1,3-Allylic migration

FIGURE 44. Schematic representation of the transition states of the degenerate 1,3-allyl migration of EH3 (E = C to Sn) with inversion (TSinv) and retention (TSret) of the EH3 group. Reprinted with permission from Reference 163. Copyright 1997 American Chemical Society... 

McComsey, D. F., Maryanoff, B. E. 3-Aza-Cope Rearrangement of Quaternary N-Allyl Enammonium Salts. Stereospecific 1,3 Allyl Migration from Nitrogen to Carbon on a Tricyclic Template. J. Org. Chem. 2000, 65,4938-4943. 

The term sigmatropic rearrangement was introduced by Woodward and Hoffmann to describe reactions in which a bond flanked by one or two n systems migrates to another position along the n system(s). A classic example would be a 1,3-allylic migration. 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

There are exceptions to this trend, as shown in the indole series [234]. The reason for this might be that the allyl migration to C-5 would forfeit all the aromatic stabilization. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

Some benzyl migrations have been observed in simple enamines,3,149 e.g., (144) (145) -y (146) allyl migrations have also been observed,3 e.g., (147) (148). 

The electrophilic nature of sulfuryl chloride is validated in a study on the chlorination of anthracene derivatives the observed substituent effects conform well with molecular orbital modeling859. Chlorination of ethyl 3,4,5-trialkylpyrrole-2-carboxylates results in a substantial amount of / -side-chain chlorinated products in addition to the expected a-side-chain chlorination860. This has been ascribed to the electrophilic nature of the reagent861 and the mechanism proposed involves an allylic migration of the halogen from the a-sigma adduct to the adjacent / -side chain. 

The Claiaen rearrangement in nitrogen heterocyclic systems has been carried out in many different solvents. The ease of allylic migration appears to vary considerably among the different systems. The solvent media commonly employed are diethylaniline, 1-methyl-naphthalene, and AjA-diethyl-m-toluidine (b.p., 231°). In other instances, where the allyl ethers are liquids, the rearrangement has also been effected without any solvent. Under these conditions, the results are not always the same as those observed with a solvent. 

The only low temperature concerted processes observed were of the jS(C-H) ester type eliminations to give olefin and acid (path a). Not unexpectedly, a Cope-type displacement of CO2 (path b) with allyl migration to form the alkyl allyl ether was not observed. At high temperatures the allyl carbonates without -hydrogens decomposed by simple bond fission free radical pathways. 

Oxidation of a terminal double bond affords a primary alcohol with allylic migration of the double bond ... 

Alternatively, oxidation of the sulfide to the sulfoxide followed by desilylation without allylic migration using CsF and sulfoxide pyrolysis gave 50, having the zygosporin G ring system in 63% yield. 

Various complexes of Fe , Ru ", Co , Rh ", Ir , Ni , and Pt have been proposed as catalysts for such allylic migrations. All of them, of course, lead to racemic product. In a recent... 

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