UBC Theses and Dissertations
Mechanistic studies of the pyrolysis of 1-pyrazolines Wigfield, Yuk-Yung
The geometric stereoisomers of 3-carbomethoxy-3-methyl-4-bromo-1-pyrazoline have been prepared. Both compounds have been found to be highly unstable and are transformed readily into the hydrobromide salt of 3-methyl-4-carbomethoxypyrazole. This transformation requires preferential migration of a carbomethoxy group over a methyl group. A stepwise mechanism is proposed for this rearranagement. Pyrolysis of 3-carbomethoxy-3-cyano-4-methyl-4-aryl-1-pyrazoline, (Z)-and (E)- gives predominantly the olefin expected for concerted migration with elimination of nitrogen. The transition state is described by a structure in which there is no bond breaking of C(5)-N bond. This mechanism is supported by (i) the lack of a C(5) secondary ⍺-deuterium kinetic isotope effect, (ii) the negative value of the entropy of activation, and (iii) the migratory aptitudes between various substituted aryl groups. The possibility of a short-lived intermediate is not completely ruled out. However, the lack of a large effect of solvent polarity on the rate of reaction and the stereospecificity of the reaction suggest the intermediate of the pyrazoline pyrolysis cannot be a zwitterion with free rotation about the C(3)-C(4) bond.
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