Allenylpalladium species

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

Transmetallation of allenylpalladium species with indium salts has been previously reported Marshall, J. A. Grant, C. M. J. Org. Chem. 1999, 64, 8214-8219. 

The reactivity of allenylpalladium species, derived from oxidative addition of propargyl alcohol derivatives to palladium (0), has attracted considerable attention in organic synthesis.283 Recently, the intramolecular reaction of the propargylic derivatives 665 with an oxygen and nitrogen nucleophile has been widely utilized for the synthesis of heterocyclic compounds. The key step of formation of a heterocyclic ring is intramolecular nucleophilic attack of a... 

Dihydropyrans. Allenylpalladium species generated from a propargyl carbonate may react intramolecularly with a remote nucleophile. Thus, a new method for alkylidenedihydropyran synthesis is developed. 

The Pd(0)-catalyzed reactions of propargylic compounds can be understood by the following mechanistic considerations. The first step in the catalytic reactions is the oxidative addition of a propargylic compound to Pd(0) species to form an inteimediate complex. By this oxidative addition, Pd(0) is oxidized to Pd(II). The intermediate Pd(II) complex undergoes further reactions with other reactants. Complex formation by stoichiometric reactions of propargylic chlorides 2 and 4 with Pd(Ph3P)4 has been studied, and the o-allenylpalladium 3 and the propargylpalladium (or c7-prop-2-ynylpalladium) 5 were isolated as yellow powders (Scheme 11.2) [2]. The allenylpalladium chloride 3 is... 

The Pd(II) species on an allenylpalladium complex A has ample Lewis acidity to activate a triple bond, on which a suitably arranged carboxylate anion attacks intramolecularly. Reductive elimination of Pd(0) species completes the formation of exo-enol lactones (Scheme The Pd-catalyzed coupling of the propargylic acetates with 4-pentynoic acids proceeds very smoothly in the presence of potassium bromide using tris(2-furyOphosphine (TFP) as the ligand (Scheme 10). 

TT-Allylpalladium complexes have long been recognized as an allyl cation equivalent and widely utilized for the allylation of a variety of hard and stabilized carbon nucleophiles. Both allenylpalladium and vinylpalladium complexes are chemically neutral, or very feebly nucleophilic, if any. Under conditions described below, however, these rr-allyl-, vinyl-, and allenylpalladium complexes alter their reactivity in mode and behave as nucleophilic species. "... 

The proposed mechanism for this cyclocarbonylation was shown to involve the insertion of Pd(0) species into the C—O bond of the substrate followed by rearrangement to the allenylpalladium intermediate 29 (as proposed for alkyl systems). Insertion of CO and subsequent reductive elimination may lead to the 2,3-dienoic acid 30, which undergoes cyclization, catalyzed by trace quantities of an acid present in the solvent, to the 2(5//)-furanone 28 (Scheme 10) (93JOC1538). 

Furthermore, highly stereoselective dithiocarbonylation of propargylic mesylates with thiols and carbon monoxide is attained by the use of tetrakis(triphenyl-phosphine)palladium(O) as the catalyst. This dithiocarbonylation is believed to proceed via allenylpalladium and allenyl thioesters, and the high stereoselectivity may be rationalized by a mechanism where nucleophilic attack of a Pd(0) species on the... 

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