N-Vinyl-2-(trifluoroacetylethynyl)pyrroles and E-2-(1-bromo-2-trifluoroacetylethenyl)pyrroles: Cross-coupling vs. addition during CH-functionalization of pyrroles with bromotrifluoroacetylacetylene in solid Al2O3 medium. H-bonding control

• Solid Al2O3 is used for trifluoroacetylethynylation or -bromoethenylation.
• Intramolecular hydrogen bonding responsible for reaction direction.
• Trifluoroacetyl group could be simply removed using solid Al2O3.

A topochemical reaction of pyrroles with bromotrifluoroacetylacetylene proceeds in solid Al2O3 at room temperature in two different directions depending on the substituent character at the nitrogen atom. While N-vinylpyrroles undergo the cross-coupling to give N-vinyl-2-(trifluoroacetylethynyl)pyrroles in up to 58% yield, unsubstituted NH-pyrroles add to the triple bond to afford exclusively E-2-(1-bromo-2-trifluoroacetylethenyl)pyrroles in 12–21% yields. It has been shown (1H NMR, quantum-chemical calculations) that the reaction is controlled by the strong intramolecular hydrogen bonding between NH and carbonyl groups.

Figure optionsDownload as PowerPoint slide