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.

Computational Study on Homolytic Bond Energies of the Ag-X (X = C, O, and H) Complexes and Hammett-Type Analysis of Reactivity

Thirty-seven calculation methods were benchmarked against the available experimental bond lengths and energies data regarding the Ag-X bonds. The theoretical protocol PBE0/VDZ//ωB97x-D/mVTZ was found to be capable of accurately predicting the homolytic bond dissociation energies (BDEs) of Ag-X complexes with a precision of 1.9 kcal/mol. With the available method in hand, a wide range of different Ag-X BDEs were estimated. BDE(Ag-CH₂X), BDE(Ag-PhX), BDE(Ag-OPhX), and BDE(Ag-OCOPhX) (X = NH₂, OMe, Me, H, Cl, and NO₂) were found to be in the ranges of 27-47, 51-54, 19-39, and 64-70 kcal/mol, respectively. Subsequently, Hammett-type analysis was carried out with reactivity parameters. Good positive linear relationships were found for BDE of Ag-O bands and decarboxylation barriers of Ag-OCOPhX with the Hammett constant σ. It suggested that electron-donating substituents could promote either the homolytic cleavage of the Ag-OPhX bond to undergo a radical process or Ag-OCOPhX decarboxylation. Moreover, ligand effects on Ag-H bonds were investigated using BDE(Ag-H) and related NPA charges on Ag. In the case of P-ligands, carbene ligands, and other small molecule ligands (i.e., CO, CO₂, and H₂O), a good negative linear relationship was found. In contrast, N-ligands could have a reverse effect. Understanding the intrinsic relationships of BDE(Ag-X) with related reactivity parameters might help gain insights into the structure-reactivity relationships in Ag-X-assisted C-H activation/decarboxylation.