Propyne Iminium Salts and Isoquinoline – 1 : 1 and 2 : 1 Adducts

The reaction of equimolar amounts of propyne iminium trifluoromethanesulfonates 1a, b and isoquinoline yielded, after hydrolytic work-up, the N-(3-oxoprop-1-en-1-yl)isoquinolinium salts 4a, b in modest yields. Monitoring of the reaction by ¹H NMR spectroscopy indicated the formation of salts 4, 3-isoquinolinio-substituted propene iminium salts 3, and N,N,N′,N′-tetramethylvinamidinium salts 5 as the major components. The expected aminoallenes (2-(3-(dimethylamino)allen-1-yl)isoquinolinium triflates) 2 could not be detected in the reaction solutions. It is possible, however, to trap the aminoallene intermediates in a polar [4 + 2] cycloaddition reaction, as shown by the isolation of 2:1 adducts 7c, d in good yield from cyclopropyl-substituted propyne iminium triflates 1c, d and isoquinoline. Hydride abstraction from 7c, d yielded the 2,4-dicyclopropyl-1,3-bis((dimethyliminio)(aryl)methyl)pyrido[2,1-a]isoquinolinium tris(triflates) 8c, d.