MECHANISM OF KOHLER SYNTHESIS OF 2-ISOXAZOLINE 2-OXIDES FROM 1,3-DINITROALKANES

Formation of 3,4,5-triphenyl-2-isoxazoline 2-oxide (3) from 1,3-dinitro-1,2,3-triphenylpropane (2) has been shown to involve an intramolecular nitrite ion displacement in the intermediate 2 mononitronate anion. Contrary to the generally accepted mechanism for this reaction, it has been shown that a nitro olefin intermediate is not involved. 1-Nitro-1,2,3-triphenylpropene (13) has been synthesized from 1,2-dinitro-1,2,3- triphenylpropane by heating with ethanolic potassium acetate. In basic solution 13 forms a nitronate anion which undergoes rapid C-1 protonation to 3-nitro-1,2,3-triphenylpropene (16); no isoxazoline 2-oxide is formed from 13 or 16. A product described by other workers as nitro olefin 13 has been shown to be a stable molecular complex composed of two cis-a-nitrostilbene molecules and one trans-stilbene molecule. Additional evidence against a nitro olefin intermediate was obtained by reaction of 2 with sodium methoxide in methanol-O-d to form 3,4,5-triphenyl-2-isoxazoline 2-oxide-5-d, and reaction of cis-α-nitrostilbene-β′-d with phnylnitromethane in methanolic sodium methoxide to form 3,4,5-triphenyl-2-isoxazoline 2-oxide-4-d. The stereochemistry of 3 has been shown to be trans. by reduction of it to known irans-3,4,5-triphenyl-2-isoxazoline. Mechanisms of isoxazole formation from 3 and alkane-1,3-bisnitronic acids are also discussed briefly. © 1969, American Chemical Society. All rights reserved.