Gold(I)-Catalyzed Cycloaddition of 1-(1-Alkynyl)cyclopropyl Ketones with Nucleophiles To Yield Substituted Furans: A DFT Study

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Abstract

By means of density functional theory (DFT), the mechanism for the synthesis of highly substituted furan (2) from 1-(1-alkynyl)-cyclopropyl ketone (1) with nucleophile (MeOH) catalyzed by Au(I) was investigated. As demonstrated, both the intimate ion-pair [AuL]+·[OTf]− (L = PPh3, PMe3, and PH3, OTf = trifluoromethane sulfonate) and the cation [AuL]+ exhibit catalysis, and the former is dominant. Moreover, the bigger the ligand coordinated to gold is, the poorer the catalysis is. The theoretical study reveals that the nucleophiles such as MeOH are important for this reaction, that the reaction between the bicyclo[4.1.0]heptan and the carbinol is a stereospecific addition reaction, and how the proton migration goes through. In the proton migration process, both the anion OTf− and another nucleophile molecule such as MeOH act as a proton shuttle to transport a proton from the nucleophile to the 2-position carbon of alkynyl. The most favorable mechanism includes the activation of the substrate, nucleophile attraction, an addition reaction of the carbonyl triple bond, a stereospecific attack of the nucleophile on the activated cyclopropane, proton migration, and regeneration of the catalyst. The substrate 1 is activated by its combination with Au(I) and a molecule of nucleophile (MeOH), which leads to a decrease in the orbital energy of π*(C1≡C2) and σ*(C3−C5) and an increase in the dipole moment of the bent bond σ(C3−C5). On the whole, the solvent effects increase the reaction barriers.

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