Initially, the phenylhydrazone designated as 1-R, where R signifies
the type of reaction: t for thermal, a for Na-protonated and
b for Nb-protonated, rearranges to the ene-hydrazine intermediate
2-R. This intermediate undergoes a [3,3]-sigmatropic rearrangement
and climbs an activation barrier to reach a transition state
(TS) termed TS-R. The energy then drops with the formation of the
non-aromatic imine 3-R that will eventually (cf. Scheme 1) lead to
the formation of an aromatic indole.