Arenes, arylation

Carbonyi-metall- Verbindimgen Ar-NO Ar—NHj Ar—N=N—Ar Ar—N=N—Ar Ar-NH-NH-Ar Nitroso-arene Aryl-amine Diaryl-diazen-l-oxide Diaryl-diazene 1,2-Diary 1-hydrazine IV/ld, 535ff. (1981) ... 

It was recognized in the 1930s that Swarts work on (trifluoromethyl)benzene opened the way to the synthesis of many side-chain-fluorinated arenes. Aryl trichloromethyl groups were found to be readily converted to trifluoromethyl groups by anhydrous hydrogen fluoride, and systematic work in the area was initiated. This included commercial investigations, and patents were filed on potential uses of both side chain and nuclear fluorinated arenes. 

The reaction rates decrease in the series C=C RX RNO, and ArN02 RNO and ArNO > C=NOH C=NR > RCN. Arenes, aryl halides, esters, and amides are not reduced. The usual catalyst poisons have no effect or only slight effect. The catalyst is recovered quantitatively and can be reused without purification. 

C-H/C-M Arene Arylation (Rh-Catalyzed Oxidative Coupling Methods). 259... 

C-H/C-X arene arylation methods (Rh-catalyzed condensation methods)... 

Third and finally, a short comparison of rhodium-catalyzed and other arene C-H arylation methods highlighting some of the challenges to overcome in the development of improved arene arylation methods will be given. 

The [(OEP)Rh]+ scaffold, while efficient at the metalation of arenes, is ill-suited for catalytic biaryl bond formation. The generation of an intermediate bearing two aryl groups cis to each other as a typical precursor to biaryls through reductive elimination is impossible due to the ligand-imposed geometry at the metal center. Nonetheless, other highly electron-deficient Rh(III) complexes can offer suitable entries into catalytic routes for electrophilic arene arylation. This is especially the case if the electrophilic Rh(III) center can be accessed in situ from the oxidative addition of Ar-X to a Rh(I) complex (Scheme 4). 

Reports of Rh-catalyzed C-H/C-H arene arylations are rare and provide limited scope and mechanistic data. In a first report, the Barrett group describes the oxidative 0,0 -dimerization of phenols in the presence of a rhodium(III) catalyst... 

Thermolysis of these complexes does not induce simple reductive elimination of the arenethiolato and hydrido ligands, but gives a mixture of arene, aryl sulhde, and phosphine sulfide. 

Naphthalene Phenylation. During the course of mechanistic studies on direct C-H arene arylations, it was observed that la led to the phenylation of naphthalene (7). The reaction afforded product 8 as a mixture of regioisomers in 31% yield (eq 5). 

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