Stoichiometric synthesis carbon-palladium bonds

Abstract Palladium-catalyzed multicomponent coupling and annulation reactions have become important tools in organic synthesis. Recently, palladium(IV) complexes have been proposed as intermediates in catalytic cycles of these transformations, although experimental evidence for their involvement is frequently lacking. Examples of such catalytic annulation reactions are discussed, followed by a review of studies performed with stoichiometric isolable or semistable palladium(IV) complexes seeking experimental evidence for feasibility of the participation of palladium(IV) intermediates in cascade carbon-carbon and carbon-heteroatom bond-forming sequences. 

The synthesis of succinic acid derivatives, /3-alkoxy esters, and a,j3-unsaturated esters from olefins by palladium catalyzed carbonylation reactions in alcohol have been reported (24, 25, 26, 27), but full experimental details of the syntheses are incomplete and in most cases the yields of yS-alkoxy ester and diester products are low. A similar reaction employing stoichiometric amounts of palladium (II) has also been reported (28). In order to explore the scope of this reaction for the syntheses of yS-alkoxy esters and succinic acid derivatives, representative cyclic and acyclic olefins were carbonylated under these same conditions (Table I). The reactions were carried out in methanol at room temperature using catalytic amounts of palladium (II) chloride and stoichiometric amounts of copper (II) chloride under 2 atm of carbon monoxide. The methoxypalladation reaction of 1-pentene affords a good conversion (55% ) of olefin to methyl 3-methoxyhexanoate, the product of Markov-nikov addition. In the carbonylation of other 1-olefins, f3-methoxy methyl esters were obtained in high yields however, substitution of a methyl group on the double bond reduced the yield of ester markedly. For example, the carbonylation of 2-methyl-l-butene afforded < 10% yield of methyl 3-methyl-3-methoxypentanoate. This suggests that unsubstituted 1-olefins may be preferentially carbonylated in the presence of substituted 1-olefins or internal olefins. The reactivities of the olefins fall in the order RCH =CHo ]> ci -RCH=CHR > trans-RCH =CHR >... 

Reductive elimination is typically the last step in a variety of stoichiometric and catalytic transformations and is thus important in the synthesis of a host of organic molecules of industrial and biological significance. Many fundamental carbon-carbon bond-forming processes, including the palladium-mediated cross-coupling of... 

Typical of the methods available for the preparation of 7t-allylpalladium complexes is the preparation of the crystalline compound 70 by heating prenyl acetate 71 in acetic acid with PdCl2 in the presence of copper(II) chloride, followed by chromatographic purification. Alkylation of 70 with the anion derived from the Ci5-sulphone 72 is then carried out in DMF in the presence of at least four equivalents of triphenylphosphine (two per Pd) and gives the crystalline C2o-sulphone 73 from which vitamin A may be obtained by ethoxide-catalysed elimination of phenylsulphinic acid [40] (Scheme 16). Despite the moderate yield (52%) in the alkylation step and the use of stoichiometric amounts of palladium, this synthesis of vitamin A (7) avoids the lengthy functionalization process that is often necessary with more conventional methods of carbon-carbon bond formation. 

More recently, Dong s group developed a palladium-catalyzed synthesis of indoles from nitroalkenes [43]. This was the first report on transition metal-catalyzed transformation of conjugated nitroalkenes into indoles. Under mild reaction conditions (1 bar carbon monoxide, 110 °C), palladium catalyzes the reductive cyclization of nitroalkenes to form a putative nitrosoalkene intermediate, which then rearranges to provide 3-arylindoles in high yields (Table 9.5). Notably, this novel C-H bond amination takes advantage of carbon monoxide as an inexpensive stoichiometric reductant and produces carbon dioxide as the major byproduct. 

Copper-catalyzed C-O, C-N, and C-S Coupling. While there is an extensive variety of palladium catalysts for C(aryl)-X bond formation (X = 0, N, and S), copper corrqtlexes have recently gained renewed popularity in these coupling processes. Use of the (CuOTf)2. benzene complex allows the formation of diaryl ethers from aryl bromides or iodides and phenols in very good yields (76-93%) (eq 121). The reaction occurs in toluene in the presence of cesium carbonate as the base and a catal)4ic quantity of ethyl acetate whose role is probably to increase the solubility of the copper species. In the case of less reactive phenols, yields can be increased by the addition of a stoichiometric amount of carboxylic acid. A slight modification of these conditions has been used in the key diaryl ether formation in the synthesis of verbenachalcone. ... 

Allylpalladium complexes find wide application in organic synthesis. With carbon nucleophiles they form new C—C bonds, thus providing a method for building carbon skeletons. Dimeric allylpalladium chlorides are not very susceptible to attack by nucleophiles, but their reactivity can be enhanced by addition of triphenylphosphine, which converts them into monomeric species. These reactions, however, are stoichiometric in palladium, which is undesirable on account of its high cost. Fortunately related catalytic procedures have been discovered. Many allylic compounds react with Pd(PPhj4 (p. 181) to form cationic -allyl complexes. These cations react with nucleophiles to form substituted allylic derivatives, while the catalyst, effectively PdL , is regenerated and reenters the cycle. 

Besides nickel and cobalt, almost all of the catalysts discussed in the last chapter which were suited for the formation of free acids can be applied, e. g. rhodium, palladium and, with certain restrictions, iron. Cobalt hydrocarbonyl catalyzes the stoichiometric ester synthesis at mild reaction conditions [35, 121]. The initially formed acylcobalt carbonyls react rapidly with alcohols even at 50 °C and, in the presence of Na-alcoholate, even at 0 °C to give esters [121]. Dienes with isolated double bonds react with carbon monoxide and alcohols at mild reaction conditions in the presence of Pd/HCl to give unsaturated monocarboxylic acid esters and at more severe conditions to give saturated dicarboxylic acid esters [508]. 

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