Cyclisation versus 1,1‐Carboboration: Reactions of B(C 6 F 5 ) 3 with Propargyl Amides

A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C 6 F 5 ) 3 were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N ‐substituted propargyl amides with B(C 6 F 5 ) 3 led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C 6 F 5 ) 3 mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species.