Allenyl complexes

Homometallic and Heterometallic Transition Metal Allenyl Complexes Synthesis,... 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

An interesting sequence, again overall an isomerization, is the stoichiometric formation of the manganese complexes 68, which, on basic alumina, isomerize to the allenyl complexes 69 from the latter the allenes 70 can be liberated with cerium(IV) ammonium nitrate (CAN) in good yields [128] (Scheme 1.30). 

Doherty, Simon, Corrigan, John P., Carty, Arthur J., and Sappa, Enrico, Homometallic and Heterometallic Transition Metal Allenyl Complexes Synthesis, Structure, and Reactivity. 

A cycloaddition process between the Rh=C bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 71-aUyl-allenyl complexes 81 (Scheme 28), the initially formed metaUacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMe3) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

Diene-A-ynes. Propargylic carbonates in reactions catalyzed by Pd(0) can react as allenyl complexes. Thus carbonylation of 2-alkynyl carbonates catalyzed by Pd(0) results in 2,3-dienylcarboxylates as the major or only product (equation I). Similarly, coupling of 2-alkynyl carbonates with terminal acetylenes in the presence of Cul and LiCl and catalyzed by Pd(0) provides l,2-diene-4-ynes in 60-83% yield (equation II). 

Although the [2+2] cycloaddition continues to dominate the methodology for the synthesis of boracyclobutene and metallacyclobutene complexes, conceptually new and potentially general alternatives have recently been introduced. In particular, the central carbon alkylation of electrophilic propargyl and allenyl complexes has significantly enriched the palette of available metallacyclobutenes, raising considerable promise for the development of new reactions of relevance to organic synthesis. 

Homometallic and heterometallic transition metal-allenyl complexes synthesis, structure, and reactivity ... 

Other variations of the [3 + 2] cycloaddition chemistry involving either SO2 or a S2O equivalent have been explored (Scheme 47) I53,i55,is6 allenyl complexes... 

The reactions of type III proceed by attack of nucleophile at the central sp carbon of the allenyl system of the allenyl complex 8. Reactions of soft carbon nucleophiles, derived from active methylene compounds such as )3-keto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediate 12, which is regarded as a palladium-carbene complex 13. The intermediate 12 picks up a proton from the active methylene compound and the jr-allylpalladium complex 14 is formed, which undergoes further reaction with a nucleophile as expected hence two nucleophiles are introduced to give the alkenes 15 (Scheme 11-5). 

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