Rationalizing the Substituent Effects in Diels-Alder Reactions of Triazolinediones with Anthracene

In this work we tackle the problem of the substituent effects in the Diels-Alder cycloadditions between triazolinediones (TADs) and anthracene. Experiments showed that aryl TADs substituted with electron-withdrawing groups (EWG) are more reactive than those substituted with electron-donating (EDG) or alkyl groups. However, the molecular origin of this preference is not yet understood. By a combination of methods including the activation strain model (ASM), energy decomposition analysis (EDA), molecular orbital (MO) theory, and conceptual density functional theory (CDFT), we disclosed the substituent effects of TADs. First, ASM/EDA analysis revealed that the reactivity of alkyl and aryl-substituted TADs is controlled by interaction energies, ΔE_int, which are ultimately defined by orbital interactions between frontier molecular orbitals. Moreover, alkyl-TADs are also controlled by the extent of strain at the transition state. The MO analysis suggested that the rate acceleration for EWG-substituted TADs is due to a more favorable orbital interaction between the HOMO of anthracene and the LUMO of the TADs, which is corroborated by calculations of charge transfer at the transition states. From CDFT, the chemical potential of anthracene is higher than those of TADs, indicating a flow of electron density from anthracene to TADs, in agreement with the results from the electrophilicity index.