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Palladium chemistry mechanisms

Hydroarylations of alkynes are catalyzed by gold complexes and these bear some resemblance to the Fujiwara Pd-catalyzed reaction. In general, when using gold chemistry, better Z/E selectivities are observed compared with palladium, lower catalyst loadings and milder conditions (neutral not TFA) are used. The mechanism involves the attack of ArH on the Au-coordinated alkyne. Flowever, electron-poor acetylenes only appear to work with palladium chemistry (Equations (75) and (76)).72... [Pg.125]

In particular, more structural data, more kinetic studies, a more thorough investigation of the platinum and palladium chemistry, the use of new techniques, such as NMR spectroscopy, and the determination of bond energies could increase our understanding of the bond nature and reactivity and make the interpretation of important catalytic mechanisms of these compounds easier. [Pg.352]

This review provides a compilation of the most well-known catalytic reactions related to organo-palladium chemistry. The references have been covered up to November 1995. The mechanisms are generally described with respect to the data in the literature and by the use of information found mainly in publications. For each catalytic cycle, the chemical parameters are listed and one or more related references are given. As an example, for the cross-coupling reactions between RX and R M, the identity of R, R, X, and M parameters are defined. The references which are quoted contain at least one example of these parameters. A few palladium-catalyzed reactions are not reported in this review because of insufficient data concerning the mechanism or because the reaction affords a complex mixture of products. [Pg.314]

Two hypothetical mechanisms have been proposed to explain the Heck reaction on the basis of Pd(II)/Pd(IV) cycles (Scheme 2.12). As discussed in Section 2.2.1, oxidative addition of aryl halides to Pd(II) precursors is both kinetically and thermodynamically difficult. The Pd(II)/Pd(IV) mechanism proposed by Shaw for the Heck reaction (Scheme 2.1) tried to elude this problem by postulating the intermediacy of anionic Pd(II) complexes with increased nucleophilicity, but it is not evident how this mechanism could be adapted to complexes containing PCP or related pincer ligands. With this problem in mind, Jensen [93] made an alternative proposal (Scheme 2.12a), which starts with the oxidative addition ofa C-H bond of the olefin to the Pd(II) pincer complex to afford a Pd(IV) vinyl-hydride intermediate. This idea was inspired by a similar reaction observed with an isostructural Ir(I) PCP complex, but such C-H bond activations are unusual in palladium chemistry. A theoretical analysis by Freeh [63] raled out such possibility, leading instead to the alternative Pd(II)/Pd(IV) cycle depicted in Scheme 2.12b. A key element... [Pg.50]

Intramolecular oxonium ylide formation is assumed to initialize the copper-catalyzed transformation of a, (3-epoxy diazomethyl ketones 341 to olefins 342 in the presence of an alcohol 333 . The reaction may be described as an intramolecular oxygen transfer from the epoxide ring to the carbenoid carbon atom, yielding a p,y-unsaturated a-ketoaldehyde which is then acetalized. A detailed reaction mechanism has been proposed. In some cases, the oxonium-ylide pathway gives rise to additional products when the reaction is catalyzed by copper powder. If, on the other hand, diazoketones of type 341 are heated in the presence of olefins (e.g. styrene, cyclohexene, cyclopen-tene, but not isopropenyl acetate or 2,3-dimethyl-2-butene) and palladium(II) acetate, intermolecular cyclopropanation rather than oxonium ylide derived chemistry takes place 334 ). [Pg.210]

An initial step of orthometallation probably also occurs when aniline is allowed to react with ethylene in the presence of a rhodium(I) catalyst. 2-Methylquinoline (10 turnovers relative to the metal) and JV-ethylaniline (30 turnovers) are formed after 72 h in what are probably two independent reaction pathways (Scheme 144).216 It is interesting to note that the intramolecular cyclization step in the proposed216 mechanism (Scheme 144) has precedent in the palladium-promoted quinoline synthesis reported by Hegedus et al.16 (see Scheme 142), but the transformation 118->119 is unusual in the chemistry of organometallic insertion reactions.106... [Pg.383]

The first examples of the use of palladium as a catalyst for carbon-carbon coupling reactions were reported almost thirty years ago [14], and over recent decades a massive effort has been devoted to the extension of the scope of palladium-catalyzed reactions. Organic and organometallic chemists have received extensive input from palladium-coordination chemistry in the task of understanding the mechanisms behind these efficient synthetic procedures [14]. [Pg.380]

KATRITZKY POZHARSKII Handbook of Heterocyclic Chemistry, 2nd Edition LEVY TANG The Chemistry of C-Glycosides LI GRIBBLE Palladium in Heterocyclic Chemistry McKILLOP Advanced Problems in Organic Reaction Mechanisms MATHEY Phosphorus-Carbon Heterocyclic Chemistry The Rise of a New Domain OBRECHT Solid Supported Combinatorial and Parallel Synthesis of Small-Molecular-Weight Compound Libraries... [Pg.403]

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... [Pg.97]

The mechanism for Pd-catalyzed C—O bond formation is similar to that of C—N bond formation. Application of this method to heterocyclic chemistry is yet to be seen, partially because the SnAt displacements of many heteroaryl halides with alkoxides are facile without the aid of palladium. [Pg.23]

Studies regarding the nature of the catalytically active species for NHC complexes in Heck-type reactions have focused on the Mizorvki—Heck reaction and have consistently revealed a palladium(O) species as the active catalyst. The induction period is shortened upon addition of a reducing agent,and postulated intermediates of the reaction were isolated and characterized as well as employed in stoichiometric and catalytic reactions. Theoretical studies using DPT calculations showed the mechanism for NHC complexes to most likely he in agreement with phosphine chemistry. ... [Pg.47]

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. [Pg.20]

Another variant of the Heck reaction which is important in heterocyclic chemistry utilizes five membered heterocycles as olefin equivalent (2.2.)7 It is not clear whether the process, coined as heteroaryl Heck reaction follows the Heck mechanism (i. e. carbopalladation of the aromatic ring followed by //-elimination) or goes via a different route (e.g. electrophilic substitution by the palladium complex or oxidative addition into the C-H bond). Irrespective of these mechanistic uncertainties the reaction is of great synthetic value and is frequently used in the preparation of complex policyclic structures. [Pg.22]


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Palladium chemistry

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