Definition of nitrogen ylides

See ylides.

ylides: Compounds in which an anionic site Y- (originally on carbon, but now including other atoms) is attached directly to a heteroatom X+ (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. NOC Rule D-5.42.

They are thus 1,2-dipolar species of the type RmX+-Y-Rn. If X is a saturated atom of an element from the first row of the periodic system, the ylide is commonly represented by a charge-separated form; if X is a second, third, etc. row element uncharged canonical forms are available: RmX=YRn.

If X is an unsaturated atom, doubly bonded to another first row element Z, the negative charge on Y may be stabilised by -conjugation, Z=X+-Y-R n Z--X+=YRn. Such ylides belong to the class 1,3-dipolar compounds. However, 1,3-dipolar compounds with only sextet-containing canonical forms (e.g. vinylcarbenes) are not ylides. See also betaines, dipolar compounds. E.g. Ph3P+-C-H2 Ph3P=CH2 (often called a Wittig reagent), (CH3)3N+-C-H2, RCN+N--R, (CH3)2S=CHPh (CH3)2S+-C-HPh.

Note that ylide is a complete word, not to be confused with the suffix -ylide, used for some radical anions.

Subclasses of ylides. Ylides RmX+-C-R2 having the negative charge on carbon are classified by citing the name of the element X before the word ylide. E.g. nitrogen ylide, phosphorus ylide, oxygen ylide, sulfur ylide. A further specification may be achieved by citing the class name of RmX before the word ylide.

Thus nitrogen ylides include amine ylides, R3N+-C-R2, azomethine ylides, R2C=N+R-C-R2, nitrile ylides, RCN+-C -R2. Some authors, who wish to express the positive charge on X, prefer e.g. ammonium ylides over amine ylides; such usage varies according to the heteroatom X and to national custom.

The ylides RmX+-Y- RmX=Y (Y = O, S, Se, Te, NR) are usually named by citing the name of RmX followed by the additive nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively).

NOC Rules C-0.3, D-5.41. E.g. amine imides; use of the less systematic synonyms amine imines and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. nitrones, Wittig reagents (synonymous with phosphonium ylides)