Mechanism of double bond migration

FIGURE 2.12 Mechanism of double bond migration on a quintet site. 

Fig. 19. Mechanism of double-bond migration by intramolecular hydrogen transfer.

Scheme 9.3 Mechanism of double-bond migration and hydrogenation of butanes over a metal catalyst.

The dissociative mechanism of double bond migration via an allylic intermediate can occur at a site, as indicated in Scheme 2, or at a two atom site such as M—which can provide the three vacant coordination positions which are required to bind both the TT-allyl, a four-electron donor, and the hydrido group. Because the surface of a metal provides sites which differ in structure, both mechanisms may operate on a given catalyst, the relative importance being a function of the metal, the alkene and the reaction conditions. - ... 

Examples for the mechanisms of double bond migration or hydrogenations reactions which occur in the systems forming catalytic cycles are given in Figures 2.10 and 2.11... 

Many isomerization reactions occur by double-bond migration. Two common mechanisms of double-bond migration are (Parshall and Ittel, 1992) (1) equilibration between internal and terminal olefins by hydrogen migration and (2) interconversion between branched and linear chains of an olefin. Another mechanism of olefin isomerization is the shift of an allylic hydrogen from the 3- to the 1-position. We confine our discussion to the more important doublebond migration mechanism. 

An alternative mechanism for double bond migration has recently been proposed by Smith (Fig. 2.12).113 It is based in part on theoretical calculations,114 in part on the recent surface science work suggesting that hydrogen occupies threefold hollows,115 and in part on the experimental observation that during hydrogenation an allylic deuterium moves 1-3 across the bottom of an adsorbed allylic system without being exchanged.116... 

On the dissociative mechanism it would reasonably be expected that only hydrogen atoms which are dissociated at the relatively low temperature of experiment, i.e., those adjacent to the double bond, would exchange with deuterium. Twigg (93) found, however, that all the hydrogen atoms in propylene and butylene were exchangeable, as may be understood by consideration of the associative mechanism for double bond migration above. 

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

Double-bond migrations during hydrogenation of olefins are common and have a number of consequences (93). The extent of migration may be the key to success or failure. It is influenced importantly by the catalyst, substrate, and reaction environment. A consideration of mechanisms of olefin hydrogenation will provide a rationale for the influence of these variables. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

The hydrogenation and isomerization of alkenes can usually be described by the classical Horiuti-Polanyi mechanism. According to that mechanism, in a deuterium atmosphere, double bond migration incorporates deuterium into the allylic position. 

Cis-trans isomerization occurs either by formation of a half-hydrogenated state (Horiuti-Polanyi mechanism) followed by rotation around the newly formed single bond and abstraction of an appropriate hydrogen onto the surface or by double bond migration (either Horiuti-Polanyi or Jt-allyl) from a cis (trans) position to an adjacent trans (cis) position (deuterium exchange studies favor the rotation mechanism). 

The mechanism of dehydration of alcohols over acidic and non-acidic alumina is the same. In the presence of the acidic alumina, however, readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. 

Fio. 13. Geometrical relationships which affect the mechanism of the metal-catalyzed double bond migration in steroids. 

A similar but different mechanism has also been proposed for the intermolecular CO-SiCaC reaction of phenylacetylene catalyzed by Rh4(CO)i2, which gives silylcyclo-pentenone 9a and 9b (Scheme 7.5) [14]. In this mechanism /9-silylethenyl-[Rh] intermediate Il.lg, arising from insertion of the alkyne moiety into the Si-[Rh] bond, reacts sequentially with a second molecule of phenylacetylene and CO, to afford <5-silylpenta-dienoyl-[Rh] complex II. Ih- Finally, carbocyclization followed by double-bond migration gives 9 a and 9 b. 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

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