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Nitrilimine

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Nitrilimines or nitrile amides are a class of organic compounds sharing a common functional group with the general structure R-CN-NR 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.

H − ≡ N + − N − − H {\displaystyle {\ce {H-\!{\equiv }}}{\color {Blue}{\ce {N+}}}\!{-}\!{\color {Red}{\ce {N-}}}\!{-}\!{\color {Red}{\ce {H}}}}
1		   H − ≡ N = N − H {\displaystyle {\ce {H-\!{\equiv }N=N-H}}}
2		   H − C − = N + = N − H {\displaystyle {\ce {H}}\!{-}\!{\color {Red}{\ce {C-}}}\!{=}{\color {Blue}{\ce {N+}}}\!{\ce {=N-H}}}
3		   H − C + = N − N − − H {\displaystyle {\ce {H}}\!{-}\!{\color {Blue}{\ce {C+}}}\!{\ce {=N}}\!{-}\!{\color {Red}{\ce {N-}}}\!{\ce {-H}}}
4		   H − C : − N = N − H {\displaystyle {\ce {H-C{:}\!-N=N-H}}}
5

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

Scheme 2. Nitrilimine formation
Scheme 2. Nitrilimine formation
 (6)

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.

References

  1. Huisgen, Rolf; Seidel, Michael; Sauer, Juergen; McFarland, James; Wallbillich, Guenter (June 1959). "Communications: The Formation of Nitrile Imines in the Thermal Breakdown of 2,5-Disubstituted Tetrazoles". The Journal of Organic Chemistry. 24 (6): 892–893. doi:10.1021/jo01088a034.
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