Superacid-Catalyzed Electrocyclization of Diphenylmethyl Cations to Fluorenes. Kinetic and Theoretical Revisit Supporting the Involvement of Ethylene Dications

In superacid media, diphenylmethyl cations bearing an α-carbonyl group generate fluorene compounds. We have obtained chemical evidence showing the acidity dependence of this fluorene cyclization process. A linear relationship was found between the rate of the fluorene cyclization and the acidity of the reaction media in kinetic studies, strongly supporting the intervention of an additional proton transfer to the diphenylmethyl monocation in the fluorene cyclization, that is, the involvement of a dication in the form of the diphenylmethyl cation bearing an O-protonated carbonyl group. Ab initio calculations employing Becke3-LYP density functional theory provided theoretical support for the idea that the dicationic fluorene cyclization is energetically more favorable than monocationic electrocyclization to give the benzofuran or the fluorene. The magnitudes of the predicted activation barriers of the dicationic fluorene cyclization are compatible with the experimental values for the fluorene cyclization and with the experimental dependence of the reaction aptitude on the α-carbonyl substituent (ester, acid, or acetyl group).