Understanding the Mechanism of the Lewis Acid Promoted [3 + 2] Cycloaddition of Propargylic Alcohol and α-Oxo Ketene Dithioacetals.

The mechanism and origin of selectivities in BF3·Et2O-catalyzed intermolecular [3 + 2] cycloadditions of propargylic alcohol and α-oxo ketene dithioacetals have been studied using density functional theory. Several possible reaction pathways were evaluated on the basis of two possible binding modes between the carbonyl or hydroxyl oxygen of substrates and catalyst. The preferred mechanism initiates dehydroxylation of propargylic alcohol by Lewis acid BF3 and generates active allenic carbocation species to provide the favorable electrophile. It then proceeds via four processes involving nucleophilic addition of C(α) on α-oxo ketene dithioacetals to the C1 of active allenic carbocation intermediate, [1,4]-alkylthio shift, H(α)-elimination, and intramolecular cyclization. This reaction sequence is in contrast to the mechanism by a previously published study, that is, [1,4]-alkylthio migration occurs prior to the cyclization. Our calculated results suggested that electrostatic attraction and hydrogen-bonding interactions between substrates and catalyst play a vital role in the [3 + 2] cycloaddition.