Stoichiometric and Catalytic C-C and C-H Bond Formation with B(C₆ F₅ )₃ via Cationic Intermediates

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C-H or C-C bond formation. The activation of alkyne-containing substrates with B(C₆ F₅ )₃ enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C-C bond formation as catalytic B(C₆ F₅ )₃ can be used to effect formal 1,5-alkyl migrations from the ester functional groups to unsaturated carbon-carbon frameworks. This metal-free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.