Electrophilic aromatic alkenylation

A second group of aromatic substitution reactions involves aryl diazonium ions. As for electrophilic aromatic substitution, many of the reactions of aromatic diazonium ions date to the nineteenth century. There have continued to be methodological developments for substitution reactions of diazonium intermediates. These reactions provide routes to aryl halides, cyanides, and azides, phenols, and in some cases to alkenyl derivatives. 

Hydroarylation can also be mediated by Au(I) and Au(III) (Scheme 33) (384). In the case of aryl substituted alkynes, the Au(III) Ji complex undergoes electrophilic aromatic substitution with the electron-rich arene to give aLkenyl-Au(III) complex, which is immediately protonated by the H generated upon C C bond formation. For the Au(I)-catalyzed hydroarylation, the cationic gold complex k coordinates the alkyne, with subsequent nucleophilic attack by the arene from the opposite face leading to an alkenyl-gold complex, which is protonated to the desired products. The nature of the reaction causes the regioselectivity of this reaction to be sensitive to electronic rather than steric factors. 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Additions of aromatic C-H bond to olefins and acetylenes result in the formation of aryl-alkyl and aryl-alkenyl bonds. This type of addition reaction is not applicable to aryl-aryl bond formation. Catellani and Chiusoli [52] reported the first example of this type of arylation in 1985. To date, several arylation reactions of aromatic rings have been developed. In almost all cases, C-H bond cleavage proceeds through electrophilic substitution with transition-metal complexes [53]. 

Arylboronic esters (Figure 5.39) and arylboronic acids (Figure 5.40) can also react with unsaturated electrophiles, which cannot be introduced in one step into metal-free aromatic compounds these reactions are with alkenyl bromides, iodides, and triflates... 

Aromatic hydrocarbons are alkenylated with allenylsilane in the presence of GaCl3 at — 90 °C. A modest level of 0r// 0-seleetivity is observed. Organogallium electrophiles generated from allenes and GaCl3 are active intermediates in this reaction. While the allenylsilane reacts exclusively at the central carbon, 1,2-alkadiene reacts at the terminal carbon predominantly (Scheme 151).446... 

Acid chlorides are suitable electrophiles. As aromatic and 1-alkenyl halides, bromides and iodides are generally good substrates. Until quite recently, the use of the corresponding chlorides, which are cheaper and often more readily accessible, had been limited to that bearing a strongly electron-withdrawing substituent at a proper position. The use of nickel catalyst [47], bulky phosphine [119], and heterocyclic carbene ligand (Scheme 23, Table 1) [116] enabled aryl chlorides to take part in the reaction. 

Application of this mechanochemical halogenation methodology to aromatics 106-108 possessing unsaturated functionalities like alkenyl or alkynyl groups led to the halogen addition to the C—C double or triple bond, without formation of electrophilic substitution products (Scheme 4.29). 

Since the early reports of the Fujiwara-Moritani reaction [2], catalytic alkenylation procedures for a broad range of aromatic substrates with various alkenes have been developed. A proposed mechanism for these Fujiwara-Moritani-type reactions is illustrated in Scheme 18.4 [4]. The reaction is initiated by electrophilic attack on an arene by a cationic palladium species [PdOAc]+, generated in situ from Pd(OAc)2, to form an arylpalladium intermediate. Subsequent alkene insertion and fi-hydrogen elimination may occur to produce an alkenylarene derivative and HPdOAc. The latter may be reoxidized by an oxidant to regenerate Pd(OAc)2. 

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