ChemInform Abstract: 1,3,5‐Triphenylbenzene as Unexpected By‐Product in the Synthesis of 2,5‐Diphenylpyrrole from Acetophenone Oxime and Phenylacetylene.

Abstract

Despite poor yield of the target product (25%), the reaction of acetophenone oxime (I) with phenylacetylene (II) in superbasic medium (Trofimov reaction) [1–4], leading to 2,5-diphenyl-1H-pyrrole (III), attracts interest from the preparative viewpoint, for alternative methods include a number of steps [5], and the overall yield (calculated with account taken of all steps) is comparable to that in the Trofimov synthesis. Determination of structure of by-products in the reaction of oxime I with acetylene II may be useful for understanding its mechanism and further optimization to enhance its preparative efficiency. We were the first to isolate from the product mixture (LiOH–DMSO, 130°C, 5 h) 6% of 1,3,5-triphenylbenzene (V) together with pyrrole III and acetophenone IV (deoximation product of initial oxime I) (Scheme 1). However, this assumption was not confirmed experimentally. We also found that under the given conditions compound V cannot be formed as a result of autocondensation of oxime I or ketone IV. Presumably, O-vinyl oxime VI which is the primary intermediate in the synthesis of pyrroles from ketone oximes and acetylene [6] (Scheme 2) undergoes rearrangement into nitrone VII (as reported in [7] for cyclohexanone oxime O-allyl ether). Enhanced CH acidity of the CH3 group in VII due to strong electron-withdrawing effect of the N-oxide fragment favors aldolization (crotonization) of VII with acetophenone IV. The condensation of azatriene VIII thus formed with the second acetophenone molecule gives azatetraene IX, and cyclization of the latter, followed by aromatization of dihydrobenzene X via elimination of vinylnitrone XI, yields triphenylbenzene V. Compound XI can be converted into acetophenone oxime (I).