Theoretical Investigation of Steric Effect Influence on Reactivity of Substituted Butadienes with Bromocyclobutenone.

The Diels-Alder reaction (DA) between various mono- and disubstituted 1,3-butadiene (Dn-1 to Dn-10) and 2-bromocyclobutenone (DPh) was carried out in gas phase using density functional theory (DFT) at the M06-2X/6-31+g** level. The reaction was found to proceed through a concerted asynchronous transition state. Further, the asynchronous and early nature of the transition state was clearly pinpointed with the frontier molecular orbital (FMO) and bond order index (BOI) analyses. The intermolecular hydrogen bonding interaction along with steric encumbrance in the transition state were found to be the predominant factors in controlling the reactivity of the dienes. Among the investigated dienes, Dn-6 was found to be the most reactive diene which is attributed to its low activation barrier due to the presence of strong intermolecular H-bonding interactions. These factors were further supported by quantum mechanical calculations using global descriptor indexes, natural bond orbital analysis, and quantum theory of atoms in molecules analysis. These theoretical results were found to be in good agreement with the previous experimental findings.