Retro Diels-Alder reactions of 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes: Experimental and density functional theory studies

Several 5,6-disubstituted-7-oxabicycloo[2.2.1]hept-2-enes (1-4) were synthe sized on greater than or equal to 0.1 mol scale. The heat-induced retro Die ls-Alder (rDA) decomposition of these derivatives was studied by thermal an alysis, and the kinetics of the rDA were measured for 4. First-order rate c onstants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures betw een 124 and 150 degrees C, were used to calculate Arrhenius activation para meters E-a (34.5 +/- 0.5 kcal/mol) and InA (1.77 +/- 0.03 x 10(4)). The obs erved activation energy was significantly larger (by 9.5 kcal/mol) than tha t previously measured for the maleic anhydride adduct 1, and this was attri buted to the difference in LUMO energies for the two dienaphiles. Modeling of the activation parameters found for 4 with density functional theory (DF T) calculations for similar compounds 5 and 6 gave close quantitative corre lations for Delta H-double dagger, Delta G(double dagger), Delta S-double d agger. The rDA reactions studied were found to be entropy-driven.