Double bond migrations allylic

Reactions of allylic systems that yield products m which double bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift... 

Allylic rearrangement (Section 10 2) Functional group trans formation in which double bond migration has converted one allylic structural unit to another as in... 

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 ... 

Homoallylic systems may isomerize under hydrogenation conditions to allylic systems, causing hydrogenolysis to occur when it would not have been expected (39b,45a-45c). In these cases, if hydrogenolysis is unwanted, it is best to avoid those catalysts and conditions that favor isomerization. Double-bond migration to an allylic position may occur even if the double bond is required to leave a tetrasubstituted position (26a). 

Double bond migration in vinylic and allylic sulphoxides can be achieved by using proper bases B (equation 375). 

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 ... 

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. 

Scheme 3 n-allyl shift mechanism for double bond migration... 

Ir(cod)(PhCN)(PPh3)]C104 (i) Double bond migration of allylic alcohols 715... 

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... 

Double bond migration occurs either by the Jt-allyl mechanism (abstraction-addition) or by the Horiuti-Polanyi mechanism (addition-abstraction). Pd is thought to favor Jt-allyl and Pt Horiuti-Polanyi mechanisms. 

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). 

Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, allene formation was observed. 

Other Pd cross-coupling reactions such as Heck [52] and Suzuki [53] reactions have also been used for macrocyclizations. The main drawback for Pd catalyzed macrocylization is the yield, that is often somewhat disappointing if compared with other established methods. Also, the introduction of the required coupling components (e.g., trialkyltin group, vinylic iodide) can be difficult in some compounds. In other cases, Pd-catalyzed side reactions such as double bond migration or allylic activation can occur. 

Dimerization of methyl crotonate has been carried out with alkaline earth metal oxides as basic catalysts 15). The reaction proceeds by Michael addition, which is initiated by abstraction of an allylic hydrogen of methyl crotonate by the basic site to form the allylic carbanion, which attacks a second methyl crotonate molecule at the jS-position to form a methyl diester of 3-methyl-2-vinylglutaric acid. The diester undergoes a double bond migration to form the final E- and Z- isomers of 3-ethylidene-3-methylglutaric acid dimethyl ester (MEG) (Scheme 22). 

D. Mechanism of ene reaction. In the ene reaction, the double bond is shifted to the allyl position with respect to the starting material according to the scheme proposed by Schenck. This involves (i) attachment-of the oxygen molecule to one of the carbon atoms of the double bond (e.g. CJ, (ii) shift of the double bond to allyl position (e.g. to C -> Cj) and (iii) migration of the allyl hydrogen atom to the terminus of the peroxy group. 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Another possible mechanism interprets cis—trans isomerization in combination with double-bond migration assuming the participation of jr-allyl intermediates54 55 (Scheme 11.2). Such dissociatively adsorbed species were originally suggested by Farkas and Farkas.56... 

Since, in the 7r-allyl complex, free rotation is not possible, cis—trans isomerisation can only occur through 7r-allyl if it is preceded by double bond migration, as shown in Fig. 10. 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

---