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[email protected]@, Z=X+–YRn\(\leftrightarrow\) Z–X+=YRn. Such ylides belong to the class 1,3 @[email protected] However, 1,[email protected]@ with only sextet-containing canonical forms (e.g. @[email protected]) are not ylides. E.g. Ph3P+–CH2\(\leftrightarrow\) Ph3P+=CH2 (often called a Wittig @[email protected]), (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, @[email protected], @[email protected], sulfur ylide. A further specification may be achieved by citing the class name of RmX before the word ylide. Thus nitrogen ylides include @[email protected], R3N+–CR2, @[email protected] R2C=N+R–CR2, @[email protected], RC≡N+–CR2. Some authors, who wish to express the positive charge on X, prefer e.g. @[email protected] over @[email protected]; 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 @[email protected] nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively). E.g. @[email protected]; use of the less systematic synonyms @[email protected] and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. @[email protected], @[email protected] (synonymous with @[email protected]).
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]