ylidemediated

Ylidemediated is an adjective used to describe chemical transformations in which a ylide intermediate acts as the key mediator. In organic chemistry, ylides are species with adjacent positively and negatively charged centers, typically phosphorus ylides (phosphoranes) and sulfur ylides (sulfonium and sulfoxonium). The term emphasizes the central role of the ylide in driving bond formation or rearrangement.

Mechanistically, ylide-mediated processes span several well-known reaction classes. In phosphorus-ylide–mediated processes such as the Wittig reaction, a phosphorane ylide adds to a carbonyl compound to form a betaine that cyclizes to an oxaphosphetane and collapses to yield an alkene with a phosphate oxide byproduct. Sulfur ylides, generated by deprotonation, can act as nucleophiles to effect epoxidation or cyclopropanation of carbonyl substrates under appropriate conditions, as exemplified by the Corey-Chaykovsky reaction family.

Representative reactions and applications include the Wittig olefination for alkene synthesis, and sulfur-ylide–mediated epoxidation or cyclopropanation of aldehydes and ketones. Ylide-mediated methods are valued for enabling diverse carbon–carbon and carbon–heteroatom bond constructions, often with access to stereochemical control through reagent design and reaction parameters.

Scope, advantages, and limitations: Ylide-mediated processes offer broad substrate compatibility and, in some cases, high selectivity. They frequently require generation of reactive ylides and byproducts such as triphenylphosphine oxide, which can affect atom economy. Reactivity and outcomes are sensitive to base strength, solvent, and substituent effects, requiring careful optimization for each transformation.

History and usage: The concept arose in mid-20th-century ylide chemistry, with the recognition of ylides as key intermediates in major transformations. Today, “ylide-mediated” remains a descriptive term in literature to denote mechanisms where the ylide drives bond formation and rearrangement.