Ylid
A ylide or ylid (/ˈɪlɪd/ or /ˈɪlaɪd/) is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. Ylides are thus 1,2-dipolar compounds. They appear in organic chemistry as reagents or reactive intermediates.
The class name "ylide" for the compound should not be confused with the suffix "-ylide".
Many ylides may be depicted by a multiple bond form in a resonance structure, known as the ylene form:
The actual electron distribution in the molecules and hence the relative importance of the ylide and ylene forms is dependent on the "onium" center and substituent pattern (the identity of the various R groups)
Phosphonium ylides are used in the Wittig reaction, a method used to convert ketones and especially aldehydes to alkenes. The positive charge in these Wittig reagents is carried by a phosphorus atom with three phenyl substituents and a bond to a carbanion. Ylides can be 'stabilised' or 'non-stabilised'. A phosphonium ylide can be prepared rather straightforwardly. Typically, triphenylphosphine is allowed to react with an alkyl halide in a mechanism analogous to that of an SN2 reaction. This quaternization forms an alkyltriphenylphosphonium salt, which can be isolated or treated in situ with a strong base (in this case, butyl lithium) to form the ylide.
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