Alkenes double-bond shift

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) ... 

Similar to the base-catalyzed double-bond shift of alkenes, the question of intra-molecularity arises in the propargylic rearrangement as well. Intramolecularity was... 

Structures in which a methyl group is held over an alkene double bond pose a special problem. Rapid rotation of the methyl group results in the NMR chemical shift representing the average environment of the three protons. Both GIAO and the... 

The four-electron system including an alkene Jt-bond and an allylic C-H o-bond can participate in a pericyclic reaction in which the double bond shifts and new C-H and C-C o-bonds are formed. This allylic system reacts similarly to a diene in a Diels-Alder Reaction, while in this case the other partner is called an enophile, analogous to the dienophile in the Diels-Alder. The Alder-Ene Reaction requires higher temperatures because of the higher... 

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... 

Some double-bond shifts and isomerizations have been observed previous to the PKR. Sometimes 1,6 enynes have reacted partially as 1,7 enynes, or 1,8-enynes have isomerized to 1,7-enynes prior to the PKR [59,60,133]. In some intermolecular examples strained alkenes have isomerized totally before the cyclization giving unexpected products. An example, in the synthesis of triquinanes like 120, depicted in Scheme 33, the starting alkene 118 was isomerized to 119 prior to the reaction with 117 [134,135]. 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

Bands due to carbon-carbon stretching may appear at about 1500 and 1600 cm for aromatic bonds, at 1650 enr for double bonds (shifted to about 1600 cm i by conjugation), and at 2100 cm for triple bonds. These bands, however, are often unreliable. (They may disappear entirely for fairly symmetrically substituted alkynes and alkenes, because the vibrations do not cause the change in dipole moment that is essential for infrared absorption.) More generally useful bands are due to the various carbon-hydrogen vibrations. 

While in propargylic systems a vinylogous reduction is the normal reactivity, in allylic systems a double bond shift on reduction is realized more generally only when terminal alkenes are produced. Especially Interesting in this respect is the Pd-catalyzed hydrogenolysis of allylic acetates (X = OAc) with ammonium formate (equation 45). ... 

The formation of the sultone (160) probably involves addition of the complex across the alkene double bond, a 1,2-hydride shift and an intramolecular nucleophilic substitution reaction. The sultone (161) is formed by addition of sulfur trioxide to give the unstable p-sultone which rearranges to the more stable y-isomer (161). Another useful route to sultones is by metallation of alkanesulfonate esters for example, butane-1,3-dimethylsulfonate (162), prepared from butanel,3-diol, yields the 8-sultone, namely 6-methyl-l,2-oxathiin-2,2-dioxide (163) (Scheme 67). 

Isomerization of Alkenes. - Solid superbasic catalysts show remarkable activity in the isomerization of alkenes. At room temperature in the liquid phase the double-bond shift in pent-l-ene and hex-l-ene, and the isomerization of the initially formed 2-alkenes takes place. The highest activity in both types of isomerization is shown by catalysts that have been prepared by deposition of alkali metal by evaporation on MgO calcined below 973 K. In this case the isomerization proceeds on very strong superbasic centres. On MgO-alkali metal calcined at temperatures above 973 K, one-electron donor centres prevail this system causes double bond shift only (Table 7). The superbasic systems are also active in the isomerization of ( )-pent-2-ene to the mixture of (Z)-pent-2-ene and pent-l-ene. The systems for which one-electron donor character prevails are completely inactive in this transformation. 

Complexes X, XI and XlVa mimic the steps of the ring-opening polymerization of five-membered cyclic alkenes and account for the reversibility of the polymerization of cyclopentene. In the structure of compound XlVb the double bond shifts so that, once more a five-membered ring is formed. 

The ene reaction is a concerted reaction in which addition of an alkene and an electrophilic olefin occurs with transfer of a hydrogen to the electrophile and with a double-bond shift. For example ... 

Rose Bengal xanthene dye photosensitizer, 277 Ruthenium, dihydrotetrakis(triphenylphosphine)- double bond shift in alkenes, 270 Ruthenium(2 +), chiral binap complexes asym. hydrogenation with, 102-103, 325-326 Ruthenium(8 +) oxide oxidation with of alcohols to ketones (catalytic), 267 of alkynes to 1,2-diones (catalytic), 117, 132 of ethers to esters, 118, 134-135... 

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