# A Case of Competitive Aromatization vs. Sigmatropic [1,5]-Hydrogen Atom Migration in a 1,2,4-Cyclohexatriene Intermediate Derived from a Bis-Enyne Cyclization

## TL;DR

This study explores a chemical reaction where a cyclohexatriene intermediate undergoes two competing processes, leading to a new non-benzenoid product.

## Contribution

The paper reveals a previously unknown reaction pathway involving a 1,2,4-cyclohexatriene intermediate.

## Key findings

- The cyclohexatriene intermediate undergoes competitive aromatization and [1,5]-hydrogen atom migration.
- A non-benzenoid pyrrole derivative is formed as a result of the [1,5]-hydrogen atom migration.
- Protic additives like MeOD and BHT influence the mechanistic pathway of the reaction.

## Abstract

1,2,4-Cyclohexatrienes are strained, reactive intermediates often formed by the tetradehydro-Diels–Alder (TDDA) reaction of a conjugated enyne bearing a tethered alkyne as the enynophile. The ene component is commonly the π-bond of an aromatic group. In this focused study, we investigated the reactivity of a symmetrical substrate in which the pair of terminal ene moieties were simple 2-propenyl groups. The intermediate 1,2,4-cyclohexatriene, now bearing a 5-isopropenyl group, underwent competitive aromatization (the most usual outcome of the strain-relieving event of the cyclohexatriene), along with an intramolecular [1,5]-hydrogen atom migration, ultimately producing a non-benzenoid, pyrrole derivative. This represents a previously unknown process for a 1,2,4-cyclohexatriene derivative. Mechanistic aspects of the competitive processes were revealed by experiments performed in the presence of various protic additives (MeOD and BHT).