Nitrilimine

Nitrilimines or nitrile amides are a class of organic compounds sharing a common functional group with the general structure Template:Chem2 corresponding to the conjugate base of an amine bonded to the N-terminus of a nitrile. The dominant structure for the parent compound nitrilimine is that of the propargyl-like 1 in scheme 1 with a C–N triple bond and with a formal positive charge on nitrogen and two lone pairs and a formal negative charge on the terminal nitrogen. Other structures such as hypervalent 2, allene-like 3, allylic 4 and carbene 5 are of lesser relevance.

$\mathrm{H}-\equiv \mathrm{N}{\phantom{A}}^{+}-\mathrm{N}{\phantom{A}}^{-}-H$ 1

$\mathrm{H}-\equiv \mathrm{N}=\mathrm{N}-\mathrm{H}$ 2

$H-\mathrm{C}{\phantom{A}}^{-}=\mathrm{N}{\phantom{A}}^{+}=\mathrm{N}-\mathrm{H}$ 3

$H-\mathrm{C}{\phantom{A}}^{+}=\mathrm{N}-\mathrm{N}{\phantom{A}}^{-}-\mathrm{H}$ 4

$\mathrm{H}-\mathrm{C}\text{:}-\mathrm{N}=\mathrm{N}-\mathrm{H}$ 5

Nitrilimines were first observed in the thermal decomposition of 2-tetrazoles releasing nitrogen:

Nitrilimines are linear 1,3-dipoles represented by structures 1 and 3. A major use is in heterocyclic synthesis. E.g. with alkynes they generate pyrazoles in a 1,3-dipolar cycloaddition. Due to their high energy, they are usually generated in situ as a reactive intermediate.