THE THERMAL AROMATIZATION OF METHYL-1,3-CYCLOHEXADIENES - AN IMPORTANT ARGUMENT AGAINST COMMONLY ACCEPTED SIGMATROPIC 1,7-H-SHIFT REACTIONS

It has been demonstrated that the methyl and ring C-atoms of methyl-1,3-cyclohexadienes interchange their positions intramolecularly during the thermal conversion to toluene at temperatures above 600-degrees-C in a quartz flow system. Gas phase pyrolysis of double C-13-labeled methyl-1,3-cyclohexadienes with C-13-labels for the primary and the tertiary C-atom gave definite C-13-distribution patterns in the aromatic ring systems of the formed toluene as well as benzene with far-reaching similarities. The NMR data of the [C-13(2)]toluene isotopomers definitely rule out that the observed integration of the methyl C-atom into the ring system proceeds via the hitherto well-established sequence: electrocyclic ring opening of the 5-methyl-1,3-cyclohexadiene to 1,3,5-heptatriene, its sigmatropic 1,7-H shift and ensuing recyclization of the 1,3,5-heptatriene to methyl-1,3-cyclohexadienes.