ylidenes

Ylides are neutral molecules containing a carbanion adjacent to a positively charged heteroatom. This specific arrangement creates a zwitterionic structure. The most common and extensively studied class are phosphorus ylides, often referred to as Wittig reagents. These are typically prepared by the deprotonation of phosphonium salts. The carbanionic carbon in an ylide exhibits nucleophilic character and can react with electrophiles.

The significance of ylides lies primarily in their application in organic synthesis. The Wittig reaction, which employs phosphorus ylides, is a fundamental method for the stereoselective synthesis of alkenes from carbonyl compounds (aldehydes and ketones). This reaction proceeds via a four-membered ring intermediate called an oxaphosphetane, which then decomposes to form the alkene and triphenylphosphine oxide. Other types of ylides, such as sulfur ylides, also find synthetic utility, for instance, in the epoxidation of $\alpha,\beta$-unsaturated carbonyl compounds. The stability and reactivity of ylides are influenced by the nature of the substituents attached to both the carbanion and the heteroatom.