1,3-Dipolar cycloaddition of methyl diazoacetate to methyl acrylate was investigated by kinetic 1 Н 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 k 1t and k 1c . The reaction rate of the isomerization of 3Нpyrazolines to 4,5-dihydro-1H-pyrazol-3,5-dicarboxylate (3Н → 1Н-pyrazoline rearrangement) was found to be sensitive to both the methyl acrylate (k 2t , k 2c ) and 1Н-pyrazoline concentrations (k 3t , k 3c ). 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 500 OVCHINNIKOV ET AL. constants (k (× 10 5 M -1 ·s -1 ), 298 K; solvent, benzene-d 6 ): k 1t = 2.3 ± 0.3, k 1c = 1.6 ± 0.2, k 2t = 1.1 ± 0.3, k 2c = 1.8 ± 0.5, k 3t = 1.2 ± 0.4, k 3c = 2.2 ± 0.7. C 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 499-

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