Superacid-Catalyzed Reductive Friedel−Crafts Reaction of Arenes Using Arenecarbaldehyde Acetals

Abstract

Reaction of 2-aryl-1,3-dioxane with arenes in the presence of a catalytic amount of trifluoromethanesulfonic acid gave the corresponding diarylmethanes in good to excellent yields. The acid-catalyzed Friedel−Crafts benzylation of arenes could altenatively be carried out using arenecarbaldehyde and 1,3-propanediol. The reaction was assumed to proceed through a redox process involving hydride shift from the cyclic acetal moiety to the benzylic carbon. The hydride shift was confirmed by the reaction with 5-ethyl-2-phenyl-4,4,6,6-tetradeuterio-1,3-dioxane, wherein more than 90% deuterium was incorporated into the benzylic carbon of the diphenylmethane. Diphenylmethyl ether Ph₂CHOCH₂CH₂CH₂OH also reacted with benzene to afford diphenylmethane under the same reaction conditions, suggesting that the ether should be the plausible intermediate that underwent the hydride shift.