A density functional theory study of the reactions of furans with substituted alkynes to form oxanorbornadienes and subsequent [4 + 2] and [2 + 2 + 2] addition reactions

Abstract

This work investigated theoretically, at the M06/6‐311++G(d,p) level of theory, the Diels–Alder reactions of furan with substituted alkynes to afford oxanorbornadienes and the subsequent [4 + 2] and [2 + 2 + 2] cycloaddition reactions leading to the formation of heterocyclic furans, deltacyclenes, and bicyclic oxanorbornadienes, which have high utility in the chemical and pharmaceutical industries. The results show that in the initial Diels–Alder reactions to afford oxanorbornadiene intermediates, reactions of alkynes bearing electron‐withdrawing substituents have lower activation barriers compared with those with electron‐donating substituents. In the [4 + 2] reactions of the oxanorbornadiene intermediates to form bicyclic oxanorbornadienes, oxanorbornadienes with electron‐donating groups favor addition across the unsubstituted olefinic bond, whereas those with electron‐withdrawing substituents favor addition across the substituted double bond, but in both cases, the exo stereoisomer is favored. The regioselectivity of the reactions are strictly dictated by the nature of substituent on oxanorbornadiene. The retrocleavage step, regardless of the addition sequence and nature of substituents, is the rate‐determining step. For the reactions of substituted oxanorbornadienes with furan along the [4 + 2] cycloaddition and the [2 + 2 + 2] cycloaddition (dimerization) of substituted oxanorbornadienes, we observed a decrease in activation barriers when electron‐withdrawing substituents are involved. The global electrophilicity indices (ω) and maximum electronic charge transfer (ΔNmax) computed for various oxanorbornadiene derivatives considered in this work is consistent with energetic trends and experimental outcomes. Perturbation molecular orbital theory was employed to rationalize the results.