Theoretical study of substituent and solvent effects on the thermodynamics for cis/ trans isomerization and intramolecular rearrangements of 2,2′-diphenoquinones.

Enthalpies (Δ H), Gibbs free energies (Δ G), and equilibrium constants (log K) for the trans → cis isomerization of various 3,3′-, 4,4′-, and 5,5′- disubstituted 2,2′-diphenoquinones with a range of electron withdrawing and releasing moieties (methyl, fluoro, chloro, bromo, trifluoromethyl, and amino) were calculated in the gas phase and in the solvent phase ( n-hexane, benzene, n-octanol, acetonitrile, and water). In the gas phase, the trans isomer of the parent and all substituted 2,2′-diphenoquinones is predicted to be more thermodynamically stable than the cis configuration, with log K ranging from −2.8 to −7.0. For all compounds, increasing solvent polarity/proticity progressively favors shifting the cis/ trans equilibrium towards greater contributions of the cis configuration and substantially increases the log K by up to 5.1 units relative to the gas phase. In polar protic and polar aprotic solvents, the estimated log K ranges as low as −0.4, indicating significant populations of the cis isomers should be present. The findings support the polar solvent phase mechanistic predictions for a cis configuration of 2,2′-diphenoquinones participating in the thermal transformation of trans-2,2′-diphenoquinones to oxepino[2,3-b]benzofurans. With limited exceptions for some amino derivatives, the cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran isomerization is expected to be thermodynamically favorable for all substituents/phases under consideration. The cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran rearrangement is predicted to become less thermodynamically favored with increasing solvent polarity/proticity. [ABSTRACT FROM AUTHOR]