1,3-Dipolar Cycloaddition of Diazo Compounds to Electron-Deficient Alkenes: Kinetics and Mechanism of Formation of Dimethyl-4,5-dihydro-1H-pyrazol-3,5-dicarboxylate

1,3-Dipolar cycloaddition of methyl diazoacetate to methyl acrylate was investigated by kinetic 1H NMR spectroscopy. It was established that the mechanism of the process includes parallel formation of trans- and cis-dimethyl-4,5-dihydro-3H-pyrazol-3,5-dicarboxylates as a result of [3 + 2]-cycloaddition of methyl diazoacetate to methyl acrylate; the corresponding rate constants were denoted k1t and k1c. The reaction rate of the isomerization of 3H-pyrazolines to 4,5-dihydro-1H-pyrazol-3,5-dicarboxylate (3H 1H-pyrazoline rearrangement) was found to be sensitive to both the methyl acrylate (k2t, k2c) and 1H-pyrazoline concentrations (k3t, k3c). Kinetic analysis showed that the proposed scheme is valid for various reagent concentrations. The numerical solution of the system of differential equations corresponded to the reaction scheme and was used to determine the complete set of reaction rate constants (k (x 105 M-1 center dot s(-1)), 298 K; solvent, benzene-d6): k1t = 2.3 +/- 0.3, k1c = 1.6 +/- 0.2, k2t = 1.1 +/- 0.3, k2c = 1.8 +/- 0.5, k3t = 1.2 +/- 0.4, k3c = 2.2 +/- 0.7.