Thermolysis of hexasubstituted-4,5-dihydro-3H-pyrazoles: Kinetics and activation parameters

A kinetics study of the thermolysis of a series of hexasubstituted-4,5-dihy dro-3H-pyrazoles (pyrazoles 1a: 3,3,4,4-tetramethyl-5-phenyl-5-acetoxy 1b: cis-3, 5-diphenyl-3, 3, 4-trimethyl-5-acetoxy; 1c: cis-3,5-diphenyl-3, 4 4- trimethyl-5-methoxy; 1d: 3,3,5-triphenyl-4, 4-dimethyl-5-acetoxy), which pr oduced the corresponding hexasubstituted cyclopropanes 2a-d in quantitative yields was carried out. The first order rate constants (k(1)) for thermal decomposition and activation parameters were determined. The relative react ivity series was found to be 1d >> 1b similar to 1c > 1a. The activation pa rameters for thermolysis were found to be: for 1a Delta H double dagger = 3 9.8 kcal/mol, Delta S double dagger = 14 eu, k(150 degrees) = 6.8 x 10(-5) s(-1); for 1b Delta H double dagger = 33.5 kcal/ mol, Delta S double dagger = 0.2 eu, k(150 degrees). = 1.7 x 10(-4)s(-1); for 1c Delta H double dagge r = 32.7 kcal/mol, Delta S double dagger = -1.8 eu, k(150 degrees). = 1.2 x 10(-4)s(-1); for 1d Delta H double dagger = 30.1 kcal/mol, Delta S double dagger = -1.6 eu, k(150 degrees) = 8.8 x 10(-3)s(-1). The effect of variati on of C3 substituents on the activation parameters for thermolysis parallel ed the trend reported for acyclic analogs. The results are consistent with the formation of a (singlet) 1,3-diradical intermediate with subsequent clo sure to yield the cyclopropanes. The mechanism of diradical formation appea rs to involve N-2-C-3 bond cleavage as the rate determining step rather tha n simultaneous two bond scission. (C) 2000 John Wiley & Sons, Inc.