triphenylphosphorane

Triphenylphosphorane, or methylenetriphenylphosphorane, is a phosphorus ylide with the formula Ph3P=CH2. It is the classic Wittig reagent used to convert carbonyl compounds into alkenes. The ylide features a formal P+–C− dipole with a P=C double-bond resonance form Ph3P+–CH2−.

Preparation of methylenetriphenylphosphorane typically involves the formation of the methyltriphenylphosphonium salt from triphenylphosphine and a methylating agent, followed by deprotonation with a strong base such as n-butyllithium or potassium tert-butoxide to give Ph3P=CH2. The reagent is a relatively stable, crystalline solid that is usually stored under dry conditions.

In typical use, Ph3P=CH2 reacts with aldehydes or ketones to form alkenes and triphenylphosphine oxide as a byproduct, in what is known as the Wittig reaction. Methylenetriphenylphosphorane is an unstabilized, highly reactive ylide and generally furnishes alkenes with the methylene fragment attached to the carbonyl carbon, producing R-CH=CH2 from aldehydes or more substituted alkenes from ketones; stereochemical outcomes depend on the reagent class and reaction conditions.

The compound is a crystalline solid, soluble in many organic solvents, and is typically stored under dry conditions. It is part of a broader class of phosphoranes used in carbon–carbon bond formation. Safety considerations are typical for organophosphorus ylides; handle with appropriate precautions to avoid inhalation or skin contact. Historically, this reagent enabled the practical synthesis of alkenes by the Wittig reaction and remains a foundational tool in organic synthesis.