Benzene derivatives oxidative coupling, arenes

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]. 

DeBoef and co-workers described the aerobic oxidative inter- and intra-molecular coupling of benzoxazole (120) with benzene derivatives in the presence of catalytic amounts of Pd(OAc)2 and heteropolymolybdovanadic acid (HPMV) (Scheme 10.40). Under these conditions, the monoarylated C2 products 121-123 were obtained in moderate to good yields in less than 2 hours extended reaction times led to formation of the undesired 2,3-diarylated products. Problematic coupling partners included anisole (which afforded regiomeric mixtures of the C2 product) and electron-poor or acidic arenes which led to no product formation. Palladation is thought to occur at the more nucleophilic C3 position before migration to the C2 position and biaryl product formation. 

DeBoef and co-workers used a strategy similar to ours in the cross-coupling of an electron-rich arene, benzofuran, with benzene [Eq. (5)]. In the initial report, both electron-rich indoles and benzofurans were regioselectively cross-coupled with benzene, or its derivatives. In this case C2-selectivity on the heterocyclic arene was observed [39]. In a subsequent report, an observation correlating C3/C2-regioselectivity with the choice of oxidant (Cu(OAc)2 or AgOAc) was made [40] and was consistent with the observations previously reported by our group [36] and presented herein (vide supra). 

Early 2011 has produced additional advances in both scope and mechanistic understanding of oxidative cross-coupling reactions, as well as, an application of this chemistry to the synthesis of a pharmacologically relevant compound. With respect to advancing the scope of compatible reaction partners in oxidative crosscoupling reactions, Beifuss communicated the direct coupling of xanthines with simple benzene and its derivatives in which the regioselectivity for the simple arene... 

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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