Low-Temperature Retro Diels-Alder Reactions of Tetramethoxyanthracenes and Tetracyanoethylene.

The retro Diels-Alder (rDA) reaction is used to prepare reactive organic molecules and, more recently, as a type of Dynamic Covalent Chemistry (DCC) for applications in polymer healing and reprocessability. Synthetic and DCC applications of the rDA are limited by the high temperatures it typically requires, but previous reports have shown that increasing the electron-donating nature of the diene and the electron-withdrawing nature of the dienophile reduces the enthalpy of formation of the DA adduct, making the rDA accessible at lower temperatures. In this work, we extend this concept to its extremes: using an electron-rich 9,10-dialkyl-2,3,6,7-tetramethoxyanthracene diene platform in combination with electron-poor tetracyanoethylene as the dienophile, we are able to observe rDA reactivity down to -80 °C as well as the longest-known C-C bond in a DA adduct formed by intermolecular cycloaddition (1.637(4) Å). Using variable-temperature NMR spectroscopy and stopped-flow UV-visible spectroscopy, respectively, we determine the thermodynamic and kinetic parameters for these DA reactions. We show that the kinetic barrier remains small while varying the 9,10-dialkyl substituent allows tuning of the enthalpy of DA adduct formation from -18 down to -8 kcal/mol.