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Olefination with Stabilized Carbanion Nucleophiles

This guide is an early version — the text is complete, and a few figures are still being redrawn. Spotted something unclear? Let us know.

The question this page answers: How can we make alkenes from carbonyls?

Deeper reading: Clayden 2e: Chapter 27 Pages 664–667 and 689–693 — see our chapter-by-chapter practice map for Clayden.

What ylides are

What is an ylide?

Ylides are molecules with a resonance form containing adjacent atoms with opposite charges and full octets.

Experimentally, the ylide resonance form is found to contribute more to the hybrid than the less charge-separated ylene one:

Experimentally, the ylide resonance form is found to contribute more to the hybrid than the less charge-separated ylene one:

While phosphorus ylides are most common, there are many other analogous ylides:

While phosphorus ylides are most common, there are many other analogous ylides:

Synthesizing ylides from onium salts

Deprotonate the onium salt

Ylides are typically synthesized by deprotonating the corresponding onium salt, which itself can be prepared by an SN2 reaction:

Ylides are typically synthesized by deprotonating the corresponding onium salt, which itself can be prepared by an SN2 reaction:

Several variations of olefination

Which name goes with which reagent?

Ylides are useful nucleophiles for converting ketones and aldehydes into alkenes. There are several variations:

Wittig olefination

Phosphonium ylide → Z alkene

The Wittig Olefination favors the formation of Z alkenes. The reaction proceeds mechanistically through an oxaphosphetane intermediate:

The Wittig Olefination favors the formation of Z alkenes. The reaction proceeds mechanistically through an oxaphosphetane intermediate:

Horner-Wadsworth-Emmons olefination

Phosphonate → E alkene

The Horner-Wadsworth-Emmons (HWE) Olefination uses a phosphonate instead of a phosphonium ylide and tends to favor the formation of E alkenes. The mechanism proceeds through two intermediates, first an alkoxide and then an oxaphosphetane:

The Horner-Wadsworth-Emmons (HWE) Olefination uses a phosphonate instead of a phosphonium ylide and tends to favor the formation of E alkenes. The mec

Julia-Lythgoe and Julia-Kocienski olefinations

Sulfones → E alkenes

The Julia-Lythgoe Olefination uses a sulfone and tends to favor the formation of E alkenes. The mechanism involves first an addition of the carbanion to the carbonyl, prior to a dissolving metal reduction/deprotonation process:

The Julia-Lythgoe Olefination uses a sulfone and tends to favor the formation of E alkenes. The mechanism involves first an addition of the carbanion

The Julia-Kocienski Olefination uses a heterocyclic sulfone instead of a phenyl one and tends to favor the formation of E alkenes. The heterocycle enables the reaction to proceed without the dissolving metal phase:

The Julia-Kocienski Olefination uses a heterocyclic sulfone instead of a phenyl one and tends to favor the formation of E alkenes. The heterocycle ena

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