Electrically neutral molecules carrying a positive and a negative charge in one of their major canonical descriptions. In most dipolar compounds the charges are delocalized; however the term is also applied to species where this is not the case. 1,2-Dipolar compounds have the opposite charges on adjacent atoms. The term 1,3-dipolar compounds is used for those in which a significant canonical resonance form can be represented by a separation of charge over three atoms (in connection with 1,3-dipolar cycloadditions). Subclasses of 1,3-dipolar compounds include:
(i) Allyl type
X=Y+-Z- -X-Y+=Z +X-Y-Z- -X-Y-Z+ (X, Z = C, N, or O; Y = N or O) See azo imides, azomethine imides, azomethine ylides, azoxy compounds, carbonyl imides, carbonyl oxides, carbonyl ylides, nitrones, nitro compounds.
(ii) Propargyl type
XN+-Z- -X=N+=Z -X=N-Z+ X-N=Z (X = C or O, Z = C, N, or O) See nitrile imides, nitrile oxides, nitrile ylides, nitrilium betaines, azides, diazo compounds.
(iii) Carbene type
:X-C=Z +X=C-Z- (X = C or N; Z = C, N, or O) See acyl carbenes, imidoyl carbenes, vinyl carbenes.
See betaines. R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, A. Padwa, Ed.,Vol. 1, Wiley, New York, 1984, p. 3.
E.g. RN--N+N RN=N+=N- RN--N=N+ ; RCN+-O- RC-=N+=O RC+=N-O-
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