And double bond shifts

With some catalyst systems, selectivity to primary metathesis products is near 100%, but side reactions (double-bond migration, dimerization, cyclopropanation, polymerization) often reduce selectivity. Such side reactions, such as oligomerization and double-bond shift over oxide catalysts, may be eliminated by treatment with alkali and alkaline-earth metal ions.26... 

Trifiro and Carra [323] used the amount of cis—trans isomerization and double bond shift as a method of investigating the type of intermediate. It is concluded that three groups of oxides exist. The first is a group of which Bi—Mo—O, Bi2W06 and Fe—Te—Mo—O are typical. These catalysts give only isomerization at temperatures at which selective oxidation also occurs, probably via the same intermediate (allylic). A second group gives isomerization at much lower temperatures (Sn—Sb—O, Fe—Sb—O, for... 

On solid acid—base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 

Typical acid-catalyzed reactions like the dehydration of alcohols and double bond shifts in olefins have been mentioned occasionally as reactions catalyzed by organic heterogeneous catalysts. An extensive kinetic study of the dehydration of tertiary butyl alcohol over pyrolized polyacrylonitrile has been describ-... 

Both cis-trans isomerization and double bond shift were incomplete under these conditions, and the product obtained from thiophene contained an excess of 1-butene over the thermodynamic equilibrium amount. 

On the other hand, H2S did not prevent cis-trans isomerization and double bond shift reactions from going to completion at 300° or 350°, and the olefin mixtures analyzed were always found to be close to thermodynamic equilibrium, whether they had been formed from thiophene, butadiene, or one of the olefins themselves. Nor did H2S prevent the total conversion of butadiene to butene, even when 10- to 20-cc. samples were used at reaction temperatures down to 200° C. and flow rates up to 10 liters per hour. This may be the explanation of the absence of butadiene from the thiophene reaction products over cobalt molybdate—that if it had been formed as it was over chromia, it would have reacted further too rapidly to survive. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

The proposed mechanism involves die carbometalation of an alkynylstannane with an a-stannylketone, protonation of fhe resultant gem-bisstannylated intermediate, and double bond shift to an allylmetal species. In contrast, the GaCl3-mediated addition of SEE to trimefhylsilylefhyne forms /i. j -unsaturated ketones, vinylation products, without isomerization to u,/I-urisaluraled ketones [272]. A similar mechanism via a gem-bismetalated intermediate formed by carbometalation has been postulated for fhe GaCh-mediated vinylation. 

Garavelli, M., Bernardi, F., Moliner, V., Olivucci, M., Intrinsically Competitive Photoinduced Polycyclization and Double bond Shift through a Boatlike Conical Intersection, Angew. Chem. Int. Ed. 2001, 40, 1466 1468. 

FT-Olefin-synthesis Secondary Olefin-hydrogenation Secondary Olefin-isomerization Sites of secondary hydrogenation and double bond shift... 

Fig. 4-29. The role of Schiff base in nonenzymic transamination. 1 pyridoxal + amino acid in presence of catalyst (metal or enzyme) yields a Schiff base. 2 Hydrogen and double-bond shifts lead to the formation of an isomeric Schiff base. 2 The base splits and leads to the formation of the keto acid and pyridoxamine. 4 The reaction is reversible. In the reversible step, the amino group of pyridoxamine is transferred to the keto acceptor...

Nickel(ll) complexes with chelating phosphines are effective catalysts for the oligomerization of ethylene and double bond shift isomerization of olefins, with high specificity for dimeric compounds rich in Unear isomers [221,222]. 

In a process analogous to the hydration of alkenes, water can be added to alkynes in a Markovnikov sense to give alcohols—in this case enols, in which the hydroxy group is attached to a double-bond carbon. As mentioned in Section 12-16, enols spontaneously rearrange to the isomeric carbonyl compounds. This process, called tautomerism, interconverts two isomers by simultaneous proton and double-bond shifts. The enol is said to tautomerize to the carbonyl compound, and the two species are called tautomers (tauto, Greek, the same mews, Greek, part). We shall look at tautomerism more closely in Chapter 18 when we investigate the behavior of carbonyl compounds. Hydration followed by tautomerism converts alkynes into ketones. The reaction is catalyzed by Hg(II) ions. 

The sequence begins with formation of an imine from the amino acid and the oxidized form of the vitamin, pyridoxal. This imine converts to an isomer via tautomerism simultaneous proton and double-bond shift (See Sections 13-7 and 13-8). Hydrolysis of the new imine gives pyridoxamine and the ketoacid. Depending on the body s metabolic needs, the pyridoxaniine thus formed may proceed to react with other ketoacids to produce required amino acids (the scheme shown above run in reverse), or it may serve to facilitate the ultimate disposal of the nitrogen by excretion. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

A peculiar dehydrofluorination occurs when tnfluoromethyl dihydropyndine derivatives are treated with organic bases A double-bond shift and a hydrogen migration convert one tnfluoromethyl group to a difluoromethyl group and aromatize the ring [22] (equation 20)... 

Enamines in which the double-bond shift is sterically prevented afford only the ammonium salts. Their spectra in the C=C stretching vibration region does not differ greatly from that of the free amine spectrum (171). For example, neostrychnine (159) has vc c 1666 cm and its perchlorate at 1665 cm . Salts of quinuclideine (92) and the polycyclic alkaloid trimethylconkurchine have similar properties. 

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

Slow double-bond shifts and little skeletal isomerization H-transfer is minor and nonselective for tertiary olefins only small amounts of aromatics formed from aliphatics at 932°F (500°C)... 

Rapid double-bond shifts, extensive skeletal isomerization, H-transfer is major and selective for tertiary olefins large amounts of aromatics formed from aliphatics at 932°F (50t) O... 

The niobium atom has a slightly distorted octahedral coordination. Interatomic distances between the niobium atom and the two oxygen atoms in trans positions, O-Nb-O are 1.81 and 2.14 A. The niobium atom is shifted from the base plane of the octahedron by 0.23 A, and this shift, in adjacent chains, is in opposite directions. Pakhomov and Kaidalova [204] concluded that the shorter Nb-O bond (1.81 A) is an intermediate between a single and double bond. 

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

Propargylic acetates, halides, and sulfonates usually react with a double-bond shift to give allenes.34 Some direct substitution product can be formed as well. A high ratio of allenic product is usually found with CH3Cu-LiBr-MgBrI, which is prepared by addition of methylmagnesium bromide to a 1 1 LiBr-Cul mixture.35... 

The Stille coupling reaction is very versatile with respect to the functionality that can be carried in both the halide and the tin reagent. Groups such as ester, nitrile, nitro, cyano, and formyl can be present, which permits applications involving masked functionality. For example, when the coupling reaction is applied to l-alkoxy-2-butenylstannanes, the double-bond shift leads to a vinyl ether that can be hydrolyzed to an aldehyde. 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

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