Allyl fragmentation reaction

Evidently, this interesting reaction starts with coordination of Pd by the allylic fragment at the site of attachment of one of the acyloxy groups. Then the anion of the nitro compound is involved in C,C-coupling with olefin (18) followed by successive ionization of the nitro fragment. Finally, Pd is mediated in 0,C-coupling... 

Generally speaking, since the a-carbon of a substituted allylic fragment is a stereogenic center, chirality may be transferred to the carbonyl compounds. Thus, very high diaste-reofacial selectivity has been obtained in the reaction of 32 with isopropyl methyl ketone due to a rigid transition state (Scheme 13.26) [54]. 

Oxidative addition involving carbon-to-oxygen bonds is of relevance to the catalysis with palladium complexes. The most reactive carbon-oxygen bond is that between allylic fragments and carboxylates. The reaction starts with a palladium zero complex and the product is a ir-allylic palladium(II) carboxylate Figure 2.16. 

Markovic and Hartwig isolated and characterized the first intermediate in iridium-catalyzed allylic substitution [100]. They isolated the metalacyclic iridium-phosphor-amidite fragment containing COD and the olefinic portion ofN- l -phenylallyl)aniline, the product of the allylic substitution reaction between cinnamyl carbonate and aniline (5 in Scheme 22). This complex containing the product of allylic substitution was first detected by NMR spectroscopy during catalytic reactions. It was then isolated, prepared independently, and shown to be chemically and kinetically competent to be an intermediate in allylic substitutions. 

Except for one recent example, all iridium-catalyzed allylic substitution reactions have been performed under an inert atmosphere with dry solvent and reagents. The iridium metalacycle is sensitive to protonation, which opens the metalacycle and results in the formation of a less-active complex containing a K -phosphoramidite ligand. A paper by Helmchen et al. addressed this issue [107]. Nearly all iridium catalysts used for allylic substitution consist of an iridium fragment chelated by COD. In the presence of a catalyst containing dibenzo[a,c]cyclooctatetraene (dbcot) in place of COD, allylic substimtion reactions occur in air with results that are comparable to those of reactions performed under an inert atmosphere (Scheme 35). 

A multi-stage synthesis of azocino[4,5,6-cd]indoles has been suggested (03M13519). From 4-bromoindole (84) with the help of successive transformations (among them the Vilsmeier-Haack reaction, Henry nitroaldole condensation, lithium aluminum hydride reduction and insertion of an allyl fragment), indole 85 has been produced in 18% yield. The cyclization of the latter on palladium... 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

SCHEME 50. Preparation and uses of allylic zinc reagents by fragmentation reaction... 

Studies with model compounds indicate that thiazole-accelerated cures tend to favor reactions at allylic positions. This has been explained (68MI11503) in terms of the structure of the polysulfidic intermediates which allows facile rearrangement and fragmentation reactions to occur (35). 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

A variety of other fragmentation reactions are potentially useful in the formation of carbon-carbon bonds, although none has yet shown the generality of the allyl- and vinyl-stannanes.146 In addition to the use of allyl sulfides discussed above (Scheme 41), allylcobalts have also been employed.147 Extensive... 

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