Further Examples of Sigmatropic Reactions

The selection rules for sigmatropic reactions are powerful predictors of thermal rearrangements. They explain how some reactions can occur ster-eospecifically and with surprisingly low activation energies, while others do not. For example, cfs-l,2-divinylcyclopropane (44) undergoes a [3,3] sigmatropic rearrangement to cis,cis-l,4-cycloheptatriene (45) with a half-life of 

Several reactions similar to the Cope and Claisen rearrangements have been developed for s)mthetic applications. A variation on the Cope 

Crawford, R. J. Can.. Chem. 1972,50,2158. Reclosure of the intermediate to 44 would then allow rapid Cope rearrangement to 45. See also the discussion in reference 67 concerning the rate-limiting step for this pathway. 

Analysis by deuterium NMR on a sample dissolved in a liquid crystal solvent gave a linear Arrhenius plot with a calculated rate constant of4600 s at300°C, AH of 13.3 kcal/mol, and AS of + 2.5 eu Poupko, R. Zimmermann, H. Luz, Z. /. Am. Chem. Soc. 1984,106,5391. The value of AS for a reaction measured in a liquid crystal solvent may include a contribution from disorder introduced into the solvent as the reaction occurs. 

Valence tautomerization of bullvalene has also been studied both in the gas phase by Moreno, P. O. Suarez, C. Tafazzoli, M. True, N. S. LeMaster, C. B. /. Phys. Chem. 1992, 96,10206 and in crystal form by Schlick, S. Luz, Z. Poupko, R. Zimmermann, H. /. Am. Chem. Soc. 1992,114,4315. 

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