Bronsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction

Lewis acid catalysts have been shown to promote carbonyl-olefin metathesis through a critical four-membered-ring oxetane intermediate. Recently, Bronsted-acid catalysis of related substrates was similarly proposed to result in a transient oxetane, which fragments within a single elementary step via a postulated oxygen-atom transfer mechanism. Herein, careful quantum chemical investigations show that Bronsted acid (triflic acid, TfOH) instead invokes a mechanistic switch to a carbonyl-ene reaction, and oxygen-atom transfer is uncompetitive. TfOH's conjugate base is also found to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction. The mechanism explains available experimental information, including the skipped diene species that appear transiently before product formation. The present study clarifies the mechanism for activation of intramolecular carbonyl-olefin substrates by Bronsted acids and provides important insights that will help develop this exciting class of catalysts.

Automated summary

In this study, it was found that Brønsted acid (such as triflic acid) differs from Lewis acid in promoting the carbonyl-olefin metathesis by invoking a mechanistic switch to a carbonyl-ene reaction, where oxygen-atom transfer is found to be uncompetitive. Additionally, the conjugate base of TfOH is able to rearrange H atoms and allow isomerization of the carbocations that appear after the carbonyl-ene reaction, providing insights into the mechanism of activation of intramolecular carbonyl-olefin substrates by Brønsted acids.