Thiophene, oxidative coupling, arenes

The oxidative coupling of thiophene, furan[338] and pyrrole[339,340] is also possible. The following order of reactivity was observed in the coupling of substituted furans[338] R = H > Me > CHO > CO Me > CH(OAc)i > CO2H. The cross-coupling of furans and thiophenes with arene is possible, and 4-phenylfurfural (397) is the main product of the cross-coupling of furfural and benzene[341]. 

A small amount of bisthiophene was isolated when thiophene was treated with Pd(OAc), [35]. The oxidative couplings of a thiophene with thiophene, furan, or substituted arenes were achieved in poor to moderate yields using PdfOAc), in HOAc [36-38]. The oxidative couplings of thiophene or benzo[Z ]thiophene with olefins also suffer from inefficiency [39]. 

Recyclable [bis(trifluoroacetoxy)iodo]arenes 66 and 70 are excellent reagents for oxidative coupling of thiophenes or pyrroles [79] and for direct oxidative cyanation of heteroaromatic compounds using Me3SiCN as source of cyanide anion [80,81]. 

The arene substrates are not limited to simple benzene derivatives. A variety of het-eroarenes can also participate in alkene arylations to generate the desired coupling products. Stoichiometric oxidative coupling of aromatic heterocycles such as furan, thiophene, selenophene, A-methylpyrrole, benzofiiran and benzothiophene with a variety of alkenes, including acrylonitrile, styrene and methyl acrylate, have been extensively studied by Fu-jiwara and coworkers [8]. Furan, thiophene, selenophene and A-methylpyrrole are easily alkenylated with alkenes to give 2-alkenylated and 2,5-dialkenylated heterocycles in relatively low yields (3 6%) [8a], while the reactions of benzofuran and benzothiophene with alkenes produced a mixture of 2- and 3-alkenylated products [8b]. 

Oxidative Coupling. An efficient C-H/C-H coupling of fu-rans and thiophenes with xanthines was developed (eq 169). A copper salt acts as the oxidant in SeAt/CMD transformation. In addition to xanthines, other heterocycles employed were imidazoles, benzimidazoles, benzoxazoles, or quinoline Af-oxides. Furthermore, xanthines can also oxidatively couple with unactivated arenes in the presence of Ag(I) and O2 as oxidants. ... 

Silver salts have been used extensively as oxidants in many different types of biaryl CDC reactions, such as the arylation of indoles, " " azoles, thiophenes," " chromones, xanthines," uracils," and pyridine-iV-oxides." " " They are also used in the inter- and intramolecular coupling of simple arenes. ° Recently, Zhang and co-workers oxidatively coupled thiophene derivatives and fluorinated arenes, using only 2.5 mol% of Pd(OAc)2 as a catalyst and AgCOs as an oxidant (Scheme 6.8)." " ... 

The photolysis of Os(CO)5 in benzene has led to (n -C6H6)Os(CO)4 which is said to be similar to, but more reactive than, the iron analogue.The dimeric halide complexes [(arene)MX2]2 (M= Ru, Os) remain a useful entry into arene complex chemistry. The development of the ligand chemistry of these complexes continues.These substrates have also been used to make metalloborane complexes, used as catalysts for the coupling of furans and thiophenes and simple derivatives such as [ (p-cymene)20s2(0H)3] used for the catalytic oxidation of aldehydes to carboxylic acids. ... 

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