Arylation of Olefins

Another methodology that is widely used for C-C bond formation is the Heck coupling (Heck, 1985 T.suji, 1995). The Heck reaction involves the palladium-catalysed arylation of olefinic double bonds (Eqn. (12)) and provides an alternative to Friedel-Crafts reactions for attaching carbon fragments to aromatic rings. 

Palladium-catalyzed arylation of olefins and the analogous alkenylation (Heck reaction) are the useful synthetic methods for carbon-carbon bond formation.60 Although these reactions have been known for over 20 years, it was only in 1989 that the asymmetric Heck reaction was pioneered in independent work by Sato et al.60d and Carpenter et al.61 These scientists demonstrated that intramolecular cyclization of an alkenyl iodide or triflate yielded chiral cyclic compounds with approximately 45% ee. The first example of the intermolecular asymmetric Heck reaction was reported by Ozawa et al.60c Under appropriate conditions, the major product was obtained in over 96% ee for a variety of aryl triflates.62... 

A remarkable dendritic effect in the Heck arylation of olefins by a dendrimer supported Pd-cat-alyst has recently reported by Portnoy et ah A. Dahan, M. Portnoy, Org. Lett. 2003, 5, 1197-1200. 

The Pd(II)-catalysed arylation of olefins by means of different aryltellurium(IV) chlorides has been more extensively investigated. ... 

Arylation of olefins can also be achieved312 by treatment with an arylpalladium reagent that can be generated in situ by several313 methods (1) by treatment of an aryl bromide with a palladium-triarylphosphine complex (ArBr —> ArPdBr") 114 (2) by treatment of an aryl iodide315 with palladium acetate316 in the presence of a base such as tributylamine or... 

In addition to the Suzuki-Miyaura reaction, the authors also found the catalyst 136 to be active toward the arylation of olefins with aryl halides as illustrated in Table 20. Again to conduct a reaction, the authors circulated a solution containing 4-iodoacetophenone 138 (1.00 mmol), the alkene (3.00 mmol) under investigation, and tributylamine 139 (3.00 mmol) in anhydrous DMF (3 ml), through the heated reactor (120 °C) at a flow rate of 2 ml min After 24h, the flow reactor was rinsed with DMF and the... 

Mella, M., Coppo, P., Guizzardi, B., Fagnoni, M., Freccero, M. and Alhini, M. (2001) Photoinduced, ionic meerwein arylation of olefins. Journal of Organic Chemistry, 66, 6344—6352. 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

The arylation of olefins in the presence of Pd(II) salts is similar to the coupling of arenes discussed above. For example, styrene and benzene afford trans-stilbene in the presence of Pd(OAc)2 orPdCl2577 ... 

Electroreduction of triphenylphosphino- or 1,2-bisdiphenylphosphinoethanenickel(II) complexes in ethanol via zero valent complexes with halobenzenes has been described87. Arylation of olefins (equation 55) can be achieved by electro generated Ni(0) complex 106, associated with triphenylphosphine and an alkene (107). The optimum conditions for the reaction include the use of one to three equivalents of triphenylphosphine and a base such as triethylamine30. 

C.C mltPFe] Pd(OAc)2 20-50 °C. Base-free arylation of olefins with arenediazonium salts catalyst stable for at least 4 runs. [76]... 

Figure 6 Scheme illustrating the Heck arylation of olefins (R — alkyl, aryl X = halide or other leaving groups L = ligand). 

Mizoroki, T., Mori, K., Ozaki, A. Arylation of olefin with aryl iodide catalyzed by palladium. Bull. Chem. Soc. Jpn. 1971,44, 581. 

Doyle, M. P., Siegfried, B., Elliott, R. C., Dellaria, J. F., Jr. Alkyl nitrite-metal halide deamination reactions. 3. Arylation of olefinic compounds in the deamination of arylamines by alkyl nitrites and copper(ll) halides. A convenient and effective variation of the Meerwein arylation reaction. J. Org. Chem. 1977, 42, 2431-2436. 

BINAP complexes have been used extensively in asymmetric synthesis, for example in hydrogenations,389,390 olefin isomerizations,390 arylation of olefins,391 and enantioselective allylation of aldehydes.392 Palladium or platinum complexes of (165) find important applications in enantioselective C—C bond formation,393-396 whilst iridium complexes are catalysts for the hydrogenation of nonfunctionalized tri- and tetrasubstituted olefins. 97... 

Another type of application of the concept of the C-0 bond cleavage of anhydrides to arylation of olefins has been reported very recently (Eq.32) [78]. In this reaction, a C-0 bond rupture was accompanied by liberation of CO. 

The exceptional sensitivity of the Heck reaction to a wide variety of factors is intriguing, crying out for a detailed investigation of the intimate mechanism of the process. The generally accepted mechanism for the Heck reaction is presented in Scheme 4. Two reports [ 126,127] have recently appeared, describing mechanistic studies of the Heck arylation of olefins. 

The palladium-catalysed arylation of olefins with aryl halides, the Heck reaction, is usually performed in polar solvents such as acetonitrile or dimethyl sulfoxide, in combination with a base and a Pd(ii) pre-catalyst that may or may not be associated with a phosphorus ligand. Given that quaternary ammonium or phosphonium salts are known to increase reaction rates, ILs emerged as promising solvents for this reaction.In the case of imidazolium-based ionic liquids, the solvent, beside providing an unusual coulombic environment. 

Meenvein reaction [1, 166, before references]. The arylation of olefinic com pounds by diazonium halides with copper salt catalysis was discovered by Meerweit (1939).8 Cupric chloride has been usually employed. Cleland,9 however, prefers cuprous bromide (MCB reagent grade) and recommends that the salt (light green) be washed with acetone until the washings are colorless and then with benzene and then with hexane. The resulting solid is dried at 120° and is only faintly colored. He... 

Arylation of olefins (6, 156).° The vinylic substitution of aryl bromides with diacetatobis(triphenylphosphine)palladium(II) as catalyst is not satisfactory with bromides containing strongly electron-donating groups (OH, NHa). Two solutions have been reported. One is to use an aryl iodide rather than the bromide and palladium acetate alone as catalyst. 

Scheme 1.23. Mechanism of palladium-catalyzed arylation of olefins (Mizoroki-Heck reaction).

Since the pioneering work by Beletskaya and co-workers [8] the intra- and (more commonly) intermolecular arylation of olefins has been shown to proceed very smoothly in aqueous medium in the presence of palladium acetate. At the beginning, the methodology seemed to be limited to aryl iodides under a strong influence of the base it was shown that the presence of potassium acetate instead of carbonate yielded lower reaction temperatures and higher rates [Eq. (3)]. 

Another method that is widely used for C-C bond formation is the Heck coupling [3]. The arylation of olefinic double bonds is mostly catalyzed by palladium complexes in homogeneous solution. Important advantages of this reaction are the broad availability of arylbromides and chlorides and the tolerance of the reaction for a wide variety of functional groups. There were also developed heterogeneous Pd/C catalysts which exhibit high activity for the Heck reaction of aryl halides with olefins. The reaction conditions are 80 200 °C, solvents (NMP, DMF, tolueneAvater), base addition is necessary (NaOAc, amines, alkali carbonates). The reaction scheme can be described as follows (Eq. 8-22). 

However, clear evidence of the IL effect was found for the regioselective arylation of olefins. It is generally accepted that the Heck reaction may proceed via two pathways, a neutral pathway leading to the preferential formation of linear olefins and an ionic counterpart more likely to give rise to branched olefins. Thus, Pd(OAc)2 and l,3-bis(diphenylphosphino)propane (DPPP) immobilized in [G4GiIm]BF4 promote the exclusive a-arylation of several classes of electron-rich olefins with a wide range of aryl iodides and bromides in the absence of halide scavengers (Scheme 3). ... 

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