Poster Abstract of Recent Results in '98
Mechanistic Study of Vinylidene Intermediates in Thermal Cyclopropene Isomerisations

by Wilhelm Graf von der Schulenburg* and Henning Hopf
Institut für Organische Chemie der Technische Universität Braunschweig, Germany
and
Robin Walsh
Department of Chemistry, University of Reading, UK.

The isomeric 1,3-dimethylcyclopropene and 1-ethylcyclopropene, and the isomeric 1,3,3-trimethylcyclopropene and 1-isopropylcyclopropene have been synthesised and the kinetics of their thermal isomerisations individually investigated in the gas-phase. Pyrolyses of each cyclopropene generates the other isomer as a transient species in quantities of several percent (up to 15%). This cyclopropene-to-
cyclopropene interconversion demonstrates clearly the involvement of vinylidene intermediates in thermal cyclopropene isomerisations.[1]

Cyclopropene-to-Cyclopropene Isomerisation
Besides the intramolecular insertion path to cyclopropene the vinylidene itself decomposes to alkyne involving a rate-affecting alkyl migration. To establish the ratio of alkyl migration of unsymmetrically substituted vinylidene carbenes, we have investigated the thermal rearrangement of the 13C-labelled 1,3-dimethylcyclopropene and 13C-labelled 1,3,3-trimethylcyclopropene. Both 13C-labelled vinylidenes rearrange either via a methyl shift or by an alkyl shift (ethyl or isopropyl) to the alkyne, allowing a direct comparison of the respective ratios of alkyl migrations. The migration preference order was found to be: ethyl > isopropyl > methyl.[2]
Alkyl Migration in the Vinylidene-to-Alkyne Path
Theoretical investigations employing high-level density functional - and ab initio -methods[3] confirm the pathways for these cyclopropene isomerizations and give the relative stabilities of the vinylidene intermediates, which cannot be obtained by experiment. 

Details of results and their further implications will be discussed at the meeting.

[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[2] W. Graf von der Schulenburg, H. Hopf, R. Walsh, Unpublished results.
[3] W. Graf von der Schulenburg, N. Goldberg, Unpublished results.
 
Email: W-K.Schulenburg@tu-bs.de
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