Understanding the domino reaction between 1-diazopropan-2-one and 1,1-dinitroethylene. A molecular electron density theory study of the [3+2] cycloaddition reactions of diazoalkanes with electron-deficient ethylenes

The reaction between 1-diazopropan-2-one and 1,1-dinitroethylene has been studied using the Molecular Electron Density Theory (MEDT) at the B3LYP/6-31G(d,p) computational level. This reaction comprises two domino processes initialised by a common [3 + 2] cycloaddition (32CA) reaction yielding a 1-pyrazoline, which participates in two competitive reaction channels. Along channel I, 1-pyrazoline firstly tautomerises to a 2-pyrazoline, which by a proton abstraction and spontaneous loss of nitrite anion yields the final pyrazole, while along channel II, the thermal extrusion of the nitrogen molecule in 1-pyrazoline gives a very reactive diradical intermediate which quickly yields the final . Analysis of the conceptual DFT reactivity indices permits an explanation of the participation of 1-diazopropan-2-one in polar 32CA reactions. A Bonding Evolution Theory (BET) study along the more favourable regioisomeric reaction path associated to the 32CA reaction allows an explanation of its molecular mechanism. The present MEDT study sheds light on these complex domino reactions as well as on the participation of diazoalkanes in polar 32CA reactions towards strong electrophilic ethylenes via a two-stage one-step mechanism.