Halides, aryl, arylation aromatic compounds

Intermolecular photoreaction of an aryl halide with another aromatic compound may lead to the formation of biaryls. In this section several examples of such reactions will be discussed. In some cases, information concerning the reaction mechanism is available but the depth to which mechanisms have been investigated varies greatly. In many cases aryl radicals formed by homolysis of the carbon-halogen bond are the reactive species. Such radicals may also be produced via electron transfer, followed by departure of halide anion. In some cases aryl cations have been proposed as intermediates. Intermolecular bond formation may also be preceded by charge transfer within an exciplex or by formation of radical ion pairs. 

Since Grignard reagents can easily be obtained from aryl halides, they are of special value in the s nthesis of many aromatic compounds, particularly as, for reasons already stated (pp. 270, 276), aromatic compounds cannot generally be prepared by means of ethyl acetoacetate and ethyl malonate. 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

The most common types of aryl halides m nucleophilic aromatic substitutions are those that bear o ox p nitro substituents Among other classes of reactive aryl halides a few merit special consideration One class includes highly fluormated aromatic compounds such as hexafluorobenzene which undergoes substitution of one of its fluorines on reac tion with nucleophiles such as sodium methoxide... 

Diaryl sulfones can be formed by treatment of aromatic compounds with aryl sulfonyl chlorides and a Friedel-Crafts catalyst. This reaction is analogous to Friedel-Crafts acylation with carboxylic acid halides (11-14). In a better procedure, the aromatic compound is treated with an aryl sulfonic acid and P2O5 in polypho-sphoric acid. Still another method uses an arylsulfonic trifluoromethanesulfonic anhydride (ArS020S02CF3) (generated in situ from ArS02Br and CF3S03Ag) without a catalyst. ... 

Schliiter et al. [161 were the first to describe coupling of aromatic compounds containing aryl-magnesium halide and aryl halide functions, catalyzed by Ni(0) compounds. Here, the authors adapted the principle of attaching solublizing side chains (in the 2- and 5- positions) and arrived at soluble and processable... 

Synthetically important substitutions of aromatic compounds can also be done by nucleophilic reagents. There are several general mechanism for substitution by nucleophiles. Unlike nucleophilic substitution at saturated carbon, aromatic nucleophilic substitution does not occur by a single-step mechanism. The broad mechanistic classes that can be recognized include addition-elimination, elimination-addition, and metal-catalyzed processes. (See Section 9.5 of Part A to review these mechanisms.) We first discuss diazonium ions, which can react by several mechanisms. Depending on the substitution pattern, aryl halides can react by either addition-elimination or elimination-addition. Aryl halides and sulfonates also react with nucleophiles by metal-catalyzed mechanisms and these are discussed in Section 11.3. 

Trimethylgermyl substituents in aromatic compounds are easily removed with halogen, yielding the corresponding aryl halide, as shown in reaction 8119. Another example of phenyl group displacement was carried out with bromine as in reaction 2 (see end of Table 2)64. 

Arylation of C-H bonds is achieved by coupling reactions of C-H bonds with aromatic compounds such as halides, triflates, and organometallic reagents. Early works in this field involve the reaction of aryl halides with norbornene. As shown in Scheme 5, the coupling reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as a... 

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