The Thermal Aromatization of Methyl-1,3-cyclohexadienes - An Important Argument against Commonly Accepted Sigmatropic 1,7-H-Shift Reactions

Article abstract of DOI:10.1002/prac.19943360303

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 deg C in a quartz flow system.Gas phase pyrolysis of double 13C-labeled methyl-1,3-cyclohexadienes with 13C-labels for the primary and the tertiary C-atom gave definite 13C-distribution patterns in the aromatic ring systems of the formed toluene as well as benzene with far-reaching similarities.The NMR data of the <13C2>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.