Poster Abstract of the Orchem'98 Tagung in Bad Nauheim
Theoretical Characterization of Vinylidenes as Intermediates in Thermal Cyclopropene Isomerizations

Norman Goldberg and Wilhelm Graf von der Schulenburg

Institut für Organische Chemie der Technische Universität Braunschweig
Hagenring 30, D-38106 Braunschweig, Germany

Several experimental studies of cyclopropene rearrangements have lead to strong evidence for the involvement of vinylidenes as reactive intermediates during the thermal ring opening and interconversion of isomeric cyclopropenes (Scheme 1).[1] However, the relative stabilities of the vinylidene intermediates could not be assessed in these experimental studies. We have therefore carried out a theoretical investigation employing high-level density functional - and ab initio -methods to further elucidate the pathways for these cyclopropene isomerizations.

We find the substituted singlet vinylidenes to lie in potential wells of about 7-11 kcal/mol (Scheme 2) with respect to reversible cyclopropene formation and alkyl migration giving rise to the corresponding alkynes. The availability of experimentally determined activation energies for the vinylidene formation from the corresponding cyclopropenes for various C4H6-, C5H8-, and C6H10 species allows for an immediate comparison of the accuracy of the theoretical energies of the transition states involved in these interconversions. The average deviation of the theoretically determined barrier heights from the experimental values is found to lie around 1.5 kcal/mol at the B3LYP/6-311+G** level of theory.
[1] H. Hopf, W. Graf von der Schulenburg, R. Walsh, Angew. Chem. 1997, 109, 415-417; 
      Angew. Chem., Int. Ed. Engl. 1997, 36, 381-383.
[2] W. Graf von der Schulenburg, H. Hopf, R. Walsh, Unpublished results.
[3] N. Goldberg, W. Graf von der Schulenburg, Chem. Commun. 1998, 2761-2762 (incl. Suppl.Mat.).
Email: W-K.Schulenburg@tu-bs.de
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