Bond Shift

Several reaction mechanisms have been proposed for bond shift isomerization. Although each of them accounts for some specific peculiarity of the 

In order to examine the possible participation of adsorbed cyclopropanes in the bond shift mechanism, the relative contributions of Paths A and B in chain lengthening were determined for a series of 2-methylalkanes (40, 43, 54). The contribution of Path B was found to decrease regularly from isopentane to 2-methylpentane and 2-methylhexane. The decreasing contribution of Path B from isopentane to 2-methylpentane is readily explained by the decreasing number of methyl substituents in the cyclopropane intermediate, but the difference between 2-methylpentane and 2-methylhexane cannot be accounted for by the cyclopropane mechanism (43). 

In an investigation of the contact reactions of highly strained cage compounds, Rooney and his co-workers (64,65) found that bicyclo(3,3,2)octane, protoadamantane, adamantene dimer, and other similar compounds were isomerized on platinum- and palladium-supported catalysts at relatively low temperatures (I50°-250°C on platinum, 2OO°-3OO°C on palladium). The ease 

Although the Rooney-Samman mechanism nicely explains the peculiarities of the isomerization of caged molecules on platinum and palladium catalysts, its extension to simpler molecules is not completely obvious. As a matter of fact, none of the above mechanisms accounts in its original form for the different behaviors of platinum and palladium in skeletal rearrangements. 

As noted above, while isobutane is isomerized on both platinum and palladium, platinum but not palladium catalyzes the isomerization of neopentane to isopentane (3 ).  

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Olivier L, Poupko R, Zimmermann FI and Luz Z 1996 Bond shift tautomerism of bibullvalenyl in solution and in the solid-state—a C-13 NMR-study J. Phys. Chem. 100 17 995-18 003... 

If a double-bond shift occurs, the number of aldehyde isomers is increased. 

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. 

Especially sensitive towards acid is the ketodiazirine (200). It decomposes on contact with O.IN acid, forming methylenecyclopentanone (213) and cyclopentanecarboxylic acid (214). The products may be formed from a developing vinyl cation (212) by C—C bond shift either to the double bond (213) or across the double bond (214) (B-67MI50800). 

These two energies are, respectively, comparable to the experimental activation energies for conformation inversion of the tub conformer and bond shifting, suggesting that the two planar structures represent the transition states for those processes. 

As seen in the previous section, if two identical knees of the (L,0)-(L, L ) family are connected together symmetrically with respect to a connecting plane, and if this connecting process is continued while maintaining the knees in a common plane, the structure obtained will close to a torus which will be completed after 10 fractional turns (Figs 7 and 9), However, if a rotational bond shift is introduced... 

Fig. 10. (a) Planar representation of a single rotational bond shift at the (9,0) to (9,0) connection of two (9,0)-(5,5) knees. This leads to a 27r/9 rotation out of the upper knee plane (b) single bond shift at the (5,5) to (5,5) connection of two (9,0)-(5,5) knees. This leads to a InjS rotation, out of the lower knee plane. The arrow indicates the location of the bond shift. 

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... 

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

Electrophilic Substitution Accompanied by Double-Bond Shifts... 

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) ... 

H/ C One-Bond Shift Correlations by Hetero-COSY (HETCOR) Experiment... 

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