Intramolecular nitrene insertions into aromatic and heteroaromatic rings. Part 9. Synthesis of 2-azidodiphenylmethanes and the kinetics of their thermal decomposition in solution

Abstract

The synthesis of some new o-azidodiphenylmethanes 3b, 10, 15, 18, 21 and 22 is described. The first-order kinetics of the thermal decomposition in trichlorobenzene of these and some previously studied azides (3a, 3c and 3d) has been studied in the range 140–180 K. The initial step is evolution of nitrogen and formation of a nitrene intermediate, which reacts to give a 10H-azepinoindole together with some amine. Rate constants for the first step were compared with those for product formation and show it to be the rate-determining factor. Electron-donating substituents para to the azide group stabilise the incipient nitrene by resonance interactions (rate constants correlate with σ_R⁺) yielding higher proportions of amine products, which result from the spin forbidden singlet–triplet nitrene transition. Substituents at the 4′-position influence the nature of the products but not decomposition rates. A new compound, 1a-methoxy-1a,9b-dihydro-1H-cyclo-propa[1′,2′:3,4]pyridol[1,2-a]indole (28), is obtained from 2-azido-4′-methoxydiphenylmethane (3d) in addition to the expected azepinoindole and amine. Kinetic measurements show that it is an intermediate in one, but not the sole, route to formation of 8-methoxyazepinoindole 27 from 3d.