Phase-transfer-catalyzed Gomberg-Bachmann synthesis of unsymmetrical biarenes: a survey of catalysts and substrates

Abstract

Two problems have hindered the Gomberg-Bachmann (GB) and Pschorr reactions of arenediazonium cations: the instability of the arenediazonium salts and side reactions. Arenediazonium tetrafluoroborate and hexafluorophosphate salts can be prepared in high yield and purity and can be stored safely. Unfortunately, these salts are insoluble in most nonpolar organic solvents. Crown ether complexation or other phase-transfer (pt) catalytic methodology can ameliorate this situation, and reactions conducted by the approaches outlined herein often afforded coupling or cyclization products in high yield and corresponding purity. The use of crown ethers, quaternary 'onium salts, lipophilic carboxylic acid salts, and even the polar cosolvent acetonitrile increase the utility of the ptGB reaction dramatically. Sixty examples of couplings are reported along with an assessment of selectivities. A number of examples are also presented of phase-transfer-type Pschorr cyclizations. In the latter case, the use of potassium superoxide, K02, is introduced to suppress indazole formation.

Radical Arylation of Phenols, Phenyl Ethers, and Furans

“…Keywords: diazonium salts · biaryls · phenols · phenyl ethers · radical reactions phase-transfer variants, [17] the application of oxygen as reoxidant in tin-mediated radical biaryl syntheses, [18] and the application of arylhydrazines and Grignard reagents [19,20] as aryl radical precursors. Biaryl compounds can also be obtained by radical addition to a nonaromatic precursor and subsequent aromatization.…”

Hydroxy‐ and Aminophenyl Radicals from Arenediazonium Salts

“…[29] Although benzene can be considered as more polar than cyclohexane, [30] still almost no transfer of radicals 1 or 2 from the aqueous to the organic phase was observed. [31] As a control experiment, we investigated the behavior of a "common" diazonium salt, such as 22, under two-phase reaction conditions (Scheme 4).…”

Modern Developments in Aryl Radical Chemistry

“…The development of phase-transfer Gomberg-Bachmann (PTGB) reactions more than 30 years ago turned out to be a major advance in radical biaryl synthesis [145]. To this day these protocols represent benchmarks, since good to high yields can be obtained with a large variety of substrates under non-demanding conditions.…”