Also contains definition of: nitrogen 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. They are thus 1,2-dipolar species of the type RmX+–YRn. 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 stabilized by π-conjugation, Z=X+–YRn↔︎ 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. E.g. Ph3P+–CH2↔︎ Ph3P+=CH2 (often called a Wittig reagent), (CH3)3N+–CH2, RC≡N+N–R, (CH3)2S=CHPh↔︎ (CH3)2S+–CHPh. 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+–CR2 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+–CR2, azomethine ylides R2C=N+R–CR2, nitrile ylides, RC≡N+–CR2. 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). 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).
See also: betaines, dipolar compounds
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1176 (https://doi.org/10.1351/pac199466051077)
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1375 (https://doi.org/10.1351/pac199567081307)