Hetero-Diels–Alder Ligation (HDAL) of C20 Nanocage and Anti-Tumor Oxoindoline in Gas Phase and Solvent Phase, Using Density Functional Theory (DFT)

In this computational research, the solvent effect is probed on HDAL of C20 with I to yield Ia compound. So, in going from the gas phase to non-polar, and in turn to the polar solvent, a good consistency appears between the dielectric constant of the solvent (ε) and the released solvation energy (∆El-g). The stability and polarity of Ia appearances are proportional to ε, and the probability of the H-bonding. While the obtained endo-isomer from HDAL is anticipated thermodynamically more stable than exo- analogue; here we found that the formation of exo-isomer is only the obtained product from this HDAL. Subsequently, exo-isomer is more stable than endo-analogue; due to the disappearance of the resulting electronic effect and ring-strain effect from π-stacking between the aromatic rings of I and nanocage. The possibility of HDAL is ruled out by the lowest energy barrier of 5.1 kcal/mol probed for exo Transition State (TS) in the gas phase, while the highest energy barrier of 9.4 kcal/mol is investigated for endo TS in H2O. Hence, the designed HDAL is distinguished as an attractive and promising procedure for ligation in biochemistry due to its higher rate and selectivity in H2O. Fascinatingly, similar to stable C60 nanostructure, exo HDAL of unstable C20 nanostructure with the scrutinized diene can be carried out thermally at room temperature and may be a potential candidate for efficient and selective HDAL in living systems.