A Computational/Experimental Investigation into the Reversibility of [6+4]/[4+2] Cycloadditions with Tropone...

While the thermoreversibility of furan-maleimide Diels-Alder adducts has been widely employed in dynamic polymer networks, the susceptibility of furans and maleimides to engage in irreversible side reactions motivated our investigations into other dynamic cycloaddition reactions. Although [6+4] pericyclic reactions of tropone derivatives and dienes have been investigated since the mid-1960s, little work has looked at the factors that may affect the energy barriers of both the forward and reverse reactions. In this work, we use DFT calculations and experimental analyses to examine the cycloaddition reactions of tropone derivatives with dienes. We find that DFT calculations at the M06-2X(D3)/6-31G(d) level of theory are in good agreement with experimentally derived values of ΔG^⧧ for the reactions of some tropone derivatives with cyclopentadiene. Likewise, calculations provide reasonable predictions of the distribution of [6+4] and competitive [4+2] adducts, with [6+4] adducts converting to the more stable [4+2] adducts at elevated temperatures. Finally, we observe that even the rather stable [4+2] adducts show partial reversion to the starting tropone. Due to the alignment between calculations and experimental kinetic and thermodynamic data, this work highlights the value of employing DFT methods to help, in the future, in the design of new reversible cycloadditions.