Abstract
3-Chloro-4,4,5-trimethyl-3,5-diphenyl-4,5-dihydro-3H-pyrazole (3b) and 3-bromo-4,4,5-trimethyl-3,5-diphenyl-4,5-dihydro-3H-pyrazole (3c) were prepared for the thermolysis project. The thermal decompositions of 3b and 3c were monitored using 1H NMR spectroscopy. Plots of ln (% starting material) vs. time (sec) were linear for at least two half lives and the first order rate constants were determined over at least a 30o temperature range. The relative reactivity was found to be 3c > 3b. The activation parameters determined for the thermal decomposition of the pyrazoline at 150oC were found to be: for 3b H‡ = 33 1.0 kcal/mol, S‡ = -2.4 0.07eu , k150 0 = 7.34 0.44 x 10 -5 s-1 ; for 3c H‡ = 300.2 kcal/mol, S‡ = -6.9 0.03 eu, k150o = 42.30.7 x 10-5 s-1. Thermal decomposition of 3b both neat and in dibromobenzene (DBB) resulted in the formation of an intermediate 2,3-diphenyl-4-methyl-1,3-pentadiene (8) as a major product and minor isomers of 8. These intermediates then thermally decomposed to 1,1,3-trimethyl-2-phenyl-1H-indene (9) via an acid catalyzed process. In order to gain a mechanistic understanding (ionic vs. radical pathways) of the thermal decomposition of 3b, a product study was conducted in protic solvents. In methanol and ethanol, 3b underwent an ionic reaction (SN1-type) with the solvent to produce 3-methoxy/ethoxy-4,4,5-trimethyl-3,5-diphenyl-4,5-dihydro-3H-pyrazole (3/3d) in good yield. The reaction of 3b with refluxing protic solvents led to the development of new method for the synthesis of alkoxy-4,5-dihydro-3H-pyrazoles which is both safe and efficient.
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