Oxidative couplings of arenes

Scheme 1. The essentials of biaryl formation by oxidative coupling of arenes.

It is difficult to make generalizations about the oxidative coupling of arenes, given the number of possible mechanisms. Factors that influence the reaction course include the oxidation potential of each aryl unit, the substitution pattern, the solvent system, and the oxidation reagents themselves. Optimization then becomes a case-by-case effort in order to obtain the best selectivities and the best yields. Nevertheless, results over the last decade have... 

Biaryls. Biaryls can be prepared by oxidative coupling of arenes with palladium(II) compounds, but the coupling is not regioselective. Regioselectivity is considerably improved by use of TTFA as oxidant and only catalytic amounts of Pd(OAc),. Formation of 4,4 -biaryls is favored from arenes substituted with either electron-donating or moderately electron-withdrawing substituents. The first step is thallation to form ArTlfOCOCF,)-. Example ... 

Aryl C-H bonds are known to be active and thns benzenes undergo auration, which has been exploited as the oxidative coupling of arenes. ... 

Scheme 8. Iron(III)-mediated oxidative coupling of arenes 25...

Scheme 17. Vanadium(V)-mediated oxidative coupling of arene 48...

Oxidative coupling of arenes and olefins, as discovered by Fujiwara and co-workes [11] ... 

In addition to the industrial apphcations, in Scheme 8.1, other reactions have been the focus of extensive research and development. For example. Chapter 12 surveys the research efforts directed toward Pd-catalyzed oxidative carbonylation of phenol affords the important monomer, diphenyl carbonate (Scheme 8.2a). Other reactions of potential industrial significance highlighted in this chapter include the oxidation of alcohols to aldehydes and ketones (Scheme 8.2b), oxidative coupling of arenes and carboxylic acids to afford aryl esters (Scheme 8.2c), benzylic acetoxylation (Scheme 8.2d), and oxidative Heck reactions (Scheme 8.2e). The chapter concludes by highlighting a number of newer research developments, including ligand-modulated catalytic oxidations, Pd/NO cocatalysis, and alkane oxidation. 

Pd-catalyzed aerobic oxidative coupling of arenes provides efficient access to biaryl compounds. The reaction exhibits near-ideal atom economy with water as the sole by-product however, it typically suffers from low regioselectivity and the blaryl products are often susceptible to further oxidation to afford oligomers. 

Oxidative coupling of arenes with alkenes consumes a stoichiometric amount of Pd(II). Efficient catalytic reaction of cinnamate (165) with benzene to afford 166 was carried out using BQ and f-butyl hydroperoxide as oxidant in AcOH [70]. The coupling proceeds smoothly in the presence of catalytic amounts of Pd(OAc)2 and molybdovanadophosphoric acid (HPMoV) under oxygen (1 atm) in AcOH [71]. 

SYNTHESIS OF BIARYLS AND POLYARYLS BY OXIDATIVE COUPLINGS OF ARENES... 

Mechanisms of oxidative couplings of arenes to biaryls and polyaryls... 

Table 2. The yields of symmetrical biaryls obtained by oxidative coupling of arenes with T1(CF3C00)3 [12]...

Another useful one-electron oxidant is cobalt(III) trifluoroacetate which, in some instances, induces the oxidative coupling of arenes, but, with aromatic substitution, trifluoroacetoxylation, as the main side-reaction [86]. 

Nitronium salts are capable to induce the OCA reaction via generation of aryl radical-cations, even with relatively electron-poor arenes, such as benzene [105]. Namely, NOBF4 in catalytic amount (1-5 mol%) efficiently catalysed the oxidative coupling reactions of polyalkoxybenzenes in dichloromethane (containing 20% trifluoroacetic acid) under exposure to air affording the corresponding biaryls in almost quantitative yields [105]. Finally, oxidative couplings of arenes can be accomplished under the classical Scholl reaction conditions [106]. For example, 1-ethoxynaphthalene (419) was coupled to binaphthyl 420 by reaction with anhydrous aluminum chloride and nitrobenzene (as oxidant) in 65% yield, Scheme 28. 

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