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Evaluation of mechanisms for the formation of cyclopropanes from pyrazolines and the interconversion of cyclopropyl ketones and dihydrofurans McKinley, James William

Abstract

A series of 3-methyl-3-acetyl-l-pyrazolines were prepared and their product distributions were determined. Part of this series included tetrasubstituted 1-pyrazolines uniquely substituted at all three ring carbons. These pyrazolines have the advantage in that they can decompose to four isomeric cyclopropanes, and thus information about the degree of retention or inversion at both C-3 and C-5 of the starting pyrazoline was obtained. The product distributions from the direct and sensitized photolysis of these pyrazolines were explained by invoking diradicals which retain the integrity of the pyrazoline precursor. The formation of dihydrofurans from cyclopropyl ketones was approached by using cyclopropanes uniquely substituted at all three ring carbons and thus information was derived about the stereochemical pathway for this conversion. The reaction was investigated both thermally and photochemically and therefore complementary results were obtained for the two modes of reaction. A step-wise mechanism for the reaction was favoured in which diradicals that retain the integrity of the cyclopropane precursor were used to explain the product distributions. In addition, the conversion of a cyclopropyl ketone to a dihydro-furan was investigated by a kinetic study and was found to proceed with a high activation energy of 48.1 Kcal/mole (log A = 14.9). These activation parameters are consistent with a step-wise mechanism involving diradicals that retain the integrity of the cyclopropane precursor.

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