Selective Dehydrogenation of 4,5,6,7-Tetrahydroindole

The novelty of this approach is in that the catalytic system, containing nanosized nickel sulfide as the only active component, has been applied for the first time for dehydrogenation of 4,5,6,7-tetrahydroindole. Earlier, such catalytic system was not employed for the aromatization of nitrogen heterocycles bearing saturated fragments. Usually, palladium- or rare-earth element-based catalysts were used in such processes [441]. 

Further, this catalyst has been modified to improve its efficacy and facilitate the technology of its preparation. To reach these goals, hydrogen sulfide is replaced by aqueous solution of sodium sulfide, and nickel chloride is used instead of nickel acetate. An industrial surfactant, sodium dodecylsulfonate, is applied for better impregnation of aluminum oxide. 

As a consequence, operation time of the catalyst increased by 4-5 times and its regeneration was significantly facilitated (heating at 370°C on air for 10 h). The catalyst allows carrying out the dehydrogenation of 4,5,6,7-tetrahydroindole into indole without the use of inert gas carrier only in a solvent vapor flow (toluene) that is important for technology. 

This development should be considered as the first technologically real synthesis of indole on the basis of industrial cyclohexanone oxime. 

SCHEME 2.21 Selective synthesis of indole from 4,5,6,7-tetrahydroindole. 

---

Trofimov, B.A., A.I. Mikhaleva, A.M. VasU tsov et al. 2010. Selective dehydrogenation of 4,5,6,7-tetrahydroindole to indole in nickel-sulfide catalyst. Bold Chem 434 (5) 636-638. 

---