A DFT study on substituents, solvent, and temperature effect and mechanism of Diels-Alder reaction of hexafluoro-2-butyne with furan.

CONTEXT

Furan and its derivatives constitute a vital class of heterocyclic chemistry used widely in organic synthesis via Diels-Alder reactions. As fluorine incorporation has been of great interest due to the limited possible pathways, the present study on [4 + 2] cycloaddition Diels-Alder reaction, between hexafluoro-2-butyne and 2-substituted (NH, OCH, OTMS, NHBoc) furans, uses the reaction as a likely route. The computational study revealed that that the reaction is feasible in all conditions and is most favorable for NH substituent in furan. The study of the effect of temperature has depicted that low temperature favors the formation of adducts, while the rise in temperature prefers ring opening to form 4-substituted-2,3-di(trifluoromethyl)phenol derivatives. The feasibility of a reaction has been determined by Gibbs energy change. The transition state study has been performed to find the activation energy, C-C single bond formation and global electron density transfer (GEDT) involved in the adduct formation. MEP plots have been used to understand the region of electrophilicity and nucleophilicity character. Furthermore, the mechanism for the formation of phenol products has been discussed. The decomposition of the NHBoc group at higher temperatures has been proved via a proposed mechanism and compared with experimental results.

METHODS

The reaction was theoretically investigated using B3LYP hybrid functional with 6-311 + G(d,p) basis sets, in gas phase and under different solvent conditions like water, acetonitrile, and THF. The transition state structures of the adduct were optimized at the lower basis set B3LYP/6-31 + G(d,p) as well as at the higher basis set B3LYP/6-311 + G(d,p) level. The changes in Gibbs energy (∆G) for the formation of products at different temperatures and in various solvents have been calculated at B3LYP/6-311 + G(d,p) level.