Arylation of alkenes

Mechanistic studies show that the arylation of alkenes proceeds via the palladation of aromatic compounds to form a rr-aryl-Pd bond (261), into which insertion of alkene takes place to form 262. The final step is i3-elimina-tion to form the arylated alkenes 259 and Pd(0). 

Palladation products formed from arylmercurials, carboalkoxymercurials, and alkylmercurials, which have no /3-hydrogen, are used in situ for the reaction of alkenes[367]. Particularly, the arylation of alkenes is synthetically useful. Styrene derivatives 402 and 403 are formed by the reaction of a... 

A suitable catalyst for carboxy-de-diazoniations was found by Matsuda s group in their work on arylations of alkenes. As in the case of alkene arylations (Sec. 10.9), they used Pd11 acetate (2 mole %) and carbon monoxide (9 atm) for reactions with benzenediazonium tetrafluoroborate and sodium acetate in acetonitrile as solvent at room temperature (Nagira et al., 1980 82-85% yield). Similar results were obtained... 

The arylation of alkenes by treatment with a diazonium chloride (or bromide) solution and cupric chloride (or bromide) is called the Meerwein arylation reaction, after its discoverer (Meerwein et al., 1939). Originally, it was discovered using a,P-unsaturated carbonyl compounds, namely coumarin (Scheme 10-43) and cinnamic derivatives (Schemes 10-44 and 10-45). As Scheme 10-45 shows, the Meerwein reac-... 

The Meerwein reaction is a valuable method for the arylation of alkenes because of the easy availability of cheap aromatic amines and compounds containing double bonds. A disadvantage is that the yield is often low (normally 20-50%, in exceptional cases reaching 80%, see Table 10-3). The reaction can be carried out in water if the alkene derivative is sufficiently soluble otherwise an organic co-solvent is necessary. Meerwein et al. (1939) used acetone, which is still the most popular solvent used today. The mechanistic function of acetone will be discussed later in this section. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Studies on heterogeneous Pd metal catalysts for the arylation of alkenes with aryl halides (the Mirozoki-Heck reaction often reported as Heck reaction) [28] continue to... 

Scheme 11.7 illustrates some arylation of alkenes by diazonium ions. Entries 1 to 4 are typical conditions. Entry 5 illustrates generation of the diazonium ion under in situ conditions. Entry 6 is an example of the reductive conditions using Ti(III). 

The palladium-catalyzed arylation of alkenes by haloarenes, and applications in furan synthesis have been described earlier (see Eq. 14 in Section IV,B, 1). By employing o-aminoiodoarenes and appropriately substituted (Z)-alkenes it has proved possible to develop an efficient synthetic route to quinolin-2-ones (Scheme 140).213... 

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck-type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinyl positions. The first report on the application of microwave methodology to this type of reaction was published by Hallberg et al. in 1996 [86], Recently, the palladium catalyzed Heck coupling reaction induced by microwave irradiation was reported under solventless liquid-liquid phase-transfer catalytic conditions in the presence of potassium carbonate and a small amount of [Pd(PPh3)2Cl2]-TBAB as a catalyst [87]. The arylation of alkenes with aryl iodides proceeded smoothly to afford exclusively trans product in high yields (86-93%) (Eq. 61). 

Arylation of alkenes with iodobenzene in the presence of supported Pd catalyst [45]. Reaction conditions acetonitrile as solvent, pressure CEM reactor. 

Van Srijdonck GPF, Boete MDK, van Leeuwen PWN (1999) Fast palladium catalyzed arylation of alkenes using bulky monodentate phosphorus ligands. Eur J Inorg Chem 1073-1076... 

Intramolecular arylation of alkenes. The intramolecular palladium-catalyzed condensation of aryl halides with an alkene group (Heck arylation5) can actually proceed more readily than the original bimolecular version. These intramolecular cyclizations typically proceed in acetonitrile at room temperature, particularly when catalyzed by Ag2C03, which also inhibits isomerization of the double bond of the product. They can be used to obtain spiro, bridged, and fused systems. Even tetrasubstituted alkenes can participate, with formation of quaternary centers.6... 

In this section we include the intramolecular arylation of the Pschorr type, the inter-molecular arylation (Gomberg-Bachmann reaction), the arylation of alkenes and alkynes (Meerwein reaction) and related processes. 

The arylation of alkenes was discovered by Meerwein146 in 1939 using ,/)-unsaturated carbonyl compounds, namely coumarin and cinnamic derivatives. Diazotizations for Meerwein reactions are made in aqueous HC1. The substitution proper may be combined with addition of HC1 to the double bond. As catalyst, CuCl2 is used. Various observations (see elsewhere7k) demonstrate that in typical Meerwein systems, part of Cu11 is reduced to Cu1. 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

Reduction of aryl diazonium ions by Ti(IEL) in the presence of a,/ -unsaturated ketones and aldehydes leads to /> arylation and formation of the saturated ketone or aldehyde. The early steps in this reaction parallel the copper-catalyzed reaction. However, rather than being oxidized, the radical formed by the addition step is reduced by Ti(IEL).109 Scheme 11.7 illustrates some typical examples of arylation of alkenes by diazonium ions. 

Arylation of alkenes catalyzed by palladium compounds is known as the Heck reaction [65]. While these reactions are very sensitive to steric effects, subtle electronic contributions to the regiochemical outcome may be assessed by comparing reactions of alkenes with similar substitution patterns. The arylating agents, being the nucleophilic arylpalladium species, tend to dictate the facility of their reaction with unsym-metrical, electron-deficient alkenes. 

A number of methods are now available for the palladium-catalyzed arylation of alkenes. Mizoroki et al. showed that, in the presence of PdCl2, iodobenzene reacted with ethylene to give styrene.623 These reactions require a stoichiometric amount of base to neutralize the hydriodic acid formed (equation 172). 

Analogous to the palladium-catalyzed arylation of alkenes with aryl halides (Section 61.2.8.4), vinyl halides can be used to prepare dienes according to equation (191).649 A base is again needed in stoichiometric quantities. It should be noted that the dienes can be isolated as such only in certain cases. When vinyl iodide itself was used, only Diels-Alder products were obtained (Scheme 74). 

Scheme 2. Oxidative arylation of alkenes by electrophilic palladium (I I).

Heck reaction is not just to make possible the alkenylation or arylation of alkenes at all, but also to guarantee that these alkenylations and arylations occur in a frans-selective fashion. 

Examples of reactions of halogenated quinones and of aryliodonium compounds, as well as arylation of alkenes and alkynes will also be mentioned in this section. 

The palladium catalysed arylation of alkenes with an organic halide was first reported Mi/orokC2 and Heck133 in the early 1970 s. As the reaction is very versatile and applicable to a wide range of substrates it has since become one of the cornerstones of palladium chemistry.134 371... 

QCdmJpFJ [QCpmJBr PdCl2 Pd(OAc)2 NaOAc 30 °C. Ligand-free, ultrasound promoted arylation of alkenes and alkynes with aryliodides palladium bis-carbenes and palladium nanoparticles ( 1 nm) are identified after catalysis product extracted with ethyl acetate/petrol ether. [66]... 

Arylation of alkenes can also be achieved by treatment with an arylpalla-dium reagent, typical generated in situ from an aryl halide or other suitably... 

As a unique method for the direct arylation of alkenes, the Heck reaction (Eq. 15) has been widely investigated, finding numerous elegant applications in organic synthesis [15,36-38,106]. 

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