- Olefin Metathesis [Catalysis]
- Mechanism of Ring Closing Metathesis
- Pure and Applied Chemistry
- Mechanism of Olefin Metathesis
- Olefin Metathesis - Chemistry LibreTexts
The reduction product 4 may arise either directly from 3 or from its isomerized counterpart 6 vide infra. Interestingly, we have previously reported that, under conditions similar to Entry 4 but using the Grubbs catalyst 1 , a nearly quantitative yield of 4 was obtained, without formation of any silylated compound [ 31 ]. Though olefin reduction with Et 3 SiH activated by Grubbs carbenes is a well-known catalytic process, its mechanism is still not very clear [ 2 , 9 , 39 , 40 ]. Cossy et al. Lastly, B or C would suffer an elimination reaction regenerating the ruthenium carbene.
In our case, hexaethyldisiloxane was detected in the final crude mixture. Formation of disiloxanes was also reported by Cossy et al. Silanes have weak Si-H bonds and formation of hexaethyldisiloxane can be explained by an oxidation of Et 3 SiH [ 41 ]. It is known that alkyldisiloxanes can be obtained by air oxidation from Et 3 SiH in the presence of Lewis acids [ 42 ]. Nevertheless, reaction was unaffected by addition of a radical inhibitor such as di- tert -butyl nitroxide DTBN , consequently a radical mechanism was ruled out.
Formation of the other reaction product, the silylated compound 5 , may occur through the following sequential steps: i isomerization of the internal olefin 3 into the terminal olefin 6 ; ii direct silylation of the latter to the corresponding vinylsilane 7 ; iii finally, a second isomerization of 7 to the allylsilane 5 Scheme 2.
An allylic mechanism, by intermediacy of allyl-Ru complexes, might also be considered in the generation of 5 from the internal 3 and terminal olefin 6 when the disilane Et 3 SiSiEt 3 would be present in the reacting system Scheme 3. As mentioned in the Introduction, solid-phase reagents are gaining popularity in synthesis due to some comparative advantages especially useful for flow chemistry.
Particularly, to the best of our knowledge, silanes immobilized on solid phase have not been applied to non-metathetic hydrogenations. Using high-loading Merrifield resin, we prepared 4- diethylsilylbutyl polystyrene resin 8 in two steps according to literature [ 44 ] Table 2. Upon reacting benzyl trans -crotonate 3 with 8 in presence of catalyst 1 or 2 , a different outcome has been attained.
Concerned are both the nature of the products and a more intricate pattern of the reactions routes, this time also involving metathesis Table 2. Only low conversions to the reduced product 4 could be attained in refluxing dichloromethane, even at long reaction times and with excess of silane. Table 2 , Entries 1 and 2. Switching to microwave heating greatly improved conversion. Under best conditions Entry 3 , approximately one-quarter of the starting material 3 was still present after reaction.
Whenever conversion to 4 has reached higher values Entries 3 and 4 , formation of a secondary product 11 was apparent from the additional signal observed in the NMR spectrum of the crude reaction mixture. Catalysis by 1 Entry 5 resulted in lesser production of 4 , while also the secondary product 11 could be evidenced by NMR.
At this point some conclusions became obvious: i always approximately one-quarter of the starting material does not react; ii catalyst 2 is more active than 1 , in agreement with its stability under the rather harsh conditions high temperature employed; iii MW-assisted heating leads to better yields than refluxing in dichloromethane; iv additional silane has no positive effect. By virtue of the starting benzyl crotonate 3 not being entirely consumed in all experiments with resin-supported silane 8 , a reasonable assumption would be that crotonate 3 partially binds to the resin.
To verify this hypothesis, a reaction was carried out Table 2 , Entry 6 using only about one-third 0. After 0. According to this result, the resin-bound compound was proposed to have the structure 10 indicating the immobilization of the benzyl crotonate Figure 2. Confirmation came from the further treatment of 10 with Et 3 SiH and catalyst 2 that allowed the catalytic cycle to continue, affording fresh product 4 Table 2 , Entry 7; Scheme 4. Compound 11 was unequivocally deduced by NMR spectroscopy to be 1,5-dibenzyl glutarate.
We assume that diester 11 arises by several successive transformations. First, isomerization species, formed in situ from the catalyst, trigger double-bond migration in 3 [ 23 , 45 , 46 ] to give the terminal olefin 6 and a trans - cis isomerization to olefin 12 [ 23 ] Scheme 6. Then, cross-metathesis between olefin 3 or 12 and 6 , promoted by the ruthenium alkylidenes originated from 1 or 2 , leads to 13 trans and cis.
Finally, saturated diester 11 is formed by hydrogenation of 13 , under activation by short-lived Ru-hydride species arising from the catalyst.
Olefin Metathesis [Catalysis]
These species transfer hydrogen atoms from the immobilized silane moieties to the C—C double bond. Formation of 13 was not detected under homogenous phase. This could be attributed to a faster reduction under triethylsilane, as compared with supported silane 8 , and therefore reduction of 3 prevents any further cross-metathesis reaction. Intriguingly however, homometathesis of 3 Scheme 7 , path b seems not to intervene, likely because of steric reasons internal double bond.
Indeed, product E that would have been formed through path b and c has not been detected among the reaction products. In this intricate catalytic process, the metathesis catalysts 1 or 2 generate prevailingly Ru species for isomerization and hydrogenation, leaving, however, some of the initial catalyst to promote cross-metathesis of 3 or 12 with 6. The validity of this interpretation is confirmed by the preponderance of compound 4 among the reaction products.
Inasmuch as olefin 3 has a disubstituted double bond, we next chose, for comparison, the mono-substituted olefin 14 acetyleugenol as starting material. Interestingly, on reacting 14 with triethylsilane in solution under the usual conditions, the isomerized compound 15 was obtained as the only product Scheme 8. Anew, this result gives evidence that double-bond migration from the terminal to the internal olefin is taking place [ 9 ] as already seen for isomerization of 3 to 6.
A plausible mechanism for olefin isomerization, as shown in Scheme 9 , involves a cyclic pathway, through the intermediacy of species II—IV, similarly to an earlier proposal by Schmidt [ 4 ]. Additional results have been obtained for reduction with resin 8 , of a further substrate, the dimethyl itaconate 16 bearing two carbonyl groups and a 1,1-disubstituted olefin Scheme This result supports our assumption that in the catalytic cycle, a C—Si bond analogous to 10 is formed between the silane and Consequently, in case of immobilized silanes, some of the starting material ends up anchored to the solid phase, causing a decrease in the yield.
Chemical reagents were purchased from commercial sources and were used without further purification unless noted otherwise. Solvents were analytical grade or were purified by standard procedures prior to use. Resins were purchased from Aldrich.
Reactions requiring inert atmosphere were carried out under a high-purity dry nitrogen atmosphere. Solvents from these reactions were transferred with syringe under high-purity dry nitrogen pressure. Spectra were run according to the literature [ 47 ]. Detection of the ions was performed in electrospray ionization, positive ion mode. ST: Statistical, selectivity achieved by excessive amount of one substrate 10 equiv.
Mechanism of Ring Closing Metathesis
S: Selective, selectivity achieved by intrinsic thermodynamics. Reactivity Matrix for Cross Metathesis. Model of E:Z unclear. Intrinsic cross product selectivity remains problematic. For complex molecules, unpredictable. Die Makromolekulare Chemie. Grela, S.
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Harutyunyan, A. Michrowska, Angew. Forbes et al. Journal of the American Chemical Society. Pulin Wang from Prof. Burke Group.
Wendy Jen from Prof. MacMillan Group. Catalysts used for this reaction  [5a-g]: The earliest heterogenous catalytic system is composed of high-valent transition metal halide, oxide and oxo-halide. Alkyl zinc or alkyl aluminum is used as the co-catalyst. Also, aluminum oxide or silica is added as support. Characteristics: Good Reactivity Extremely poor functional group tolerance Why? Because it is a lewis acid, it will be poisoned Complicated and ambiguous catalytic mechanisms 2.
Characteristics: Lower functional group tolerance Air and water sensitive Multi-substituted and hindered substrates accessible Why high reactivity?
Pure and Applied Chemistry
Because the coordinative and electronic unsaturation along with the bulky ligands make them good electrophilic agents and reduce the bimolecular decomposition 4. Categories of Olefin Metathesis: 1. Cross Metathesis The transalkylidenation of two terminal alkenes with release of ethene is catalyzed by the Grubbs catalyst. Mechanism  : 2. Mechanism: 3. Ring-Opening Polymerization Metathesis ROMP [11a-c] Ru based catalysts can open the strained ring with a second alkene via the cross-metathesis mechanism to form products containing terminal vinyl groups. Mechanism [12a] : 4.
Enyne Metathesis [12b-c] Alkyne and alkene can have similar reaction to produce 1,3-diene, and this intermolecular process is called cross-enyne metathesis, whereas the intramolecular reactions are referred to as ring-closing anyone metathesis RCEYM. Detailed mechanistic studies of Grubbs group catalysts in Cross Metathesis Classification of Olefins: Type I:Fast homodimerization Olefins homodimerize rapidly and both the homodimers and monomers can enter the catalytic CM cycle. Type II:Slow homodimerization Slow olefin homodimerization and the homodimers can be only partly consumed in latter CM catalytic reactions.
Mechanism of Olefin Metathesis
Styrene Secondary allylic alcohols Increasing Electron Deficiency. Quaternary allylic carbon-containing olefins Hint: Some reactants are called bridge type olefins and the reactivity depends on the substituent pattern and catalyst choice s. The chelating oxygen atom replaces a phosphine ligand, which in the case of the 2nd generation catalyst, gives a completely phosphine-free structure.
The 1st generation Hoveyda—Grubbs catalyst was reported in by Amir H. Hoveyda 's group,  and in the following year, the second-generation Hoveyda—Grubbs catalyst was described in nearly simultaneous publications by the Blechert  and Hoveyda  laboratories. Siegfried Blechert 's name is not commonly included in the eponymous catalyst name. The Hoveyda—Grubbs catalysts, while more expensive and slower to initiate than the Grubbs catalyst from which they are derived, are popular because of their improved stability.
Hoveyda—Grubbs catalysts are easily formed from the corresponding Grubbs catalyst by the addition of the chelating ligand and the use of a phosphine scavenger like copper I chloride : . The second-generation Hoveyda—Grubbs catalysts can also be prepared from the 1st generation Hoveyda—Grubbs catalyst by the addition of the NHC: .
In one study a water-soluble Grubbs catalyst is prepared by attaching a polyethylene glycol chain to the imidazolidine group.
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The rate of the Grubbs catalyst can be altered by replacing the phosphine ligand with more labile pyridine ligands. By using 3-bromopyridine the initiation rate is increased more than a millionfold. The principle application of the fast-initiating catalysts is as initiators for ring opening metathesis polymerisation ROMP.
Because of their usefulness in ROMP these catalysts are sometimes referred to as the 3rd generation Grubbs catalysts. Grubbs catalysts are of interest for olefin metathesis. It is mainly applied to fine chemical synthesis. Large-scale commercial applications of olefin metathesis almost always employ heterogeneous catalysts or ill-defined systems based on ruthenium trichloride.
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Olefin Metathesis - Chemistry LibreTexts
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