Science of Synthesis

Schobert, R.; Gordon, G. J., Science of Synthesis, (2004) 27, 1050.

Phosphacumulene ylides ‌156‌ (X = NR1, O, S) and hydrogen halides[‌568‌] (and alkyl halides[‌569‌]) form highly reactive ketenylium salts. By reactions typical of ketene systems in general, these intermediates undergo immediate [2+2] cycloadditions with a second equivalent of the starting ylide to give “dimeric” salts such as ‌157‌ (‌Scheme 56‌); however, when the ylide ‌156‌ (X = O) is reacted with alcohols, amines, and thiols the intermediate ketenylium cations are intercepted by the more strongly nucleophilic counter anion (alkoxide, amide, thiolate) to yield monomeric “acyl” ylides ‌158‌. The ylide ‌156‌ (X = NPh) also reacts with carboxylic acids to give the corresponding acyl ylides ‌159‌ upon heating.[‌249‌] As acyl ylides enter into Wittig alkenation reactions far more quickly than the starting ylides ‌156‌ (X = NR1, O), multicomponent or domino reactions between the latter, an acidic component (alcohol, amine, thiol), and a carbonyl compound become possible leading to α,β-unsaturated carbonyl derivatives. Variants employing difunctional carbonyl derivatives bearing such acidic groups yield (hetero)cyclic products.

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References

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• 8.Meeeee-Meee, (8888) 8/8e, 8888.
• 8.Meeeee-Meee, (8888) M 8, 888.