing the small methyl moiety on the nitrogen, revealed as the best performing catalyst, even surpassing the parent complex GII-SIMes.
Catalyst 32 , having a bulky N - tert -butyl substituent on the NHC, displayed a considerably lower activity than the other tested catalysts. The replacement of the mesityl group by a 2,6-diisopropylphenyl group as in complexes 24a and 33 led preferentially to bis NHC -coordinated complexes, which showed metathesis activity only at elevated temperatures . These results underline that steric differences in N -alkyl NHC ligands are more important than differences in their donor capacities in determining the activity and selectivity of the corresponding catalysts.
Quite recently, on the basis of a previous work, Verpoort et al. For all of the complexes, two rotamers were observed in solution, and the most abundant species was identified as the isomer with the indenylidene moiety located under the mesityl group. Solid-state structures of the complexes showed, consistently, the same relative orientation between the indenylidene and mesityl unit.
Complexes 41 — 43 were tested in various representative metathesis reactions of standard substrates and compared to the benchmark catalysts IndII-SIMes. Indeed, besides its faster initiation, complex 41 offers a less encumbered NHC for the approach of substrates to the metal center during the metathesis process. Although the benzylidene complex GII-SIMes exhibited a faster initiation than the indenylidene complex 41 with all the used catalyst loadings, the latter outperformed GII-SIMes in the overall catalyst efficiency, especially at the lowest catalyst loading of 0.
In , Blechert and Buchmeiser et al. Figure Grubbs-type complex 44 and its monopyridine derivative 45 containing a chiral uNHC.
Scheme 6: Alternating copolymerization of 46 with 47 and The selectivity in the copolymerization was mainly ascribed to the steric interaction between the 2-phenylethyl substituent at the nitrogen and the growing polymer chain. Figure Pyridine-containing complexes 49 — 52 and Grubbs-type complex Complexes 49 and 52 were obtained as monopyridine adducts, while complexes 50 and 51 were obtained as a mixture of mono- and bis pyridine adducts.
ChemInform Abstract: A Novel Class of Ruthenium Catalysts for Olefin Metathesis.
In terms of initiation efficiency, the pyridine-derivatives turned out to be more efficient than the corresponding phosphine-containing complexes. However, the molecular mass of the copolymers was far lower than the theoretical value, suggesting that competitive chain-termination reactions occur. The pronounced steric bulk on the pentiptycenyl side of the NHC ligand compared to the other less hindered side determines two differently accessible active sites around the metal and different rates of monomer incorporation, thus dominating the selectivity in the formation of alternating copolymers.