Alkenes palladium salts

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

Palladium salts will attack C-H bonds in functionalised aromatics such as acetoaniline to form palladium-carbon bonds that subsequently undergo insertion of alkenes [31], (3-Hydride elimination gave styryl derivatives and palladium hydride, which requires re-oxidation of palladium by benzoquinone. The reaction can be regarded as a combined Murai reaction (C-H activation, if electrophilic) and a Heck reaction (arylalkene formation), notably without the production of salts as the cross-coupling reactions do. An example is shown in Figure 19.15. 

When the Pd bears chiral ligands, these reactions can be enantioselective.1448 ir-Allylmo-lybdenum compounds behave similarly.1449 Because palladium compounds are expensive, a catalytic synthesis, which uses much smaller amounts of the complex, was developed. That is, a substrate such as an allylic acetate, alcohol, amine, or nitro compound1450 is treated with the nucleophile, and a catalytic amount of a palladium salt is added. The rr-allylpal-ladium complex is generated in situ. Alkene-palladium complexes (introducing the nucleophile at a vinylic rather than an allylic carbon) can also be used.1451... 

Trialkylamines are used as additives in the telomerization of butadiene and water in a two-phase system (103). The catalyst comprises a palladium salt and tppms or tppts. The amines may build cationic surfactants under catalytic conditions and be capable of micelle formation. The products include up to five telomerization products (alcohols, alkenes, and ethers), and thus the reaction is nonselective. 

For additional examples and a discussion of the mechanism of electrophilic addition to alkenes with palladium salts, see Volume 4, Chapter 3.4. 

Caiboxylate ions are also effective nucleophiles in palladium-catalyzed reactions of alkenes and several classes of lactones including 7-pyrones (equation 11)17 and isocoumarins (equation 12)18 have been made in this manner. These early studies used stoichiometric amounts of palladium salts, since efficient redox systems had not yet been developed. However, with more modem techniques catalysis in these systems should be relatively straightforward. The more recent catalytic cyclization-caibonylation process in equation (13) is indicative of this.19... 

The direct palladation procedure is limited to substitution with aryl and heterocyclic groups. The met-allation is an electrophilic process and therefore does not work well with deactivated aromatics. When possible, mixtures of isomers may be obtained. Since the palladium salts employed for the metallation are moderately strong oxidizing agents, the reaction cannot be used with easily oxidizable alkenes or aromatics. The only effective method for making this procedure catalytic is to reoxidize the palladium in situ with oxygen under pressure an inconvenient and potentially dangerous procedure. 

Organo-iron and -cobalt phthalocyanines are reported to transfer the organic groups to palladium salts, and they have been used to substitute alkenes.46... 

The catalyst The amount of catalyst required in an aryl bromide or iodide alkene substitution varies widely with the reactants and the reaction conditions. Most examples reported have used 1-2 mol % of palladium salt relative to the aryl halide, but much lower amounts are sufficient in some instances. In an extreme case, where very reactive p-nitrobromobenzene was added to the very active alkene, ethyl acrylate and sodium acetate was the base in DMF solution at 130 C with a palladium acetate-tri-o-tolylphos-phine catalyst in 6 h the palladium turned over 134 000 times and ethyl p-nitrocinnamate was obtained in 67% yield.63... 

Palladium(II) salts apparently oxidize arylamines to arylpalladium salts since alkenes are arylated by reaction with only an aromatic amine and a palladium salt. However, yields are generally low.100 Much better yields are obtained if /-butyl nitrite is added and, of course, this forms the diazonium salt in situ. This not only saves a step but some diazonium salts which are too unstable to be isolated may be used as well. The reactions are carried out in the presence of acetic or chloroacetic acid with 5-10% bis(di-benzylideneacetone)palladium as catalyst (equation 41).101... 

V,(V-Dichloro-p-toluenesulfonamidc (TsNCL) has been found to be an efficient nitrogen source for the aziridination of unfunctionalized alkenes using palladium catalysts. Among the palladium salts, (AcO Pd was the most effective catalyst.81... 

Heteropolyoxametalates are often used in combination with palladium salts as catalysts in oxidation processes using dioxygen as the oxidant. Indeed, the oxidative coupling reaction of benzenes with alkenes was also successfully achieved by use of the Pd(OAc)2/molybdovanadophosphoric acid (HPMoV)/02 system [14a]. For example, reaction of benzene with ethyl acrylate using this catalytic system in acetic acid afforded ethyl cinnamate as a major product in satisfactory yield. Typically, the reaction is conducted in acetic acid at 90 °C under 1 bar of 02. After 6 h the TON is 15. This number was recently improved to 121 [14b]. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

Palladium catalysts are best known for oxidizing alkenes to ketones or vinyl derivatives. However, formation of a,P-unsaturated carbonyl compounds by UV irradiation of oxygenated solutions of alkenes in the presence of catalytic amounts of palladium salts has been observed by Muzart. - This reaction is believed to proceed through a ir-allylpalladium trifluoroacetate complex, e.g. (77). 

Palladium salts can bring about oxidative carbonylation of alkenes in the presence of copper(II) salts which can reoxidize Pd° to Pd . Oxidative carbonylation is favored over simple hydroesterification by the presence of bases and by low temperatures (25 C) and low pressures (3-15 bar). The products can be a, -unsaturated esters, dicarboxylic acid esters or -alkoxy esters. By careful optimization of the conditions (25 °C, 4 bar CO, methanol solvent, CuCh reoxidant and sodium butyrate buffer) high yields of diesters can be obtained (equation 35). ... 

Palladium salts also promote the addition of nucleophiles to alkenes and alkynes. The Pd-catalyzed additions of nucleophiles to alkynes, which is useful for intramolecular cyclizations such as the isomerization of 2-alkynylphenols to benzofurans, proceeds by exactly the same mechanism as does the Hg-catalyzed reaction. However, the Pd-catalyzed additions of nucleophiles to alkenes takes the course of substitution rather than addition because alkylpalladium complexes are unstable toward /3-hydride elimination. The Pd-catalyzed nucleophilic substitutions of alkenes are discussed later in this chapter (Section 6.3.6). 

This catalytic system consisted of an aqueous acidic solution of a palladium salt with a cop-per(I) salt and oxygen or air as an oxidant however, application of the Wacker process to longer chain alkenes has been a challenge due to their low solubility in aqueous media. This has led to research in biphasic systems using tetraalkylammonium salts,93 polyethylene glycols,94 as well as cyclodextrins.95,96... 

Nucleophilic cyclisations onto palladium-complexed alkenes have been used to prepare indoles, benzofu-rans and other fused systems. The process can be made catalytic in some cases by the use of reoxidants such as p-benzoquinone or copper(II) salts. 

Dicarboration of conjugated dienes can occur in either a 1,2- or a 1,4-addition mode. Several examples of stepwise stereoselective c -l,4-dicarborations of open-chain and cyclic dienes, via palladium-allyl complexes, which use stoichiometric amounts of palladium salts and soft, delocalized carbanions have been reported65 67. Catalytic dicarboration of 1,3-butadiene, stereoselectively leading to ( )-alkenes with up to 70% yield, is achieved using aryl iodides and delocalized carbanions of the malonate type in the presence of palladium(II) chloride and l,2-bis(diphenylphosphino)ethane68. A Tt-allyl-palladium intermediate is thought to determine the E geometry of the product. 

The reaction of aryl and alkenyl halides (bromides and iodides) such as 79 with alkenes and alkynes, in the presence of a catalytic amount of a palladium salt, results in substitution of the halide by the alkenyl or alkynyl group, and is referred to as the Heck reaction [37,39]. The reaction is initiated by oxidative addition of the halide to a Pd(0) species to give 80, followed by alkene (or alkyne) insertion to give 81, which has a Pd-C bond, and cis Pd-H elimination to liberate the product 82 (Scheme 19). 

Based on the proposed mechanism of the stoichiometric alkene arylation, it was anticipated that the same reactions could be achieved using substoichiomelric amounts of palladium salts. In the presence of an oxidant, palladium(O) could be reoxidized to palladium(II), which could then reenter the reaction, thus establishing a catalytic cycle for the arene vinylation reactions. 

For reviews on the Fujiwara-Moritani reaction, see (a) Fujiwara, Y. (2002) Palladium-promoted alkene-arene coupling via C—H activation, in Handbook of Organopalladium Chemistry in Organic Synthesis, Vol. 2 (eds E.-i. Negishi and A. de Meijere), John Wiley Sons, Inc., New York, pp. 2863-71 (b) Jia, C., Kitamura, T. and Fujiwara, Y. (2001) Catalytic functionalization of arenes and alkanes via C—H bond activation. Acc. Chem. Res., 34, 633-9 (c) Fujiwara, Y. and Jia, C. (2001) New developments in transition metal-catalyzed synthetic reactions viaC—H bond activation. PureAppl. Chem., 73,319-24 (d) Moritani, I. and Fujiwara, Y. (1973) Aromatic substitution of olefins by palladium salts. Synthesis, 524-33. 

Palladium-catalyzed alkene Copper salt Reoxidizes the palladium... 

Allylpalladium halides are formed very readily, for example from alkenes and palladium salts. The reaction is assisted by a base such as sodium acetate, which removes hydrogen chloride. Yields are improved if copper(II) acetate is added to reoxidize any palladium metal which may be formed in side reactions (cf. p. 380). Allylpalladium reagents have found considerable application in organic synthesis, both in stoichiometric and catalytic reactions (p. 261). 

In [51], Wacker oxidation of olefins was studied in the presence of catalytic systems comprising water-soluble calixarenes (sulfonated and glycydylated derivatives), palladium salt, and copper salt. The presence of nonpolar cavities in these molecules enables binding nonpolar substrates and their transfer into the aqueous phase where the reaction takes place. The activity of these catalysts depends on the complementarity between the cavity size of the host molecule and the size of the guest molecule. Therefore, substrate selectivity was exhibited. For example, the addition of calixarene increased the reaction rate for linear 1-alkenes which size corresponded to the size of the calixarene cavity (1-hexene for calix[4]arene and 1-octene for calix[6]arene). The activity of catalytic system applied for the oxidation of substituted styrenes also depended on the ratio of the size of the substrate molecule and that of the calixarene cavity. 

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