Palladium-vinyl bond

As described above for the palladium(0)-catalyzed reactions, carbon-carbon bonds can be obtained by insertion of an olefin into a palladium-vinyl bond (vinylpalladation). This approach has been applied in palladium(II)-catalyzed exchange reactions of olefins by generating the vinylpalladium species from chloropalladation of an acetylene [115,116]. This technique for generating vinylpalladium was later applied to the palladium-catalyzed 1,4-oxidation of conjugated dienes [117]. Thus, the use of substrate 91 in a palladium(II)-catalyzed oxidation in the presence of LiCl afforded 94 in 65% yield [Eq.(49)]. The... 

An interesting Pd(0)-catalyzed tandem cyclization of aIkyn-A-(2,4-dienyl)amides 7 was recently reported by Xie and Ln (Scheme An arylpalladinm species, formed via oxidative addition of Phi to Pd(0), adds to the acetylene to give a vinylpalladium intermediate 8. Insertion of the coordinated donble bond of the diene into the palladium-vinyl bond to give a rr-allyl complex followed by amine attack wonld acconnt for the product 9. 

Mechanism. The insertion of acetylene into a palladium-halogen bond occurs as the first step and subsequently allyl halide inserts into a palladium-vinyl bond. The /3-elimination of PdXj gives a codimer and regenerates the active palladium catalyst. In the case of unsubstituted acetylene, the cotrimer is formed by the successive insertion of acetylene and allyl halide into the palladium-vinyl bond (Scheme 10). 

Carbene Insertion into the Palladium-Vinyl Bond... 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

In perfluorinated trialkylethenes the single vinylically bonded fluorine is hydrogenolyzed with hydrogen over palladium though only partially, but with complete regioselectivity.35 In contrast, in perfluorinated tetraalkylethenes allylic fluorine is eliminated while the hydrogen atom attacks the double-bonded carbon.36... 

Four oxidation states of palladium are encountered in organometallic chemistry see Palladium Inorganic Coordination Chemistry) In order of importance, they are Pd , Pd , Pd, and Pd . With the reduction of palladium from Pd to Pd , the metal changes its reactivity from electrophile to nucleophile. However, unlike main group nucleophiles such as thiolates or cyanide, Pd complexes react with both alkyl halides and aryl or vinyl halides. Reactions of Pd complexes with these latter sp halides generate new Pd aryl or vinyl bonds through the process of oxidative addition. 

The most generally accepted mechanistic speculations center about a y3-acetoxyalkylpalladium intermediate (VI) formed by an insertion reaction of ethylene into a palladium-oxygen bond (Reaction 3). It is proposed that this intermediate would then decompose via a palladium-assisted hydride transfer to vinyl acetate (Reaction 4) (36). While Reaction 4 would appear more awkward than a simple )3-hydrogen... 

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

One of the most common examples of this approach involves the coupling of aryl or vinyl halides with palladium-catalyzed cydization. This chemistry typically employs palladium(O) catalysts, which are postulated to undergo an initial oxidative addition of aryl or vinyl halides to form a palladium(II) complex to mediate cydization. This is followed by reductive elimination of the heterocyde-aryl or heterocyde-vinyl bond. This approach has been employed to construct a range of polysubstituted indoles. For example, Cacchi has shown that the Pd(PPh3)4 catalyzed coupling of trifluoroace-tanilides with aryl halides or vinyl halides/triflates proceeds to substituted indoles in good yield (Scheme 6.14) [18]. 

These highly useful complexes can be generated in other ways. For instance, they can be formed by a modification of the Heck reaction. Insertion of dienes into vinyl or aryl-palladium(II) bonds generates -ir-allyl complexes. These can be trapped by external nucleophiles, but the product of p-hydride elimination will also be obtained (Scheme 9.73). After insertion of one double bond of the diene 9.258 into the carbon-palladium bond of the t -intermediate 9.262, p-hydride elimination may occur to give the expected Heck product 9.260. However, the t -intermediate 9.265 is in equilibrium with its -equivalent 9.266 and, if an appropriate nucleophile is present, this complex may be trapped to give the allylated product 9.261. Intramolecular trapping is more efficient. This has been used, for instance, to prepare dihydroindoles 9.269 (Scheme 9.74). 

Insertion reactions of monoenes into Pd-R bonds also lead to j -allylic palladium complexes provided that either the monoene or the R group contain an unsaturated functionality. Thus, the insertion of an alkene into a Pd—vinyl bond leads to 77 -allylic palladium compounds. The insertion of styrene and other vinyl arenes into a Pd—R bond leads to 77 -benzylic palladium complexes. The aryl ring in these derivatives gets involved in bonding, and the 77 -benzylic coordination can be looked at as a particular case of 77 -allyl. Nonetheless, the adoption of cr-coordination is more facile in -benzylic derivatives than in j -allyls due to the extra stabilization provided by the aromaticity of the aryl ring in the n-form. 77 -Benzylic palladium complexes have been prepared by insertion of styrene into Pd-Pd-Me, and Pd-SiRs bonds, and styrene derivatives into Pd-Me bonds (Equation (33)). Insertion of vinylarenes into a usually non-detected Pd-H bond leads to the synthesis of other -benzylic palladium derivatives. 

Scheme 10 Formation of five-membered chelate complexes by the insertion of polar vinyl monomers into a palladium-acetyl bond.

Substitution at the Carbon—Chlorine Bond. Vinyl chloride is generally considered inert to nucleophilic replacement compared to other alkyl halides. However, the chlorine atom can be exchanged under nucleophilic conditions in the presence of palladium [7440-05-3] Pd, and certain other metal chlorides and salts. Vinyl alcoholates, esters, and ethers can be readily produced from these reactions. 

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