Structural Investigations into the retro-Diels-Alder Reaction. Experimental and Theoretical Studies

The manifestations of the retro-Diels Alder reaction in the ground-state structures of a range of cyclopentadiene and cyclohexadiene cycloadducts 915 have been investigated by a combination of techniques. These include low-temperature X-ray crystallography, density functional calculations (B3LYP/6-31G(d,p)) on both the ground states and transition states, and the measurement of 13C−13C coupling constants. We have found that the carbon−carbon bonds (labeled bonds a and b), which break in the rDA, are longer in the cycloadducts 915 than in their corresponding saturated analogues 9s15s, which cannot undergo the rDA reaction. The degree of carbon−carbon bond lengthening appears to be related to the reactivity of the cycloadduct, thus the more reactive benzoquinone cycloadducts 5b and 13 have longer carbon−carbon bonds. Those cycloadducts 14 and 15 which are predicted to undergo asynchronous reactions show differing degrees of carbon−carbon bond lengthening, reflecting the differing degrees of bond breaking at the calculated transition states for the rDA.