Nitrilimine

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:^[1]

6

One alternative synthesis technique is an elimination reaction of hydrazoyl halides.^[2]

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.

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