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Mechanistic studies of the decomposition of 1-pyrazolines Szilagyi, Sandor

Abstract

A series of pyrazolines having a carbomethoxy group on C-3 and a bridge (number of the carbon atoms = 3, 4, 5) connecting C-3 and C-4 was synthesized and studied. l-Carbomethoxy-2,3-diazabicyclo (3.3.0)oct-2-ene-5d₁ was also prepared and the overall deuterium kinetic isotope effect determined. The calculations concerning the specific deuterium kinetic isotope effects for the different processes involved in the thermolysis led to values similar to those obtained from other experiments with the exception of that for cyclopropane formation which was found to have a "inverse" isotope effect (kH/kD = 0.71). 3-tert-Butyl-3-carbomethoxy-1-pyrazoline was synthesized and its thermal and photo decomposition studied. The effect of the size of the ester group on product distribution was also examined by undertaking a product study on 3-methyl-3-carbethoxy-1-pyrazoline and 3-methyl-3-carbo-tert-butoxy-1-pyrazoline. cis- and trans-3-Methyl-3-carbomethoxy-l-pyrazoline-4d₁ and trans-3-methyl-carbethoxy-1-pyrazoline-4d₁ were prepared. Kinetic studies were done on the trans isomers and the specific deuterium kinetic isotope effects were calculated from the measured overall kinetic isotope effects. The n.m.r. studies of 1-methyl-l-carbomethoxy-cyclopropane-2d₁ obtained by both the thermal and sensitized photo-decomposition of cis- and trans-3-methyl-3-carbomethoxy-l-pyrazoline-4d₁ revealed that each of the samples isolated (collected from the gas chromatograph) from the four decomposition products contained equal amounts of cis- and trans-1-methyl-1-carbomethoxycyclopropane- 2- d₁. Especially these results, but also the thermal and photo-decomposition of cis- and trans-3-methyl-4-tert-butyl-3-carbomethoxy-l-pyrazolines, suggested that the cyclopropane formation occurred via a diradical intermediate in which the free rotation about the carbon-carbon bond would be reduced or prevented by bulky substituents. The formation of deuterated β,ƴ -olefinic esters and ethyl and methyl angelate-4d₁ as well as ethyl and methyl tiglate-3d₁ from the trans-isomers corroborated McGreer's suggestion for stereospecific olefin formation. Kinetic and product studies done on a series of cis- and trans- 3- methyl-4-alkyl-3-carbomethoxy-l-pyrazolines (alkyl = isopropyl, isobutyl and tert-butyl) provided additional proof for stereospecific olefin formation and also substantiated the idea of restricted or prevented rotation about the carbon-carbon bond in the 1,3-diradical intermediate. Since both cis- and trans-3-methyl-4-tert-butyl-3-carbomethoxy-l-pyrazolines gave only cyclopropane product upon thermolyses in contrast to most pyrazolines it became possible to undertake a direct measurement of the secondary g deuterium kinetic isotope effect for cyclopropan formation.

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