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-dihydro-3H-pyrazoles (pyrazolines 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 produced the corresponding hexasubstituted cyclopropanes 2a–d in quantitative yields was carried out. The first order rate constants (k₁) for thermal decomposition and activation parameters were determined. The relative reactivity series was found to be 1d >> 1b ∼ 1c > 1a. The activation parameters for thermolysis were found to be: for 1a ΔH‡ = 39.8 kcal/mol, ΔS‡ = 14 eu, k_150° = 6.8 × 10⁻⁵ s⁻¹; for 1b ΔH‡ = 33.5 kcal/mol, ΔS ‡ = 0.2 eu, k_150° = 1.7 × 10⁻⁴s⁻¹; for 1c ΔH‡ = 32.7 kcal/mol, ΔS‡ = −1.8 eu, k_150° = 1.2 × 10⁻⁴s⁻¹; for 1d ΔH‡ = 30.1 kcal/mol, ΔS‡ = −1.6 eu, k_150° = 8.8 × 10⁻³s⁻¹. The effect of variation of C3 substituents on the activation parameters for thermolysis paralleled the trend reported for acyclic analogs. The results are consistent with the formation of a (singlet) 1,3-diradical intermediate with subsequent closure to yield the cyclopropanes. The mechanism of diradical formation appears to involve N₂-C₃ bond cleavage as the rate determining step rather than simultaneous two bond scission. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:299–302, 2000