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UBC Theses and Dissertations

Pyrolysis and photolysis of cis and trans-3, 5-dimethyl-3-acetyl-delta1-pyrazoline and cis and trans-3,5-dimethyl-3-carbomethoxy-delta1-pyrazoline. Chiu, Norman Wing Kwai


The thermal and photolytic decomposition of cis and trans_-3,5-dimethyl-3-acetyl-Δ¹-pyrazoline have been found to give six products. They have been separated and identified as 2,3,5-trimethyl-Δ²-dihydrofuran, cis and trans-1,2-dimethyl-1- acetyleyclopropane, cis and trans-3-methyl-3-hexen-2-one and 3-methyl-4-hexen-2-one. The formation of cyclopropanes by photolysis showed some degree of stereospecifIcity. Both pyrolysis and photolysis yielded olefins with high degree of stereospecificity. 2,3,5-TrImethyl-Δ²-dihydrofuran was a major product from the decomposition of cis-3,5-dimethyl-3-acetyl-Δ¹ -pyrazoline only. Pyrolysis gave a higher ratio of olefins to cyclopropanes than photolysis. These decomposition reactions gave results analogous to those of cis and trans-3,5-dimethyl-3-carbomethoxy-Δ¹-pyrazoline. The product compositions from both the pyrolysis and photolysis of cis and trans_-3,5-dimethyl-3-carbomethoxy-Δ¹-pyrazoline have been found to show a small and regular influence of the solvent and this has been related to the dielectric constant of the solvent. Pyrolysis and photolysis gave an olefin to cyclopropane ratio of 57:43 and 21:79, respectively, in formamide, and 7:93 and 5:95, respectively, in cyclohexane. A small kinetic solvent effect has been observed for the pyrolysis of 3,5-dimethyl-3-carboraethoxy-Δ¹-pyrazoline. The rate of pyrolysis as followed by the rate of nitrogen evolution has been found to decrease for the following series of solvents: dl-n-butyl ether, tetralin, nitrobenzene and form-amide. These rates were all within a factor of three. The absence of rate enhancement in a solvent of high dielectric constant has been used as an argument against an ionic Intermediate in these reactions. Liquid phase photolysis of trans-3,5-dimethyl-3-carbo- methoxy-Δ¹-pyrazolIne at various temperatures ranging from -55ᵒ to 58ᵒ did not show appreciable change in the product compositions attributable to the influence of temperature. The solvent temperature therefore does not affect the amount of quenching of any "hot" intermediate. Photolysis and pyrolysis of cis and trans-3,5-dimethy1-3-carbomethoxy-Δ¹ -pyrazoline under identical conditions did not give the same product composition. This suggested the two reactions do not have a common Intermediate. No isomerization between the cis and trans-3,5-dimethyl-3-carbomethoxy-Δ¹-pyrazolines has been observed as shown by the partial pyrolysis and photolysis of the trans-pyrazoline. These results are discussed In view of current mechanistlc proposals for the pyrolysis and photolysis of Δ¹-pyrazolines.

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