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Thermal Rearrangements, XXV . The Automerization of Benzene as A Radical‐Initiated Reaction
Author(s) -
Zimmermann Gerhard,
Nüchter Matthias,
Hopf Henning,
Ibrom Kerstin,
Ernst Ludger
Publication year - 1996
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.199619960911
Subject(s) - chemistry , isomerization , benzene , radical , intramolecular force , photochemistry , medicinal chemistry , ring (chemistry) , pyrolysis , hydrogen , stereochemistry , organic chemistry , catalysis
The thermal isomerization of [1,4‐D 2 ]‐ ( 3a ) and [1,2‐ 13 C 2 ]benzene ( 1a ) has been studied in excess hydrogen at 750–850°C with contact time less than 1.2 s and very low partial pressure in a quartz flow system. In both cases, the main isomerization products are the corresponding meta isomers. The data suggest a radical intramolecular interchange of the benzene carbon atoms by 1,2‐C shifts. The multistep reaction cascade is initiated by H addition to the benzene ring followed by transannular homoallyl rearrangements involving the intermediate formation of bicyclo[3.1.0]hexenyl and cyclopentadienylmethyl radicals. This pathway constitutes a side reaction competing with the direct stabilization of the cyclohexadienyl radicals formed preferentially at high temperature.