Carbon-palladium complexes, alkene/alkyne insertion

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The reactions shown in Scheme 1 require activation of the aromatic C-H bond by a metal and subsequent insertion of an alkene or alkyne in the aryl-carbon palladium bond (Chapter III.1.3.2.5). C-H activation has been the topic of many studies since the 1960s and several metal complex systems are known to induce... 

In more complex reaction cascades an additional alkyne-insertion step can occur. Thus starting with intramolecular carbopalladation of a vinyl iodide to a carbon-carbon triple bond, followed by two intramolecular alkene-insertion steps and termination with dehydropalladation, a palladium-catalyzed synthesis of l-(5 -methylbicyclo[3.1.0]hex-T-yl)-5,5-bis(carboethoxy)cyclo-hexadiene (52) starting from l-iodo-4,4-bis(carboethoxy)-ll-methyldodeca-l,ll-dien-6-yne (51) is achieved. ... 

Interception of the Tr-allyl palladium complex by soft nucleophiles, particularly malonates, has been described above. Alkenes, alkynes and carbon monoxide can also insert into the Tr-allyl palladium complex, generating a u-alkyl palladium species. When an internal alkene is involved, a useful cycbzation reaction takes place (sometimes called a palladium-ene reaction).Addition of palladium(O) to the allylic acetate 225 gave the cyclic product 226 (1.225). The reaction proceeds via the -ir-allyl palladium complex (formed with inversion of configuration), followed by insertion of the alkene cis- to the palladium and p-hydride elimination. In some cases it is possible to trap the a-alkyl palladium species with, for example, carbon monoxide. 

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. 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

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