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Mechanism of pyrolysis of 3,3,4,5-tetrasubstituted 1-pyrazolines. McKinley, James William

Abstract

A series of tetrasubstituted 1-pyrazolines uniquely substituted at all three carbon centers has been prepared and decomposed thermally. The 1-pyrazolines in which three of the substituents occupy a pseudo equatorial position and the remaining substituent occupies a pseudo axial position give as the major product the cyclopropane with retention of configuration. On the other hand,the 1-pyrazolines in which two substituents are pseudo equatorial and two are pseudo axial give a random distribution of cyclopropanes. Evidence is presented that the former set of pyrazolines have a larger degree of folding between the two planes defined by C-3, C-4, C-5 and C-5, N-1, N-2, C-3. This suggests that the larger the degree of folding in the pyrazoline molecule the more stereospecificity there is. Two mechanisms are proposed to account for the cyclopropane formed with retention of configuration - one involving a concerted mechanism and the other involving an intermediate resembling a pyramidal diradical. In the case of one pair of C-5 isomeric pyrazolines, the pyrazoline in which three substituents are pseudo equatorial and one is pseudo axial gave 99% cyclopropane products whereas the C-5 isomer in which there are two substituents both equatorial and axial gave 67% olefin products. This supports the mechanism involving concerted migration of the hydrogen at C-4 that is trans to the leaving nitrogen.

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