Reductive elimination allene derivatives

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

The proposed reaction mechanism is as follows (Scheme 16.83). Zinc metal reduces Ni(II) species to Ni(0). A nickelacyclopentadiene may be produced via coordination of two molecules of propiolates and regioselective head-to-head oxidative cyclometallation. Coordination and subsequent insertion of an allene into the Ni(II)-carbon bond give rise to a nickelacycloheptadiene intermediate. Finally, a benzene derivative is produced via reductive elimination followed by isomerization. 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

MH, undergo the transmetallation with 8 to generate 10 (Scheme 11-4). Subsequent reductive elimination gives the allene derivative 11 as a final product. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

Intermediate 2 can coordinate an additional alkyne (or olefin) to give 5 after insertion into the Ni-C bond (Scheme 7). Reductive elimination affords the corresponding cyclodecatriene derivative. Several related stoichiometric reactions with certain Ni complexes and alkynes or allene have been observed, thus confirming the proposed catalytic cycle (route ( )) [48] (eqs. (17) and (18)). 

The second type of reactions proceed by transmetallation of the complexes 1. MR (M = main group metals) and metal hydrides MH undergo the transmetallation with 1 to generate 6. Subsequent reductive elimination gives the allene derivative 7. Also reactions of 1 with 1-alkynes in the presence of Cul to afford allenylalkynes belong to this type. 

Alper and Xiao [45] synthesized thiochromanones by palladiumotalyzed carbonylative ring-forming reactions of 2-iodothiophenol derivatives, allenes, and CO. The fhiochroman-4-ones were achieved in good to excellent isolated yields with high regioselectivity, which was probably caused by electronic effects (Scheme 1.24). This catalytic heteroannulation comprises the regioselective addition of the sulfur moiety on the more electrophilic carbon center of the allene, arylpalladium formation, CO insertion, subsequent intramolecular cydization, and, finally, the reductive elimination. 

In 1999, Alper and Xiao developed a novel access to thiochroman-4-one derivatives by palladium-catalyzed carbonylative heteroannulation of o-iodothiophenols with allenes. The reaction afforded the thiochroman-4-ones in good to excellent isolated yields with high regioselectivity (Scheme 3.76). The catalytic heteroannulation may involve regioselective addition of the sulfur moiety of the reactants on the more positive end of the allene, atylpalladium formation, CO insertion, subsequent intramolecular cyclization, and then reductive elimination. 

In the case of thiols (RSH), the corresponding hydrothiocarbonylation takes place successfully at the terminal double bond, as shown in Scheme 11.8 [11]. A possible pathway is as follows (1) formation of an H-Pd-SR species via oxidative addition of H -S R to Pd(0) (2) coordination of the allene double bond and subsequent insertion into a Pd-H bond to form an allypalladium complex intermediate (3) CO insertion resulting in the formation of an acylapalladium carbonyl species and (4) reductive elimination affording the thioester derivates. 

---