Pyrazolines XI. Determination of the Stereochemical Changes at Carbons-3 and -5 of 1-Pyrazolines on Their Thermal Conversion to Cyclopropanes

Abstract

Six 3,3,4,5-tetrasubstituted pyrazolines have been prepared and the product distribution from their pyrolysis reactions have been determined. This has permitted for the first time the unambiguous evaluation of the stereochemical fate of C-3 and -5 of the pyrazolines on conversion to cyclopropanes. A stereoselectivity is noted for these systems which can be related to the structure of the initial pyrazoline. This suggests that the transition state for reaction retains many of the stereochemical factors present in the pyrazoline. Such a condition would be the case for concerted breaking of the two C—N bonds with a developing overlap of the new bonds being formed at the transition state. That this reaction is not restricted by symmetry constraints suggests that the three carbon fragment can act as a symmetric or antisymmetric unit and that these are not separated by more than a few kcal of energy.