Fosforüülide

Fosforüülide, also known as phosphonium ylides, are a class of organophosphorus compounds that contain a phosphorus atom with a positive charge bonded to a carbon atom bearing a negative charge. This unique arrangement of charges results in a zwitterionic or dipolar structure. The general formula for a fosforüülide is R3P=CR2R', where R, R', and R'' are organic groups. The phosphorus atom typically has three organic substituents and is formally double-bonded to a carbanion.

The reactivity of fosforüülides is largely attributed to the polar nature of the phosphorus-carbon double bond. The carbanionic character of the carbon atom makes it a strong nucleophile, readily attacking electrophilic centers. This nucleophilicity is a key feature that underpins their most important application: the Wittig reaction. In this reaction, a fosforüülide reacts with an aldehyde or ketone to form an alkene and triphenylphosphine oxide (if triphenylphosphine was used to form the ylide). This reaction is highly versatile for the synthesis of carbon-carbon double bonds and is widely employed in organic synthesis.

Fosforüülides are typically prepared by deprotonating a phosphonium salt. Phosphonium salts are formed by the reaction of a tertiary phosphine, such as triphenylphosphine, with an alkyl halide. The resulting phosphonium salt, which has an acidic alpha-hydrogen adjacent to the positively charged phosphorus atom, is then treated with a strong base, like an alkoxide or an organolithium reagent, to generate the corresponding fosforüülide. The stability of fosforüülides can vary depending on the nature of the organic groups attached to the phosphorus and carbon atoms.