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Comparative study of the cope rearrangement of hexa‐1,5‐diene and barbaralane
Author(s) -
Baumann Harold,
VoellingerBorel Aline
Publication year - 1997
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19970800711
Subject(s) - chemistry , hexa , diene , transition state , steric effects , computational chemistry , cope rearrangement , state (computer science) , stereochemistry , crystallography , organic chemistry , algorithm , catalysis , natural rubber , computer science
The classical rules for Cope rearrangements predict a transition state with chair form to be favored over the boat form. On the other hand, bridged homotropylidenes, which allow only a boat‐form transition state by steric reasons, have extremely low barriers. A controversy about the correct pathway and the different possible intermediates and transition states of the reaction has gone on for years. In this work, the hypersurfaces of barbaralane, in comparison with the boat ‐ and chair ‐form of hexa‐1,5‐diene, are computed by the ab inito CASSCF (6,6)/6‐31G** method starting with UMP2/6‐31G** natural orbitals (NO's). All three hypersurfaces show characteristic features, and, moreover differ from each other. A hitherto undiscussed intermediate, bicyclo[2.2.0]hexane, was localized on the boat ‐hexa‐1,5‐diene pathway. So it is noteworthy that our transition state for the boat ‐hexa‐1,5‐diene does not correspond to the transition states found by other authors for this conformation. The computed enthalpies of activation of boat ‐ and chair ‐hexa‐1,5‐diene, and barbaralane are in good agreement with the experimental data.