Uncatalyzed Sigmatropic 1,5-Shift of Acyl Groups in the Thermolysis of 5-Acyl-5-methyl-1,3-cyclohexadienes

Article abstract of DOI:10.1002/hlca.19810640318

Four different 5-acyl-5-methyl-1,3-cyclohexadienes 1a-d (R=COOCH3, COCH3, COC6H5, CHO) have been shown to yield mixtures of 1,3-disubstituted cyclohexadienes 2-7 and 1,3-disubstituted aromatic product 8 upon thermolysis at 150-300 deg C in solution and at 350-500 deg C in the gas phase in a flow system.Two reaction pathways are considered for the rearrangement of the C-Skeleton.For the ester 1a 13C-isotopic substitution shows that products arise to 75-86percent through a 1,5-sigmatropic shift of the methoxycarbonyl group and to 14-25percent through a sequence of reaction steps involving a 1,7-H-shift reaction in an acyclic intermediate.For the more reactive compounds 1b-d isomerization is assumed to follow the 1,5-sigmatropic pathway exclusively.A kinetic study yields the following sequence for the migration tendency of acyl groups toward sigmatropic 1,5-shift: COOCH3< COCH3< COC6H5< CHO.