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-@C01267@, Z=X+–YRn\(\leftrightarrow\) Z–X+=YRn. Such ylides belong to the class 1,3 @D01753@. However, 1,3-@D01753@ with only sextet-containing canonical forms (e.g. @V06620@) are not ylides. E.g. Ph3P+–CH2\(\leftrightarrow\) Ph3P+=CH2 (often called a Wittig @R05190@), (CH3)3N+–CH2, RC≡N+N–R, (CH3)2S=CHPh\(\leftrightarrow\) (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, @P04563-1@, @O04379@, sulfur ylide. A further specification may be achieved by citing the class name of RmX before the word ylide. Thus nitrogen ylides include @A00275@, R3N+–CR2, @A00565@ R2C=N+R–CR2, @N04153@, RC≡N+–CR2. Some authors, who wish to express the positive charge on X, prefer e.g. @A00292@ over @A00275@; such usage varies according to the heteroatom X and to national custom. The ylides RmX+–Y\(\leftrightarrow\) RmX=Y (Y = O, S, Se, Te, NR) are usually named by citing the name of RmX followed by the @A00134@ nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively). E.g. @A00271@; use of the less systematic synonyms @A00272@ and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. @N04164@, @P04563-2@ (synonymous with @P04563-3@).
See also:
dipolar compounds
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]