Ring Expansion of 2-Alkylidenedihydroquinolines to 2-Iminodihydro-1-benzazepines by Phenyl, Methanesulphonyl, and Trifluoromethanesulphonyl Azide[1]

2-Alkyl-1-methylquinolinium hexafluorophosphates 1 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinolines 2, which are investigated by NMR spectroscopy. 1,3-Dipolar cycloaddition of phenyl azide to 2 yields the spirocyclic products 10. While, at 80-110 °C, the [3 + 2] cycloaddition that afforded (u)-10f is reversible and accompanied by epimerisation to give (l)-10f, thermolysis of the dimethyl compounds 10b and d affords the ring-expanded products 14b and d, respectively, in good yields along with molecular nitrogen. Irradiation of 10d with light of λ > 320 nm results in the formation of similar amounts of 14d and [3 + 2] cycloreversion products, viz. 2-diazopropane (5b) and the N-phenylimine 15d. - Trapping of 2 by methanesulphonyl azide (18a) gives mixtures of the products of ring expansion (21b, d-f, 10-50 %) and [3 + 2] cycloreversion (22a, d, 10-80 %) of the apparently very labile intermediate spirocyclic cycloadducts 19. The ratio of 21 vs. 22 is significantly improved when 18a is replaced by trifluoromethanesulphonyl azide (18b), which affords the iminodihydrobenzazepines 21i-k in 50-75 % yield. The structures of the products are based on NMR evidence and X-ray diffraction analyses performed with 21b, d, and (ax,E)-21e.