A Molecular Electron Density Theory Study of the Reactivity and Selectivities in [3 + 2] Cycloaddition Reactions of C,N-Dialkyl Nitrones with Ethylene Derivatives

The zw-type [3 + 2] cycloaddition (32CA) reactions of C,N-dialkyl nitrones with a series of ethylenes of increased electrophilic character have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Both, reactivity and selectivities are rationalized depending on the polar character of the reaction. Due to the strong nucleophilic character of C,N-dialkyl nitrones, the corresponding zw-type 32CA reactions are accelerated with the increased electrophilic character of the ethylene, which also plays a crucial role in the reaction mechanism, thus determining the regio- and stereoselectivities experimentally observed. While, in the 32CA reactions with nucleophilic ethylenes, the reaction begins with the formation of the C-C single bond, determining the ortho regioselectivity, in the 32CA reactions with strong electrophilic ethylenes, the reaction begins with the formation of the C-O single bond involving the β-conjugated carbon of the ethylene, determining the meta regioselectivity. The present MEDT study also provides an explanation for the unexpected ortho regioselectivity experimentally found in the 32CA reactions involving weak electrophilic ethylenes such as ethyl acrylate and acrylonitrile.