Arylation of styrene

Under Lewis-acid-catalyzed conditions, electron-rich arenes can be added to alkenes to generate Friedel-Crafts reaction products. This subject will be discussed in detail in Chapter 7, on aromatic compounds. However, it is interesting to note that direct arylation of styrene with benzene in aqueous CF3CO2H containing H2PtCl6 yielded 30-5% zram-PhCH CHR via the intermediate PhPt(H20)Cl4.157 Hydropheny-lation of olefins can be catalyzed by an Ir(III) complex.158... 

E)-stilbene was the exclusive product of the Pd colloid-catalyzed Heck arylation of styrene with chlorobenzene. Recently, a polymer-mediated self-assembly of functionalized Pd and Si02 nanoparticles have been found to be highly active catalysts for hydrogenation and Heck coupling... 

C4)4N]Br Palladium(O) carbene complexes NaOAc 140-160 °C. Phosphine-free arylation of styrene with arylchlorides excellent conversions with both activated and non-activated arylchlorides product extracted with Et20. [67]... 

C4)4N]Br Supported Pd nanoparticles Bu3N 130 °C. Phosphine-free arylation of styrene with arylchlorides palladium immobilised on layered double hydroxide microwave irradiation leads to significant rate acceleration product isolated by distillation. [81]... 

The reaction between aryl or alkenyl halides or arenediazonium salts and al-kenes catalyzed by palladium complexes, the so-called Heck reaction, has been performed in aqueous media. Arylation of styrene or acryHc acid derivatives occurred in high yields in the presence of a free-ligand palladium complex as catalyst and a base (Na2C03 or K2CO3) [96-98] and, eventually, a quaternary ammonium salt [Eq. (14)] [99,100]. 

The development of Pd colloids as catalyst for C—C coupling reactions is rather recent [5]. The first example was reported by BeUer et al. in 1996 they used preformed Pd coUoids stabihsed by tetra-octylammonium bromide prepared following the Bonneman procedure in the Heck arylation [6]. The colloidal system was effective for the Heck arylation of styrene or butyl acrylate by activated aryl bromides, but showed only moderate to little activity for deactivated aryl bromides and aryl chlorides. To obtain these results, the authors found that the colloidal pre-catalysts must be added slowly to the reaction mixture to avoid the formation of inactive palladium black at the beginning of the reaction. 

Fujiwara and Moritani discovered in 1%7 that when styrene-palladium chloride complex 1 was heated in benzene (2a), toluene (2b) or p-xylene (2c), in the presence of acetic acid, tra 5-stilbene (3a), rram -methylstilbene (3b) or tra -2,5-dimethylstilbene (3c) were produced respectively, albeit in low yields (Equation (9.1)) [3], No stilbene derivative was obtained, however, in the case of mesitylene, which may be attributed to the steric hindrance of three methyl groups on the benzene ring [3]. Shortly thereafter, Fujiwara and coworkers [4] found that arylation of styrene (4a) occurred much more efficiently in the presence of stoichiometric palladium acetate instead of the styrene-palladium chloride complex (1). Thus, equimolar amounts of styrene (4a) and palladium acetate were refluxed in benzene (2a) in the presence of acetic acid, affording a 90% yield of rran -stilbene (3a). In the case of toluene (2b) and p-xylene (2c), fran5-4-methylstilbene (3b, 58%) and rran5-2,5-dimethylstilbene (3c, 47%) were obtained (Equation (9.2)). 

Scheme 10.2 Copper-catalysed intermolecular arylation of styrene (2).

With stoichiometric amounts of Zn powder (27) as reducing agent, [NiCl2(PPh3)2] (28) allowed for catalytic arylations of styrene (2), and, albeit less selectively, of ethyl acrylate (29) [31], The catalytic system exhibited a relatively broad scope, enabling the use of aryl iodides, bromides and chlorides (Scheme 10.9). When using iodobenzene (13) as electrophile, an increase in isolated yield was achieved through the addition of water. 

The use of pyridine (31) as additive allowed for more selective and efficient nickel-catalysed arylations of styrenes (Scheme 10.10) [32, 33]. Aryl and alkyl bromides gave good yields of isolated products. With respect to the latter, secondary alkyl bromides proved superior to primary ones. However, use of methyl acrylate (1) as substrate yielded predominantly products originating from conjugate additions, rather than Mizoroki-Heck-type reactions. 

On the contrary, for layered highly efficient arylations of styrenes and methyl acrylate (1) were accomplished with the inorganic base NaOAc (Scheme 10.19) [45]. As for the previously reported platinum-based catalysts, high reaction temperatures were mandatory. The reusable catalyst was found to be further limited to the use of iodoarenes as electrophiles, since bromoarenes gave no Mizoroki-Heck-type coupling at all. 

Scheme 10.19 Platinum-catalysed intermolecular Mizoroki-Heck-type arylation of styrene...

A heterogeneous cobalt catalyst was employed for arylations of styrene (2) and two acrylates with aryl iodides. Generally, isolated yields were significantly lower than those observed for heterogeneous nickel catalysts [24]. Further, a silica-supported poly-y-aminopropylsilane cobalt(II) complex was reported as a highly active and stereoselective catalyst for Mizoroki-Heck-type reactions of styrene (2) and acrylic acid (16) using aryl iodides [23,25]. 

Scheme 1.16 Palladium-catalyzed arylation of styrene (35) (Heck, 1972).

The reactions discussed so far involved attack of soft nucleophiles on rr-alkene palla-dium(II) complexes. Hard carbon nucleophiles such as methyl lithium also react with these complexes.f The alkylation proceeds here by initial attack at the palladium atom. Subsequent di-insertion of the olefin into the Pd—C bond and di-/S-hydride elimination result in liberation of the alkylated alkene. The arylation of styrene by Grignard reagents can also be performed,the reaction being realized in the presence of stoichiometric or catalytic quantities of palladium chloride (Scheme 14). 

Pd catalysts also promote the decarbonylative Mizoroki-Heck reactions of even poorly reactive p-nitrophenyl aryl carboxylates with olefins to give the vinyl arenes in good to excellent yields, along with CO and the corresponding phenols [40]. Mizoroki-Heck-type arylation of styrene and acrylate esters by use of aroyl chlorides can also be performed in the presence of PdCljlPhCN) / (PhCHjlBUjNCl as the catalytic system without adding base [41]. 

Photoaddition. Ir(ppy)3 (ppy = 2-phenylpyridine) was used as a photoredox catalyst in the oxyatylation, aminoarylation, and o [Pg.68]

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