Homocoupling phenolates

Novel heteroquaterphenoquinones were synthesized by a stepwise cross-coupling reaction or by a more convenient one-pot oxidative homocoupling reaction of the heterocycle-substituted phenols (Scheme 20, <96JOC4784 see also 95TL8055>). 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

Oxidative biomimetic aryl coupling reactions of electron-rich (phenolic) aromatic hydrocarbons have occasionally been reported for selected examples see the reviews by Bringmann et al. [93 a] and the review on the ellagitannin chemistry by Quideau and Feldman [93b]. Oxidative homocoupling has also been achieved with palladium(II) acetate via the 5-arylpalladium complexes [78 d], but stoichiometric amounts of the palladium reagents are often required and acetylation can occur as a side reaction [93a]. 

Anodic Homocoupling of Anions, at-Complexes, Organometallics and Phenolates... 

Pd nanoparticles supported on PANI-NFs are efficient semi-heterogeneous catalysts for Suzuki coupling between aryl chlorides and phenylboronic acid, the homocoupling of deactivated aryl chlorides, and for phenol formation from aryl halides and potassium hydroxide in water and air [493], PANl-NF-supported FeCl3 as an efficient and reusable heterogeneous catalyst for the acylation of alcohols and amines with acetic acid has been presented [494]. Vanadate-doped PANI-NFs and PANI-NTs have proven to be excellent catalysts for selective oxidation of arylalkylsulfides to sulfoxides under nuld conditions [412]. Heterogeneous Mo catalysts for the efficient epoxidation of olefins with ferf-butylhydroperoxide were successfully synthesized using sea urchin-Uke PANI hollow microspheres, constructed with oriented PANI-NF arrays, as support [495]. Pt- and Ru-based electrocatalyst PANI-NFs—PSSA—Ru—Pt, synthesized by the electrodeposition of Pt and Ru particles into the nanofibrous network of PANI-PSSA, exhibited an excellent electrocatalytic performance for methanol oxidation [496]. A Pt electrode modified by PANI-NFs made the electrocatalytic oxidation reaction of methanol more complete [497]. Synthesis of a nanoelectrocatalyst based on PANI-NF-supported... 

Apart from aryl halides, other very easily available substrates for nickel-catalysed biaryl (II) synthesis are aryl sulfonates (XII). Method D is very efficient in the homocoupling reactions of substituted aryl sulfonates in good to excellent yields [15], Table 6. Substituted aryl sulfonates are readily obtained from phenols and trifluoromethanesulfonic anhydride, benzenesulfonyl-, tosyl- or methanesulfonyl chloride in pyridine, or in a suitable inert solvent such as dichloromethane in the presence of triethylamine or Hiinig s base. Among other nickel complexes, Ni(dppe)Cl2 and Ni(dppf)Cl2 have been used (10 mo1%) as slightly less versatile catalysts for the homo-couplings of naphthyl sulfonates in refluxing THF, DMF or their mixtures [42]. 

Recent advances have enabled the participation of phenol-derived substrates, including triflates and mesylates, to participate in homocoupling processes to generate functionalized biaryls. In these processes, the addition of a tetraalkylammonium iodide was beneficial in enhancing the rate of homocouplings. A broad range of symmetrical biaryls may be generated efficiently by this procedure. 

Pd(OH)(OOH)(PPh3)2 yields a homocoupling biaryl and hydrogen peroxide. Thus, this type of dimerization competes with the formation of phenols (Eq. 55). However, even if the experimental operation strictly controls oxygen, another slow process leading to such a dimer is unavoidable when naked palladium is used as the catalyst. 

Electrosynthesis Using Diamond Electrode, Scheme 2 Phenol homocoupling reactions in iluorinated media... 

In none of our extensive test reactions of aryl halides with benzoic acids carried out in the presence of diverse palladium catalysts, we were able to detect even traces of the unsymmetrical biaryl. Instead, we observed mostly homocoupling and dehalogenation products in some cases along with phenol esters. The conversion of aryl triflates with potassium benzoates did not lead to the desired biaryls, either, but to the phenol esters instead. 

The participation of aryl triflates in this reaction is important from a synthetic standpoint as many substituted phenols are readily available from either commercial sources or well established synthetic procedures. The first report of the homocoupling of aryl triflates utilized ultrasonication to effectuate the reaction (26,27). Later, reaction conditions were developed which did not require ultrasonication. Alternatively, Pd-catalysts were effective in the electrochemically (52) and chemically (55) driven coupling of aryl triflates. Unfortunately, the high... 

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