Aryl palladium species

The Heck reaction is a synthetically powerful reaction wherein a carbon-carbon bond is formed between two sp hybridized carbon atoms. The syn nature of the addition of vinyl-aryl palladium species to carbon-carbon double bond precludes a syn p-hydride elimination for cyclic olefins. As a result, a new chiral center is formed. Shibasaki and Vogl provided a comprehensive review of this subject in 1999 440 Overman and Donde reviewed the intramolecular version of this reaction in 2000. Pfaltz and co-authors specifically reviewed this reaction using PhosOx ligands. ... 

In the presence of palladium(II) and silver(I) salts, arene carboxylates could be converted to aryl palladium species, which were engaged in Heck coupling reactions.90 Since the more electrophilic palladium trifluoroacetate proved to be the bestcatalyst, decarboxylation probably occurred by aromatic electrophilic substitution... 

The aryl palladium species formed attacks one of the double bonds present in 13. 

When the initial aryl-palladium species has more than one possible proton in the vicinity, the C—H activation can take place several times. Carretero and coworkers [77] observed three C—H activations after the initial Mizoroki-Heck reaction nsing o, /i-nnsatnrated snlfones 134 and iodobenzene. Under normal conditions, the expected Mizoroki-Heck product 135 is formed however, using an excess of iodobenzene, 136 is obtained in high yield. In this transformation, three molecules of iodobenzene are incorporated into the final product. On the other hand, a later study showed that, under the same conditions, similar electron-deficient alkenes as enones do not nndergo this domino reaction instead, only the Mizoroki-Heck product is obtained [78]. A computational analysis of the transformation explained this finding with the difference in the energy of the transition states that ultimately lead to the five-membered ring palladacycle PdCl (Scheme 8.35). 

Based on a transformation described by Catellani and coworkers [80], Lautens s group [81] developed a series of syntheses of carbocycles and heterocycles from aryl iodide, alkyl halides and Mizoroki-Heck acceptors. In an early example, the authors described a three-component domino reaction catalysed by palladium for the synthesis of benzo-annulated oxacycles 144 (Scheme 8.37). To do so, they used an m-iodoaryl iodoalkyl ether 143, an alkene substimted with an electron-withdrawing group, such as t-butyl acrylate and an iodoalkane such as -BuI in the presence of norbomene. It is proposed that, after the oxidative addition of the aryliodide, a Mizoroki-Heck-type reaction with nor-bornene and a C—H activation first takes place to form a palladacycle PdCl, which is then alkylated with the iodoalkane (Scheme 8.37). A second C—H activation occurs and then, via the formation of the oxacycle OCl, norbomene is eliminated. Finally, the aryl-palladium species obtained reacts with the acrylate. The alkylation step of palladacycles of the type PdCl and PdCl was studied in more detail by Echavarren and coworkers [82] using computational methods. They concluded that, after a C—H activation, the formation of a C(sp )—C(sp ) bond between the palladacycle PdCl and an iodoalkane presumably proceeds by oxidative addition to form a palladium(IV) species to give PdC2. This stays, in contrast with the reaction between a C(sp )—X electrophile (vinyl or aromatic halide) and PdCl, to form a new C(sp )—C(sp ) bond which takes place through a transmetallation. 

Several studies have used palladium catalysis in the arylation of benzoxazoles. A palladium catalyst with a phosphine ligand allows their reaction with aryl mesylates without the requirement for acid or copper additives. In the reaction with arene-sulfonyl chloride, palladium is used in combination with copper. A plausible mechanism involves initial cupration of the benzoxazole followed by copper—palladium exchange and oxidative addition of the sulfonyl chloride to palladium to give (84). This intermediate may lose sulfur dioxide to give an aryl palladium species, which, on reductive elimination, yields 2-arylbenzoxazole. The arylation of benzoxazoles and benzthiazoles with aryl boronic acids is also catalysed by a combination of palladium... 

Heck-Matsuda reaction. This reaction was first described in 1977 by Tsutomu Matsuda, who pointed out the unique reactivity of the arenediazonium salts in the Heck reaction, allowing a fast and direct generation of cationic aryl-palladium species in the reaction medium. Indeed, the high levels of chemo- and stereocontrol observed in these reactions usually do not depend on the use of halide scavengers (such Ag" or Tl salts), dry solvents, inert atmosphere, or phosphane hgands (Scheme 1) ... 

The ruthenium catalyzed reactions of aromatic ketones and alkenes or a,o)-dienes maybe related mechanistically to the palladium catalyzed Heck reaction of aryl halides with alkenes (77). Insertion of palladium into the C-X bond of the aryl halide leads to a reactive aryl palladium species which is the key intermediate in this reaction. The Heck reaction has also been applied to the synthesis of polymers (72-76). 

Or//io-substituted arenes can serve as suitable templates for CH-activation. While in 4.17 the concomitant reaction of aryl palladium species with carbon electrophiles is discussed, in this paragraph we discuss the oxidation of arenes to phenol and aryl halide derivatives. 

The overall process for fluorination consists of two steps and is easy to perform. Initial transmetallation of the boronic acid to afford the intermediate aryl palladium species is carried out in basic methanol/benzene and proceeds in >70% yield for the substrates evaluated. The subsequent fluorination is performed using Selectfluor in either acetonitrile or acetone at 50 °C with the desired aryl fluorides being isolated in yields ranging from 31-82% (Scheme 15.66). The reaction is rapid, being completed within 30 minutes and shows no significant sensitivity to air or moisture. 

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