Kinetics an Stereochemistry of the Thermal Interconversion of 4,5-Dimethyl-2,6-octadienes

Article abstract of DOI:10.1021/jo00232a026

Gas-phase pyrolysis of threo-4,5-dimethyl-cis,cis-1,1,1,8,8,8-hexadeuterio-2,6-octadiene over the temperature range 220.0-260.0 deg C resulted in formation of the threo, trans, trans isomer with log k = 11.36 - 36000/2.303RT.NMR analysis with Simplex minimization of the residuals from a Gear numerical integration provided a nearly identical rate constant for the degenerate interconversion of the deuterium isomers of the threo,trans,trans diastereomer at 240 deg C.All six 4,5-dimethyl-2,6-octadienes are interconverted at temperatures above 290 deg C.Mass spectral analysis of the reaction products from a 1:1 mixture of protio and D₆ diene provided evidence for cleavage-recombination as the pathway for conversion to erythro,trans,trans,erythro,cis,cis and threo,trans,cis isomers.Competing with the cleavage-recombination is the boatlike shift to the erythro,trans,cis isomer.NMR analysis of the (-)-α-phenylethylamine bis salt of the threo-2,3-dimethylsuccinic acid derived from 33.5-h 240 deg C pyrolysis of optically pure hexadeuterio starting material provided evidence for little incursion of antarafacial-antarafacial 3,3-shifts via a twist transition state competing with the chair transition state.An analysis of the energy surface for all interconversions reveals that at 300 deg C, the boat transition state is ca. 6 kcal/mol higher in energy than the sterically most favorable chair transition state, and the cleavage reaction transition state is only 0.6 kcal/mol higher in energy than the boat transition state.