Department of Chemistry, Oregon State University, Corvallis, Oregon 97331 USA
Abstract:
Four trienes of known stereochemistry have been prepared and their thermal cyclization studied to ascertain the general stereochemistry of thermal triene electrocyclic reactions. A mixture containing (Z)-1,2-dicyclohexenylethylene was obtained by semihydrogenation. Spectra evidence for thermal cyclization of the triene was obtained, but inability to isolate pure materials canceled plans for stereochemical study.A stereoselective synthesis for (E,Z,E)-2,4,6-octatriene was devised and a pure sample was cyclized thermally to give a 5,6-dimethylcyclohexadiene. Oxidative cleavage of this diene gave meso-2,3-dimethylsuccinic acid. Pure samples of (Z,Z,E)- and (Z,Z,Z)-2,4,6-octatrienes were prepared and found to interconvert thermally at 110°. Cyclization of the equilibrium mixture occurred at 178° to give a 5,6-dimethylcyclohexadiene which gave rac-2,3-dimethylsuccinic acid upon oxidative cleavage. Evidence is presented to suggest that the (E,Z,Z) isomer is the actual reactant undergoing cyclization. Both cyclohexadienes were shown to undergo a 1,5-hydrogen shift to form 1,6-dimethyl-1,3-cyclohexadiene.Rates of the electrocyclic reactions were measured in pentane solution, and thermodynamic activation parameters were ascertained. The results are: (E,Z,E) k(132°) = 4·45 × 10?5 sec?1, ΔH = 28·6 kcal/mole and ΔS = ?7 eu; (E,Z,Z)+(Z,Z,Z) k(178°)= 1·53 × 10?5 sec?1 or (E,Z,Z) k(178°) = 2·13 × 10?5 sec?1, ΔH = 32 kcal/mole, ΔS = ?9 eu; cis-5,6-dimethyl-1,3-cyclohexadiene (1,5-H shift), k(178°) = 1·77 × 10?5 sec?1, ΔH = 33 kcal/mole, ΔS = ?7 eu; trans-5,6-dimethyl-1,3-cyclohexadiene (1,5-H shift), k(178°) = 7·7 × 10?6 sec?1, ΔH = 36 kcal/mole, ΔS = ?3 eu.
