Halopalladation alkynes

Most of the useful reactions initiated by halopalladation are concentrated on the alkynes. Halopalladation of alkynes will give the vinylpaUadium complexes, which are versatile reactive intermediates in a number of catalytic reactions. " Another way of generating a vinylpaUadium complex is by the oxidative addition of a vinyl halide or tri-flate to paUadium(0).f f While the latter approach has had many applications in catalytic reactions,the application of the former one is stiU Umited. 

In the literature, many works reported that alkynes seem to undergo both cis- and trans-chloropalladation.f The stereochemistry of halopalladation of alkynes was recently studied in detail by Backvall et al. The chloropalladation of a few substituted alkynes has been studied under different conditions. 

The results are in accordance with those obtained from the trapping with allyl chloride.f Halopalladation of alkynes having a heteroatom at suitable position will change the stereochemistry of halopalladation due to the coordination of the heteroatom to the palladiumf [ i [ i (Scheme 3). 

B.iv. Halopalladation-Initiated Codimerization of Alkynes and Allyl Halides... 

As discussed in Sect. V.3.5, halide ions can also add to the palladium 7r-complexes, and the process may be termed halopalladation. Addition of Pd and a halogen, such as Cl and Br, to olefins, acetylenes, allenes, and conjugated dienes has been reported. However, most of the currently useful reactions initiated by halopalladation involve the use of alkynes as the substrates. Halopalladation of aUcynes gives vinylpalladium intermediates, which can undergo intramolecular carbopalladation to form cyclized products. Pd-catalyzed cyclization of ally lie aUcynoates can produce 7r-alkylidene-y-butyrolactones, which represent a basic structural unit in a wide variety of biologically active natural products. 

Huang et al. described paUadium/copper-cocatalyzed cascade halopalladation/ decarboxylation/carbon-carbon fonning reactions for the synthesis of 5-halo-6-substituted benzo[it]naphtho[2,l-d]furans 267 [95] (Scheme 6.71). The protocol represents the first example of trapping the a-vinylpalladium intermediate, generated from halopalladation of alkynes, by the decarboxylative coupling reaction. 

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