YlidesIUPAC Term

Compounds in which an anionic site Y sup class minus /sup (originally on carbon, but now including other atoms) is attached directly to a heteroatom X sup class plus /sup (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. They are thus 1,2-dipolar species of the type R sub i m /i /sub X sup class plus /sup –Y sup class minus /sup R sub i n /i /sub . 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 R sub i m /i /sub X YR sub i n /i /sub . 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 sup class plus /sup –Y sup class minus /sup R sub i n /i /sub Z sup class minus /sup –X sup class plus /sup YR sub i n /i /sub . 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. Ph sub 3 /sub P sup class plus /sup –C sup class minus /sup H sub 2 /sub Ph sub 3 /sub P sup class plus /sup CH sub 2 /sub (often called a Wittig reagent), (CH sub 3 /sub ) sub 3 /sub N sup class plus /sup –C sup class minus /sup H sub 2 /sub , RC N sup class plus /sup N sup class minus /sup –R, (CH sub 3 /sub ) sub 2 /sub S CHPh (CH sub 3 /sub ) sub 2 /sub S sup class plus /sup –C sup class minus /sup 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 R sub i m /i /sub X sup class plus /sup –C sup class minus /sup R sub 2 /sub 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, i sulfur ylide /i . A further specification may be achieved by citing the class name of R sub i m /i /sub X before the word ylide. Thus nitrogen ylides include amine ylides, R sub 3 /sub N sup class plus /sup –C sup class minus /sup R sub 2 /sub , azomethine ylides R sub 2 /sub C N sup class plus /sup R–C sup class minus /sup R sub 2 /sub , nitrile ylides, RC N sup class plus /sup –C sup class minus /sup R sub 2 /sub . 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 R sub i m /i /sub X sup class plus /sup –Y sup class minus /sup R sub i m /i /sub X Y (Y O, S, Se, Te, NR) are usually named by citing the name of R sub i m /i /sub X 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).