Halopalladation

The allylic 2-alkynoates 77 are a group of special enynes with an ester linkage between their double bond and triple bond. When halopalladation of the triple bond is followed by C-C double bond insertion and the cleavage of the carbon-palladium bond, a series of y-lactones (78-81) can be obtained. 

Lu X (2002) Palladium-Catalyzed Reaction via Halopalladation of jr-Compounds. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 2267... 

Stereoselective intramolecular carbopalladation of ( )-4-halo-2-butenyl 2-propynoates 17 with bis(benzonitrile)dihalopalladium catalysts gives (Z)-3-(halomethylene)dihydro-4-vinyl-2(3//)-furanones 18 in excellent yields and high stereoselectivities44,45,56,57. Although resembling metallo-ene reactions (see Section 1.5.8.3.7.), this conversion is interpreted as proceeding via a primary tram- halopalladation of the triple bond. 

The halopalladation of MCPs gives CP-Pd and CPC-Pd intermediates depending on the reaction conditions. Thus, the isomerization of alkylidene cyclopropyl ketones to 4ff-pyran derivatives takes place in the presence of a palladium chloride catalyst via chloropalladation to form a CPC-Pd and the successive P-carbon elimination (Eq. 10) [25]. In contrast, the addition of Nal changes the reaction pathway dramatically. Under the conditions, the reaction proceeds through a CP-Pd intermediate and results in the formation of furan derivatives. 

Utilization of heteropalladation (halopalladation ox5q>alladation amino-palladation acetyloxypalladation) of unsaturated hydrocarbons in organic synthesis, particularly, in s5mthesis of heterocycles 07CJO819. 

D.iv. Halopalladation, Oxypalladation, Aminopalladation, and Other Heteropalladation Reactions... 

Halopalladation is a reaction in which a halide ion, as a nncleophile, adds to the palladium complexed unsaturated compounds. Chloropalladation of unsaturated organic substrates is an important reaction in organopalladium chemistry. Additions of Pd—Cl to allenes, " olefins,acetylenes, " and conjugated dienes have been reported. While a lot of works concerning the halopalladation reactions of unsaturated substrates were studied, only a few papers related to the catalytic reactions initiated by halopalladation of organic substrates were reported until recently. 

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.ii. Halopalladation-Initiated Carbonylative Cyclization of Yndiols to /8-Chloro-y-butenolides... 

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

For the halopalladation reaction of 7-methylenebicyclic[2.2.1]hept-2-enes with the complexes PdX2(PhCN)2 (X = Cl, Br) using both solvents of low polarity (CH2CI2 and CHCI3) and methanol, the products result from tran -addition of Pd—to the exocyclic double bond (Scheme 35). 

E. SYNTHESIS OF NATURAL PRODUCTS VIA HALOPALLADATION OF PALLADIUM tr-COMPLEXES... 

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. 

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