The case of the missing acetylene. The mechanism of an intramolecular SN(V) reaction and a new route to 1-methylbenzo[de]quinolines

1-Halogeno-2-(1-naphthyl)alkenes with a dimethylamino group in the peri position are smoothly converted to 1-methylbenzo[de]quinoline under mild conditions, in a process involving the loss of MeBr rather than HBr. The Z-bromide is 45 times more reactive than its E-isomer, and 7 times more reactive than the Z-chloride. These systems were designed to show efficient elimination of HBr, but acetylene is not a likely intermediate. There is good evidence to support both of two alternative addition–elimination mechanisms: a 6-endo-dig route which would involve a “normal” addition–elimination process in an unusual setting; and a tandem 5-exo-dig Michael addition–carbene rearrangement. The evidence so far does not permit a final choice between the two.