Palladium-coordinated alkyne

Many domino Mizoroki-Heck reactions start with the formation of vinyl palladium species, which are generally formed by an oxidative addition of vinylic halides or triflates to palladium(O). S uch an intermediate can also be obtained from an addition of a nucleophile to a divalent palladium-coordinated alkyne or allene. In most of these cases some oxidant must be added to regenerate palladium(II) from palladium(O) in order to achieve a catalytic cycle. However, Liu and Ln [155] have successfully applied a protonolysis reaction of the C—Pd bond formed in the presence of excess halide ions to quench the C—Pd bond with regeneration of a palladinm(II) species. In this way, reaction of 306 and acrolein in the presence of Pd(OAc)2 and LiBr gave predominantly 307 (Scheme 8.76). Depending on the snbstitution pattern and reaction conditions, 308 was formed as a side prodnct. 

Hydroarylations of alkynes are catalyzed by gold complexes and these bear some resemblance to the Fujiwara Pd-catalyzed reaction. In general, when using gold chemistry, better Z/E selectivities are observed compared with palladium, lower catalyst loadings and milder conditions (neutral not TFA) are used. The mechanism involves the attack of ArH on the Au-coordinated alkyne. Flowever, electron-poor acetylenes only appear to work with palladium chemistry (Equations (75) and (76)).72... 

Examples of palladium- and rhodium-catalyzed hydroaminations of alkynes are shown in Equations 16.90-16.92 and Table 16.9. The reaction in Equation 16.90 is one of many examples of intramolecular hydroaminations to form indoles that are catalyzed by palladium complexes. The reaction in Equation 16.91 shows earlier versions of this transformation to form pyrroles by the intramolecular hydroamination of amino-substituted propargyl alcohols. More recently, intramolecular hydroaminations of alkynes catalyzed by complexes of rhodium and iridium containing nitrogen donor ligands have been reported, and intermolecular hydroaminations of terminal alkynes at room temperature catalyzed by the combination of a cationic rhodium precursor and tricyclohexylphosphine are known. The latter reaction forms the Markovnikov addition product, as shown in Equation 16.92 and Table 16.9. These reactions catalyzed by rhodium and iridium complexes are presumed to occur by nucleophilic attack on a coordinated alkyne. 

Nucleophilic attack of coordinated alkynes is another synthetic approach toward alkenylpalladium compounds. This step plays a role in several palladium-catalyzed addition reactions. The palladium-catalyzed cyclization of 6-aminohex-l-yne gives rise to putative ammonium-alkenylpalladium(ii) complexes. ... 

X-ray crystal structures of many new palladium 77 -alkyne complexes have been determined Pd(0) complexes are mainly trigonal planar showing in-plane coordination of the alkyne. Pd(ll) derivatives exhibit out-of-plane 77 -alkynes, rotated to an angle that is much smaller than 90° so that a perpendicular arrangement is never reached a sum of factors, not only electronic, may play a role in deciding the actual angle adopted, as mentioned above. 

Model stoichiometric reactions of [PdCH3(CO)(Pr DAB)] [B 3,5-(CF3)2C6H3 4] (Pr DAB = l,4-diisopropyl-l,4-diaza-l,3-butadiene) with alkynes and carbon monoxide have been investigated by NMR spectroscopy and DFT studies to identify the putative intermediates involved in the cydocarbonylation of alkynes [47]. Addition of but-2-yne (R = CH3) or 1-phenylpropyne (R= Ph) results in regioseledive insertion into the Pd-acyl bond to afford a five-membered palladacycle 54 that undergoes rapid cydocarbonylation at low temperature to afford a palladium-coordinated, q -allylic lactone 55. The a,f -unsaturated y-lactone could be liberated either by proton abstraction with a stoichiometric amount of Na[BEt3H] or by nucleophilic addition... 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

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