(Triphenylphosphoranylidene)ketene: The Bestmann Ylide

Introduction

(Triphenylphosphoranylidene)ketene (1), also known as the Bestmann ylide, is a reagent with intriguing properties and proven synthetic utility.[ ¹ ] After early reports of this compound by others,[ ² ] H. J. Bestmann popularized the use of 1 in the formation of a wide range of (triphenyl­phosphoranylidene)acyl derivatives.[ ³ ] The Bestmann ylide (1) is now commercially available and used in a wide range of reactions. Alternatively, 1 can be prepared by the deprotonation of methyl (triphenylphosphoranylidene)acetate (2, Scheme [1)] [ ⁴ ] A variety of strong bases have been used to perform this transformation,[ ⁵ ] with NaHMDS being particularly efficient and convenient.

Scheme 1 Preparation of the Bestmann ylide

Interestingly, the X-ray crystal structure of 1 shows a 145.5° P=C=C bond angle and a remarkably short C=C bond length (1.21 Å).[ ⁶ ] These properties are derived from the combination of three resonance structures (Scheme [2]), which highlight the two overlapping, orthogonal π-systems that stabilize the molecule. This makes the reactivity of the Bestmann ylide significantly different from typical ketenes and phosphorus ylides. The Bestmann ylide is surprisingly stable and can be stored under inert atmosphere at ambient temperature for months.[ ⁴ ]

Scheme 2 Resonance structures of 1

• References

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