Mechanism of dediazoniation of arenediazonium salts with triphenylphosphine and trialkyl phosphites. Generation of cation radicals from trivalent phosphorus compounds and their reactions

Abstract

4-Nitro-, 4-methyl-, and 4-methoxy-benzenediazonium fluoroborates are readily dediazoniated with triphenylphosphine or trialkyl phosphites (alkyl = methyl or ethyl) in alcoholic solvents at room temperature in the dark under a nitrogen atmosphere to give the corresponding arenes and the corresponding oxidation product from the phosphine or the phosphites, triphenylphosphine oxide or trialkyl phosphates, respectively, along with a small amount of biaryls. In the reaction with the phosphite, dialkyl arylphosphonate is also formed. The stoichiometry of the reactions and the results of the reactions in deuteriated methanols, together with other evidence, indicate that the reactions proceed by a radial-chain mechanism initiated by single-electron transfer from the trivalent phosphorus compound to the diazonium salt, during which the cation radical is generated from the former. The product distribution shows that this cation radical reacts with the solvent alcohol to give a phosphoranyl radical, which eventually affords the final product, the phosphine oxide or the phosphate, from the phosphine or the phosphite, respectively. The cation radical from the phosphite also undergoes radical coupling with the aryl radical Ar˙ generated during the reaction to yield dialkyl arylphosphonate via a phosphonium intermediate, whereas no radical coupling of the cation radical from the phosphine with Ar˙ takes place.