Arenes cross-coupling with aryl halides

Aryl-aryl coupling is an important synthetic reaction. Efficient aryl-aryl coupling is possible by Pd-catalyzed cross couplings of aryl halides with aryl metal (Mg, Zn, B, Sn, Si) compounds, and the methods have wide applications. On the other hand, a simple Pd-catalyzed arylation of arenes with aryl halides has been known. Although less attention has been paid, the importance of the reaction is increasing as more efficient methods of aryl-aryl bond formation become available. The arylation of arenes with aryl haUdes is treated in this section. 

Negishi and his co-workers have developed several approaches to allylated arenes involving cross-coupling reactions. Preliminary studies established that aryl-aluminium, zirconium, or zinc compounds readily undergo palladium-catalysed cross-coupling with allylic halides or acetates (Scheme 19). Later in the year it was shown that allylic alcohol derivatives containing OAlR2,... 

The synthesis of alkene-substituted arenes by cross-coupling can, in principle, be achieved either by the reaction of alkenyhnetals with aryl halides and related electrophiles or by that of arylmetals with alkenyl electrophiles. For the sake of simplicity, the former reaction is termed the alkenyl-aryl coupling, while the latter is termed the aryl-alkenyl coupling in this Handbook. Similar terms may be devised by linking the carbon groups of the organometal and organic electrophile with a dash in this order. 

Pentacoordinated silicate TASF, one of the best activators of organosilicon compounds in the Pd-catalyzed cross-coupling reaction as we have seen, is found to be involved in the coupling reaction with aryl halides in the absence of other organosilanes to give methylated arenes (Scheme 24). Recently, DeShong and co-workers also reported that tetrabutylammonium triphenyldifluorosilicate, another pentacoordinated silicate, is applicable for the phenylation of aUyl benzoates and aryl halides in the presence of a palladium catalyst. 

B-Alkyl Suzuki coupling offers tremendous opportunities for one pot CTOSS-coupling between any alkene and aryl halides, as the intermediate alkylboranes are typically not isolated and can be used in situ for further cross-coupling. Some of the representative examples of this coupling include the synthesis of pyridyl alcohols 148 (Scheme 28.41), synthesis of alkyl arenes 149 (Scheme 28.42), and coupling of cyclopropyl boronic acid with aryl halides to obtain cyclopropyl arenes 153 (Scheme 28.43). ... 

Perfluoroarenes were also found to be highly reactive coupling partners in intermolecular direct arylation [68, 69]. A wide range of aryl halides can be employed, including heterocycles such as pyridines, thiophenes, and quinolines. A fluorinated pyridine substrate may also be cross-coupled in high yield and it was also found that the site of arylation preferentially occurs adjacent to fluorine substituents when fewer fluorine atoms are present. Interestingly, the relative rates established from competition studies reveal that the rate of the direct arylation increases with the amount of fluorine substituents on the aromatic ring. In this way, it is inversely proportional to the arene nucleophilicity and therefore cannot arise from an electrophilic aromatic substitution type process (Scheme 7). 

In the cross-coupling reaction, starting from the simple arene (with directing group), palladation by a Pd(II) salt would lead to the formation of the palladacyclic complex (Ar1Pd(II)L) (Scheme 3). After the transmetallation and reductive elimination processes, the biaryl product is obtained together with Pd(0). If the Pd(0) can be further oxidized to Pd(II) catalyst, a catalytic cycle will be formed. By accomplishing this, arenes (C-H) are used to replace the aryl halides (C-X). Similarly, arenes (C-H) can be used to replace the aryl metals (C-M). 

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