Dienes, from allylic halides coupling

The first reported radical reaction promoted by tellurium reagent was probably the conversion of allylic halides into the coupled 1,5-dienes by treatment with telluride anions. The reaction, which gives the best results when employing the reagent prepared in situ from elemental tellurium and lithium triethylborohydride, proceeds through the intermediacy of the thermally unstable bis-allylic telluride followed by extrusion of tellurium and coupling of the formed allylic radicals. 

In another method for the coupling of two different allylic groups,1346 a carbanion derived from a p.-y-unsaturated thioether couples with an allylic halide.1347 The product contains an SPh group that must be removed (with Li in ethylamine) to give the 1,5-diene, but this... 

The complex formed on addition of cuprous iodide to a solution of a lithium dialkylamide in ether or tetrahydrofuran is effective in the reductive coupling of allylic halides to give 1,5-dienes with preservation of stereochemistry. This method has been used5 for the stereospecific synthesis of all-trans-squalene and (E,Z,Z,E) squalene from (E,E)- and (Z,JE)-farnesyl bromides, respectively. In an attempted synthesis of (3S)-squalene-2,3-epoxide, 4-[(4R)-2,2,5,5-tetramethyl-l,3-dioxolan-4-yl]butan-2-one (1) and the phosphonium iodide (2) were prepared.6 Unfortu-... 

Reductive coupling of allylic halides. This cobalt complex (1 equiv.) effects reductive coupling of allylic halides to form 1,5-dienes with preservation of the geometry of the double bonds/ The major product from coupling of terpenoid allylic halides is that formed by head-to-head coupling. The triphenylphosphine liberated during the reaction is removed as methyltriphenylphosphonium iodide, obtained by reaction with methyl... 

Coupling of allylic compounds. Cross-coupling of alkenylstannanes with allylic carbonates provides 1,4-dienes. Allyltriethylgermane is obtained from Et,GeSnBuj and allylic halides. ... 

Alkenyl-aluminium or -zinc species cross-couple with allylic halides or derivatives of allylic alcohols (c.g. acetates or phosphates) under the influence of Pd(PPh3)4 to give 1,4-dienes. In favourable examples coupling occurs with complete retention of stereo- and regio-chemistry of both alkenyl and allyl substrates, and it is free from formation of homocoupled products (Scheme 73). However, partial allylic inversion was observed in reactions involving E-crotyl... 

When 1,3-diene is the radical acceptor, a three-component coupling reaction of alkyl halide, 1,3-diene, and trimethylsilylmethyl-magnesium chloride takes place to afford homoallylsilane The final step of the three-component coupling is reductive elimination from allyl(trimethylsilylmethyl)cobalt that predominates over conceivable /3-hydride elimination. 

This palladium-catalyzed three-component coupling reaction leading to the formation of aryl-substituted allylic amines was recently adapted to solid-phase synthesis (Scheme 8.23). Amines were chosen to attach to a solid support (Rink resin) in this three-component coupling process and were reacted with a variety of aryl halides and linear or cyclic non-conjugated dienes, the reaction being carried out at 100 °C for two days in the presence of palladium acetate and diisopropylethyl-amine. A wide variety of aryl-substituted allylic amines were then obtained after cleavage from the solid support by trifluoroacetic acid [60],... 

Allylic zinc reagents undergo cross-coupling reactions with reactive halides leading to 1,5-dienes. Usually the new carbon-carbon bond is formed from the more-substituted end of the allylic system (Scheme 9-11) [30]. 

Even simple alkenes, such as 1-hexene 9.270, can yield ir-allyl complexes on reaction with vinyl halides under Heck conditions (Scheme 9.75). This is only apparent when a nucleophile is present, such as a secondary amine or a malonate anion. The ir-allyl complex 9.279 is generated by isomerization of the corresponding T -complex 9.278 that is itself generated by a -hydride elimination-reinsertion sequence from the initial insertion product 9.275. Nucleophilic attack on the Tr-allyl complex gives the three component coupling product 9.273, while the Heck product, diene 9.272, can arise from dissociation from t -complex 9.276 or p-hydride elimination from the -intermediate 9.278. Again, the reaction is particularly effective in an intramolecular sense. °°... 

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