Computational insight into the carbenic character of nitrilimines from a reactivity perspective

Nitrilimines (R-CNN-R) can be described through a carbenic valence bond structure, and although intermolecular carbenic reactions from nitrilimines are unknown, intramolecular reaction products from ortho-vinyl MeCOO-CNN-Ph (1) and Ph-CNN-Ph (2) that seem to have followed two typical carbene reaction mechanisms, [1+2] cycloaddition and C-H insertion, have been reported. This study sheds light on whether such reaction mechanisms are tenable, using the electron density and its Laplacian (QTAIM), natural bond orbital (NBO) descriptions, and reaction profiles. It is shown that the reaction in 1 is distinctly different from the [1+2] cycloadditions of ethene with the typical singlet carbene CF(2) and the nitrilimine F-CNN-F with its large carbenic character, and the formal [1+2] cycloaddition product from 1 is in fact not the primary product. Similarly, it is shown that the reaction in 2 is fundamentally different from the C-H insertions of CF(2), F-CNN-F, and even H-CNN-H with its small carbenic character into methane, and again the formal C-H insertion product from 2 is not the primary product. The small model reactions from CF(2), F-CNN-F, and H-CNN-H were analyzed using B3LYP, MP2, and B2PLYP with the aug-cc-pVTZ basis set, whereas the full study was performed with B3LYP/6-31+G(2d,2p), as it was shown to be sufficient.