Biscyclization Reactions in Butadiyne- and Ethyne-Linked Triazenes and Diazenes: Concerted versus Stepwise Coarctate Cyclizations

A series of alkyne-linked bis-2H-indazoles has been prepared by the double cyclization of ethyne-
or butadiyne-linked phenyltriazene or phenyldiazene moieties. Even though there are two five-membered
ring cyclizations and several triple bond shifts involved, the reactions proceed rapidly under neutral conditions
with mild heating, affording the heterocycles in excellent yields. DFT calculations, in agreement with
experimental observations, indicate that the reactions:  (1) occur via a very short-lived carbene intermediate,
(2) are concerted via an asymmetrical transition state, or (3) are even synchronous, with as many as 16
bonds that are made or broken simultaneously. The biscyclizations presented herein strikingly illustrate
the concept of coarctate reactions, the stabilization of transition states by coarctate Möbius aromaticity,
the ethynologation principle, and the stereochemical rules.