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.

${\displaystyle {\ce {H-\!{\equiv }}}{\color {Blue}{\ce {N+}}}\!{-}\!{\color {Red}{\ce {N-}}}\!{-}\!{\color {Red}{\ce {H}}}}$ 1

${\displaystyle {\ce {H-\!{\equiv }N=N-H}}}$ 2

${\displaystyle {\ce {H}}\!{-}\!{\color {Red}{\ce {C-}}}\!{=}{\color {Blue}{\ce {N+}}}\!{\ce {=N-H}}}$ 3

${\displaystyle {\ce {H}}\!{-}\!{\color {Blue}{\ce {C+}}}\!{\ce {=N}}\!{-}\!{\color {Red}{\ce {N-}}}\!{\ce {-H}}}$ 4

${\displaystyle {\ce {H-C{:}\!-N=N-H}}}$ 5

Nitrilimines were first observed in the thermal decomposition of 2-tetrazoles releasing nitrogen:^[1]

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.