An unexpected mode of cationic oligomerization of 1-vinyl-4,5,6,7-tetrahydroindole

1‐Vinyl‐4,5,6,7‐tetrahydroindole in the presence of Brønsted acids (HCl, H2SO4, HOAc) and Lewis acids (BF3 · OEt2, FeCl3, TiCl3, SnCl4, SiCl4, Et3SiCl, Me2, PhMeSiCl2) undergoes an alternative oligomerization involving both the double bond and the α‐position of the pyrrole ring to form the dimer 1‐vinyl‐2‐[1‐(4,5,6,7‐tetrahydro‐1‐indolyl)ethyl]‐4,5,6,7‐tetrahydroindole in a yield up to 68% and oligomers of the same backbone, soluble (major product) and insoluble (minor product) in hot hexane, the latter being enriched by oxidized and hydrolyzed tetrahydroindole units. The molecular weights of the former oligomers were assessed from 1H NMR spectra to range from 1000 to 2000.