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Mechanism of pyrolysis of 1-pyrazolines Masters, Ian M.E.

Abstract

The kinetics for the liquid-phase thermal decomposition of a number of 1-pyrazolines have been determined. Factors determining the relative rates of decomposition were: i) the nature of the polar substituent at the C-3 position; ii) the position and stereochemistry of alkyl substitution on positions C-4 and C-5; and iii) the polarity of the solvent employed. The pyrolysis rates were found to increase with the increasing electron-withdrawing ability of the C-3 substituent. Alkyl substitution at C-4 decreased the rate of pyrolysis relative to hydrogen at C-4 but this decrease also depended on the stereochemistry of the C-4 substituted pyrazoline. Alkyl substitution at C-5 increased the rate of pyrolysis relative to hydrogen at C-5 but the stereochemistry of substitution at C-5 had little effect on the rates. A drastic polarity increase upon going from n-butyl phthalate to formamide solvent decreased the rate of pyrolysis of 3-cyano and 3-carbomethoxy substituted 1-pyrazolones by a factor of about two. However, the rate of pyrolysis of 3-acetyl substituted 1-pyrazolines was increased by a factor of about two upon going from n-butyl phthalate to formamide solvent. These results are discussed in relation to the current views on pyrazoline pyrolysis. Deuterium substitution at the C-5 position was found to decrease the pyrolysis rate due to a secondary deuterium kinetic isotope effect. A comparison of the magnitudes of these effects with those found for similar systems indicated that there is considerable breakage of the C (5)-N bond in the transition states for pyrolysis in both n-butyl phthalate and formamide solvents. Deuterium substitution at C-5 had very little effect on the product distribution. A pyrazoline with deuterium at the C-4 position was prepared, 3-mothyl-: 3-carbomethoxy- 1-pyrazolone-4, 4-d₂, The product distribution, compared to the natural compound, showed that olefin formation was decreased due to the deuterium substitution. The kinetic isotope effect on the rate of pyrolysis was found to be 1. 36 in n-butylphthalate solvent. It has been previously suggested that there may be separate transition states for olefin and cyclopropane formation. Calculations using the above isotope effect and the product distributions for the deuterated and natural pyrazoline gave a value of 1. 94 for the deuterium isotope effect on the olefin-forming reaction. This supports a mechanism for olefin formation where the migration of a C-4 hydrogen to the C-5 position is concerted with breakage of the carbon-nitrogen bond. In formamide solvent, however, the kinetic isotope effect was found to be only 1. 06. This was taken as an indication that C (4)-H bond breakage was not advanced in the transition state for pyrazoline pyrolysis in this polar solvent. However, the effect of deuterium substitution on the product distribution suggests that a nitrogen-free intermediate is formed in the rate-determining step. The deuterium isotope effect on the olefin forming step, largely primary, was estimated to be 2.2. The synthesis and decomposition products of a new pyrazoline, 3, 5, 5, -trimethyl-3-acetyl-1-pyrazoline, are described. The preparations of the deuterated pyrazolones are described and their nuclear magnetic resonance spectra arc discussed in detail.

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