Arenes coupling

Complex iron(III) salts are frequently used in oxidative arene coupling reactions and quinone formation and tetra-n-butylammonium hexacyanoferrate(III) has several advantages in it use over more conventional oxidative procedures. When used as the dihydrogen salt, Bu4N[H2Fe(CN)6], it oxidizes 2,6-di-z-buty 1-4-methylphenol (1) to the coupled diarylethane (2), or aryl ethers (3) and (4) (Scheme 10.4), depending on the solvent. It is noteworthy that no oxidation occurs even after two days with the tris-ammonium salt. 

Friedel-Crafts alkylations and acylations, followed by reduction of the C=0 group, are most frequently used to synthesize arenes. Coupling reactions can also be employed. 

A catalytic arene-arene coupling can occur in the presence of cooxidants such as Cu11, Fe111 and heteropolyacids along with molecular oxygen, but this reaction is still not selective enough for industrial use. 

Reinhout et al. prepared a (3-CD-calix[4]arene couple 64 in which a dansyl unit is attached to calix[4]arene and oriented toward the secondary face of (3-CD [56], This compound exhibits a fluorescence peak at 538 nm, and its intensity was hardly influenced by guest molecules. The examination of molecular models suggested a very strong self-inclusion of the dansyl group into the CD cavity. 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

Herrmann, W. A. Arene coupling reactions. Applied Homogeneous Catalysis with Organometallic Compounds (2nd Edition) 2002, 2, 822-828. 

The 2-naphthylamine-appended jS-CyD-calix[4]arene couple 49 showed sensitivity for analytes such as steroids and terpenes with different selectivity from the native CyD. On the other hand, the dansyl-appended j8-CyD-calix[4]arene couple 50 did not show any change in fluorescence intensity upon the addition of guests because of the strong inclusion of the dansyl group into the CyD cavity [166]. 

An early example of this arene coupling was reported by Ames and BuU,t who noted the formation of benzofuro[3,2-c]cinnoline in the reaction of 3-bromo -phenoxycinnohne with ethyl acrylate in the presence of a catalytic amount of paUadium(n) acetate (Scheme 2, Eq. 1). Subsequent studies showed that ethyl acrylate is not necessary, and the arylation reaction was achieved by using 0.1 equiv of palladium acetate, 0.2 equiv of triphenylphosphine, and 3 equiv of sodium acetate in DMA at 170 °C (Scheme 2, Eq. 

For reviews on the Fujiwara-Moritani reaction, see (a) Fujiwara, Y. (2002) Palladium-promoted alkene-arene coupling via C—H activation, in Handbook of Organopalladium Chemistry in Organic Synthesis, Vol. 2 (eds E.-i. Negishi and A. de Meijere), John Wiley Sons, Inc., New York, pp. 2863-71 (b) Jia, C., Kitamura, T. and Fujiwara, Y. (2001) Catalytic functionalization of arenes and alkanes via C—H bond activation. Acc. Chem. Res., 34, 633-9 (c) Fujiwara, Y. and Jia, C. (2001) New developments in transition metal-catalyzed synthetic reactions viaC—H bond activation. PureAppl. Chem., 73,319-24 (d) Moritani, I. and Fujiwara, Y. (1973) Aromatic substitution of olefins by palladium salts. Synthesis, 524-33. 

Scheme 16.24 Copper catalysis for alkene amination/arene coupling.

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