Stability and Stereochemistry in the Decomposition of Pentasubstituted 1-Pyrazolines Controlled by Interactions between Bulky Vicinal Substituents

Article abstract of DOI:10.1021/jo00266a027

1,2-Diacyl-1-chloroethylenes were reacted with several disubstituted diazomethanes to give the pyrazolines and the cyclopropanes.The terminal decomposition of the isolated pyrazolines was carried out.The thermal stability of the pyrazolines increases with the variation of the substituents at C-5 in the order biphenylylene < Ph, Ph < Ph, Me < Me, Me, while pyrazolines bearing bulky vicinal substituents at C-3, C-4, and C-5 in the cis configuration are substantially more stable than other isomers.This abnormal stability is explained by the reasonable expectation that bulky C-4 substituents partly inhibit conformations of the conjugated substituents at C-3 or C-5 favorable for the decomposition, which increases the activation energy.Although most of the thermal transformations of the pyrazolines to the cyclopropanes retain the stereochemistry of the starting materials, some pyrazolines bearing bulky vicinal cis groups gave mixtures of stereoisomeric cyclopropanes.The mechanism for the thermolysis of these pyrazolines is explained on the basis of stereochemical distribution of decomposition products by (90,90) trimethylene intermediates.