Arylations of Electron-Rich Heteroarenes

Palladium-catalyzed arylations of simple electron-rich five-membered heteroarenes with one heteroatom, such as furans, thiophenes, and pyrroles, with aryl iodides, bromides, or chlorides are among the most frequently studied direct arylation reactions [31, 39, 85]. These reactions usually afforded five-membered heterocycles, which were arylated at the position adjacent to the heteroatom in moderate to good yields. These reactions were mainly accomplished with electrophilic catalysts and proceeded more efficiently using aryl bromides with electron-withdrawing groups. This is in agreement with an electrophilic SEAr-type mechanism relying on a palladium(0)/palladium(ll) manifold [86]. Selected results of catalytic direct arylations of simple electron-rich five-membered heteroarenes (124—126) with aryl iodides, bromides, or chlorides are summarized in Table 9.2. 

In contrast, N-methylpyrrole underwent direct arylations atthe 3-position [93], and a comparable result was obtained in direct arylations of N-phenylpyrrole employing the electron-deficient rhodium complex 114, in combination with Ag2C03 under microwave irradiation [74b]. Selected examples of regioselective arylations of substituted five-membered heteroarenes are summarized in Table 9.3. 

Direct arylations of benzo[fo]furans, benzo[b]thiophenes, and indoles displayed different regioselectivity patterns. The chemical behavior of benzo[b]furans and benzo[fo]thiophenes are rather similar to those of non-benzannelated furans and thiophenes, respectively (Table 9.4). 

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Regioselectivity for direct arylation of electron-rich heteroarenes ... 

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