Thermal (Iodide) and Photoinduced Electron-Transfer Catalysis in Biaryl Synthesis via Aromatic Arylations with Diazonium Salts

Abstract

The dediazoniative arylation of various aromatic hydrocarbons (Ar‘H) with diazonium salts (ArN2 +) in acetonitrile can be readily effected to biaryls (Ar-Ar‘) in high yields, simply by the addition of small (catalytic) amounts of sodium iodide. [In the absence of Ar‘H, the competitive iodination to ArI is nearly quantitative.] Iodide catalysis of biaryl formation is efficiently mediated by aryl radicals (Ar•) that participate in an efficient homolytic chain process in which ArN2 + acts as a 1-electron oxidant. The complex kinetics of such an electron-transfer chain or ETC process (Scheme 1) is quantitatively verified by computer simulation of the Ar‘H-dependent (a) competition between arylation vs iodination and (b) catalytic efficiency of iodide, using the GEAR algorithms. ETC catalysis also pertains to the alternative photochemical procedure for arylation (in the absence of iodide), in which the deliberate irradiation of the charge-transfer band of the precursor complex [ArN2 +, Ar‘H] initiates the same homolytic chain arylation. The latter underscores the mechanistic generality of the ETC formulation for various types of catalytic dediazoniations of aromatic diazonium salts.

Radical Arylation of Phenols, Phenyl Ethers, and Furans

“…Methyl 4-(5-methylfuran-2-yl)benzoate (25): [50] Compound 25 was prepared from methyl 4-aminobenzoate and 24 according to the modified procedure described for the preparation of 23. To increase the solubility of 24 in the aqueous phase, CH 3 CN (18 mL) was added.…”

“…Several examples have been described with pentafluorophenyl diazonium salts. [25] The perfluorinated diazonium salt, moreover, had the advantage that no attack of the aryl radical on the diazonium salt itself, either in the sense of a biaryl coupling or in the sense of an azo coupling, was observed.…”

Pentakis(trifluoromethyl)benzenediazonium Cation: A Useful Building Block for the Syntheis of Trifluoromethyl‐Substituted Derivatives

“…[23] Diazonium salt 1, with yield 49 %, was prepared from aniline 2 using NOBF 4 in dry MeCN and precipitated with dichloromethane as previously described. [11] During diazotization, phenol 4 (19 %) appeared in the reaction mixture most of the time, which indicated the high reactivity of diazonium salt 1 with water derived from the reaction itself and the moisture in the solvents. At the same time, crystals of 1 could be isolated.…”

“…[4] Arenediazonium compounds, next to halides, are often used to produce aryl radicals, [5,2] which are intermediates for various amino acids, [6] pharmaceuticals, [7] heterocycles, [8] arylolefins, [9] biaryls, [10,11] and other useful materials.…”

Modern Developments in Aryl Radical Chemistry

“…Electron-deficient aryl diazonium salts such as the pentafluoro derivative can offer the attractive option to conduct radical arylations as chain reactions with an S RN 1 mechanism (Scheme 35) [151]. In these special cases, only catalytic amounts of an initiating reductant, such as sodium iodide, are required.…”