Gomberg-Bachmann biphenyl synthesis

Gomberg-Bachmann biphenyl synthesis. Reaction of stable arenediazonium tet-rafluoroborates or hexafluorophosphates in an aromatic solvent with potassium acetate (2 equiv.) and a phase-transfer catalyst results in biar Is in high yield. Crown ethers, Aliquat 336, and tetrabutylammonium hydrogen sulfate arc all effective catalysts. The reaction is useful for synthesis of unsymmetrical biaryls. The ortho-isomer predominates in reactions with a monosubstituted benzene. The most selective method is to couple a substituted arenediazonium salt with a symmetrical arene. 

GOMBERG-BACHMANN BIPHENYL SYNTHESIS Phase-transfer catalysts. 

Due to the relatively slow addition of aryl radicals to substituted benzenes [7], inter-molecular Gomberg-Bachmann type arylations often require favorable reaction conditions in which the substrate arene is used as solvent to be efficient. To achieve good control and selectivity in the addition step, radical biphenyl synthesis can alternatively be conducted in an intramolecular fashion by taking advantage of a removable tether. In the first examples, sulfonamides, sulfonates [130], and azetidinones [131] were employed as linkers between the two aryl moieties. Recently, phosphinates [132] and siloxanes [133] have been used for this purpose (Scheme 25). 

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