Alkynylpalladium complex

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

The shift towards the monoaminocarbonylation, obtained under these conditions, as compared with the dialkoxycarbonylation observed in alcoholic solvents, is due to the basicity of the amine, which favors the formation of an alkynylpalladium complex (Scheme 37). This species then inserts carbon monoxide with formation of an alkynoylpalladium intermediate, which undergoes nucleophilic displacement by R2NH to give the final 2-ynamide. 

The phenylacetylene formed would then re-enter the catalytic cycle for the second arylation. Cul is believed to assist the formation of the alkynylpalladium complexes. [Pd(PPh3)4] (103) and [PdCl2(diphos)] in the presence of Cul also catalyze the reaction, which requires only normal temperature and acetylene pressure. 

Although catalytic amounts of Ag20 have been found to promote the palladium-catalyzed coupling of aryl boronic acids and terminal alkynes, the authors in this case do not attribute the desired reactivity to the formation, and subsequent transmetalla-tion, of a silver acetylide. Rather, it is proposed that the Ag20 activates the alkynylpalladium complex to allow transmetallation from the boronic acid, and that any competing formation of the silver acetylide results in a homocoupling of the alkynes (Scheme 1.55).123... 

No appreciable development had been noted in the palladium-catalyzed polymerization of isocyanide until Takashi s group reported a new palladium initiator for the polymerization of aryl isocyanides [32-34]. They tested a series of alkynylpalladium complexes, including mononuclear palladium acetylides 21, ( -ethynediyl) dipalladium complexes 22, and Pd-Pt heteronu-clear -ethynediyl complexes 23, as initiators for the polymerization of isocyanides (Fig. 1) [34]. Although the mononuclear acetylides 21 only under-... 

C.ii.a. Alkyl-, Benzyl-, Aryl-, Alkenyl-, and Alkynylpalladium Complexes. Alkyl- and benzylpalladium complexes (Table 8) can be obtained by a variety of synthetic methods. 

Aryl-, alkenyl-, and alkynylpalladium complexes (Table 9) are generated in manners similar to the preparation of alkyl- and benzylpalladium derivatives. However, oxidative addition is much more widely applicable and satisfactory than in the preparation of alkyl-palladium derivatives, since aryl, alkenyl, and alkynyl halides are not only much more reactive toward Pd(0) complexes than their alkyl counterparts but are also uncomplicated by j8-elimination. 

These polymers are synthesized in ways similar to the preparation of alkynylpalladium complexes from Pd(II) species and alkynylmetals, such as alkynyltins and alkynylcoppers (Scheme Concentrated solutions of these polymers have been found to form ly-... 

The mechanism of this cyclization can be rationalized in terms of the formation of TT-alkynylpalladium complexes and intramolecular nucleophilic attack by amides. The trifluoromethyl group must effectively attenuate the nucleophilicity of the nitrogen base. 

Alkynylpalladium complexes [Pd(C=CPh)(Ac -PP2)][BF4] and [Pd(C=CPh)(/ -PP2)][OAc] with the tridentate phosphine ligand (PPh2 = PhP(GH2GH2PPh2)2) were obtained from [Pd(PP2)][03SGF3]2 and from [Pd(OAc)2] and PP2 with phenyl acetylene, respectively, Several stable alkynylpalladium complexes [Pd(PNO)(G=GR)] (R = Ph, Bu ) 18-22 have been obtained. NMR and an X-ray crystal structure of 19 confirmed the coordination of the PNO ligand and the position of the alkynyl group trans to the imine nitrogen. " ... 

The mechanism of the palladium-catalyzed non-oxidative heterocoupling of alkynes was examined recently." NMR studies revealed oxidative addition of haloalkynes to [Pd L2] via a [Pd(r7 -XG=GPh)L2] complex 23. The observed drastic acceleration of the oxidative addition by added GuX and the m-configuration of the resulting [PdX(G=GPh)L2] 24 led to the suggestion of intermediate heterobimetallic complexes 25. Intermediates underway to a dialkynylpalladium species from 24 were not supported by NMR it could proceed through transmetallation by alkynylcopper(i) species, or via complex 26." An alternative mechanism involves transmetallation between two alkynylpalladium complexes m-[PdH(G=GR )L2] and m-[PdX(G=GR )L2], to provide the dialkynylpalladium complex [Pd(G=GR )(G=GR )L2] from which reductive elimination takes place." " ... 

Other reactions that presumably involve alkynylpalladium complexes are insertions of alkynes into the Si-Si bond of 3,4-benzo-l,2-disilacyclobut-3-ene to give 5,6-benzo-l,4-disilahexa-2,5-dienes, and activation of unstrained G-G bonds by / -alkynyl elimination in the palladium-catalyzed reaction of propargylic alcohols with ethyl acrylate to give enynes. ... 

Gascade procedures and Sonogashira conditions have been used to obtain dendrimer building blocks like 27-30 and metallodendrimers. Luminescent heterodimetallic rhenium-alkynylpalladium complexes 31 and 32 were obtained through the use of m situ prepared alkynylsilver(i) species that reacted with common palladium precursors with GuGl/Et3N as the co-catalyst. Gomplex 31 was authenticated by an X-ray crystal structure. " ... 

Novel routes to oligomeric alkynylpalladium complexes have also been devised according to Scheme 7. In this instance, the procedure has been particularly followed for the synthesis of oligomers like 36a based on diethynylthio-phene. The novel route gives less of the bimetallic byproduct 36b compared to the dehydrohalogenation... 

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