Nature of Lewis Base Catalysis of 1,3-Dipolar Cycloaddition of Methyl Diazoacetate to Methyl Acrylate; NMR Kinetic Spectroscopy and DFT Study

Abstract

The effect of Lewis base (LB) in the domino reaction between methyl diazoacetate and methyl acrylate has been studied. This domino process is initialized by a [3+2]-cycloaddition reaction to generate 3H-pyrazoline followed by a subsequent 1,3-H shift reaction forming 1H-pyrazoline as the more stable isomer. The rate of the first step is not sensitive to the presence of LBs (THF, Py, DMAP, DBU, and triphenylphosphine) as it was evidenced by kinetic nuclear magnetic resonance spectroscopy and quantum chemical modeling. LBs manifest remarkable catalytic effect on the second step of the reaction only acting as proton acceptor. DFT calculations reveal fine correlation between enthalpy of proton transfer from trans-3H-pyrazoline to LB and basic strength of the latter described in terms of BF-affinity scale. Under conditions of LB catalysis the reaction rate of the first step (methyl diazoacetate and methyl acrylate interaction) limits the rate of 3H → 1H pyrazoline isomerization and, therefore, restricts catalytic efficiency of LB. An alternative mechanism for catalysis of the 1,3-dipolar cycloaddition through formation of triazene-like intermediate during reaction of diazo acetate with LB and following Michael addition of intermediate to alkene was carefully analyzed. This reaction scheme was not confirmed in our experiments.