Palladium-catalyzed alkynyl-aryl coupling

Palladium-mediated cydization based on the reactivity of o-alkynyl or alkenyl-phenyl isonitriles have been developed [105]. On the basis of their earlier studies on the three-component synthesis of allyl aryl cyanamides [106], Yamamoto and co-workers reported a palladium-catalyzed three-component coupling reaction of 2-alkynylisocyanobenzenes 122 with allyl methyl carbonate and trimethylsilylazide leading to N-cyanoindoles 125 [107]. One of the key steps of the proposed mechanism is the formation of 7i-allylpalladium carbodiimide 123 and its isomerization to rc-allylpalladium cyanamide complex 124 (Scheme 8.50). 

The palladium-catalyzed carbonylative Negishi coupling of ort/w-disubstituted aryl iodides with an alkynyl zinc reagent was investigated by Martin and colleagues in 2008 [34]. Alkynones were produced in good yields under mild conditions (Scheme 4.42). 

Water as a green solvent has been successfully applied as a reaction medium for palladium-catalyzed carbonylative Sonogashira reactions (Scheme 5.11) [36]. Instead of using alkynes, activated acetylenes stibanes can also be applied as coupling partners in carbonylations. An example is the palladium-catalyzed carbonylative Sonogashira coupling of alkynyl stibanes with aryl iodides that was published by Kakusawa and Kurita in 2006 [37], The reaction was carried out under 1 bar of CO in DMAc using 5 mol% of Pd(OAc)2 and 20 mol% of PPhs. 

R = alkenyl, aryl, allyl, benzyl, propargyl R = alkenyl, aryl, alkynyl, alkyl, benzyl, allyl The Negishi Coupling was the first reaction that allowed the preparation of unsymmetrical biaryls in good yields. The versatile nickel- or palladium-catalyzed coupling of organozinc compounds... 

The presumed catalytic cycle for this coupling is the following Once formed from 23, the highly coordinatively unsaturated 14-electron palladium(O) complex 24 participates in an oxidative addition reaction with the aryl or vinyl halide to give the 16-electron palladium(II) complex 25. A copper(I)-catalyzed alkynylation of 25 then furnishes an aryl- or vinylalkynyl palladium(II) complex 27. Finally, a terminating reductive elimination step reveals the coupling prduct 9 and regenerates the active palladium(O) catalyst 24. 

A catalytic amount of CuCl was found to activate alkynyl(trimethyl)silanes in the palladium-catalyzed coupling reaction with aryl triflates (Eq. 7) [12]. The catalytic cycle is considered to involve the transfer of an alkynyl group from an alkynylsilane to Cu(I) and then to palladium(II). A sequential palladium-catalyzed reaction of trimethylsilylacetylene gives unsymmetrical diarylacet-ylenes (Eq. 8). 

Stille reaction the palladium-catalyzed reaction of an aryl or vinyl halide with an organotin reagent, where one of the organic groups is alkynyl, aryl, or vinyl, to give a cross coupled product... 

Palladium-catalyzed cross-coupling involving alkynylmetals and related alkynyl nucleophiles alkynyl-aryl, alkynyl-alkenyl, and alkynyl-alkynyl coupling... 

The Negishi palladium-catalyzed cross-coupling reaction of alkenyl, aryl, and alkynyl halides with unsaturated organozinc reagents provides a versatile method for preparing stereodefined arylalkenes, arylalkynes, conjugated dienes, and conjugated... 

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