The catalytic effects of a thiazolium salt in the oxa-Diels-Alder reaction between benzaldehyde and Danishefsky's diene: a molecular electron density theory study.

The oxa-Diels-Alder (ODA) reaction of benzaldehyde with Danishefsky's diene in the presence of a [thiazolium][Cl] salt, as a model of an ionic liquid, has been studied within Molecular Electron Density Theory (MEDT) at the M06-2X/6-311G(d,p) computational level. The formation of two hydrogen bonds (HBs) between the thiazolium cation and the carbonyl oxygen of benzaldehyde modifies neither the electrophilic character of benzaldehyde nor its electronic structure substantially but accelerates the reaction considerably. This ODA reaction presents an activation energy of 4.5 kcal mol-1; the formation of the only observed dihydropyranone is strongly exothermic by -28.8 kcal mol-1. The presence of the [thiazolium][Cl] salt decreases the Gibbs free energy of activation of the ODA reaction between benzaldehyde and Danishefsky's diene by 5.9 kcal mol-1. This ODA reaction presents total para regioselectivity and high endo stereoselectivity. This ODA reaction takes place through a highly asynchronous polar transition state structure (TS) associated with a non-concerted two-stage one-step mechanism. ELF analysis of para/endo TSs associated with the ODA reactions in the absence and presence of the [thiazolium][Cl] salt shows that the formation of the HBs at the TSs does not modify their electronic structure substantially. This MEDT study makes it possible to conclude that the acceleration found in the ODA reaction of benzaldehyde with Danishefsky's diene in ILs is a consequence of an increase of the global electron density transfer at TS3-pn, resulting from HB formation, and the greater strength of the HBs at the polar TS3-pn compared to that at the benzaldehyde : [thiazolium][Cl] complex, and that the strength in the HB formed is more relevant that than an increase of the electrophilic character of the interaction between reagent.