Simple, convenient, and direct conversion of anilines and anilides into arynes

Abstract

Reaction of aniline with pentyl nitrite and acetic anhydride in benzene, or other solvents, gives benzyne in up to 32% yield as measured by trapping with anthracene, and tetraphenylcyclopentadienone and its 2,5-di-p-tolyl analogue. The role of the anhydride is to by-pass the suppressive effect of water on benzyne formation from the diazonium acetate ion pair. Reaction in benzene of aniline–acetic anhydride or (better) acetanilide with p-chloro-benzoyl nitrite (PCBN)(from silver p-chlorobenzoate and nitrosyl chloride) gives benzyne in up to 71% yield as measured by trapping as before and by 9,10-dimethoxyanthracene, and by the ene reaction with methyl methacrylate to give ethyl 2-methylene-3-phenylpropionate (31%). meta-Substituted anilines and acetanilides (m-XC₆H₄·-NHAc; X = F, Cl, Br, I, CO₂Et, MeO, Bu^t, CF₃, or NO₂) and 3-aminopyridine behave like aniline and acetanilide, the para-isomers give lower yields, and the ortho-isomers give little or no arynes, with the exception of ethyl anthranilate and 2,5-dimethoxy- and 2,5-dichloroaniline.