https://doi.org/10.1351/goldbook.Y06728
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
Sources:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1176 [Terms] [Paper]
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1375 [Terms] [Paper]