Pathways to Functionalized Heterocycles: Propargyl Rearrangement using B(C6F5)3

The reactions of propargyl amides, ureas, carbamates, and carbonates with B­(C6F5)3 proceed via an intramolecular 5-exo-dig cyclization across the alkyne unit to yield the corresponding vinyl borate species. The generated sp2 carbocation is stabilized by the flanking heteroatoms, allowing for isolation of oxazoline intermediates. The fate of these intermediates is strongly dependent upon the propargyl-functionalized starting material, with the carbamates and carbonates undergoing a ring-opening mechanism (propargyl rearrangement) to give cyclic allylboron compounds, while prolonged heating of the urea derivatives shows evidence of oxazole formation. In a deviation away from the reactivity of carbamates stated previously, the benzyl carbamate substrate undergoes dealkylation at the benzylic position, liberating 5-methyloxazol-2-(3H)-one.