Phosphacumulene ylides

Phosphacumulene Ylides are Phosphorus Ylides and at the Same Time Cumulenes H. J. 

Addition of an electrophile to the lone pair of oxo-, thioxo- and imino-vinylidenephosphoranes transforms the nucleophilic system into the dipolar ir -ir system of a ketene. The resulting phosphonium salt becomes a true dipolar ketene which, as such, reacts in a known manner (equation 101). Whenever the anion Nu is a stronger nucleophile than the original cumulated ylide, the new alkylidene-phosphorane will be formed, in which compound ElNu has added to the parent vinylidenephosphorane. If the starting phosphacumulene ylide is a stronger nucleophile than Nu , the intermediate salt always reacts with a second molecule of unreacted ylide in a [2 + 2] cycloaddition to give 1,3-cyclobutanedione derivatives. 

The resulting dimers of the original phosphacumulene ylides are of particular interest because one of their resonance forms represents another type of push-pulT cyclobutadiene. 

In the reaction of OH-, NH-, SH- and CH-acidic compounds with phosphacumulene ylides the anion Nu of the initially formed phosphonium salt (c/. equation 101) is so nucleophilic, that addition of the anion to give a new alkylidenephosphorane is faster than cycloaddition of a second mole of starting ylide (equation 105). ° ... 

The addition of acidic compounds to phosphacumulene ylides yielding substituted alkylidenephosphoranes is of particular interest in the reaction of those acidic molecules which carry, besides the Y— bond, a group capable of cyclization with the ylide function formed by addition. " ... 

H. J. Bestmann, Phosphacumulene Ylides and Phosphaalene Ylides , Angew. Chem. Internat. Edn., 1977,16, 349. 

Abstract Phosphacumulene ylides of the general formula Ph3P=C=C=X [X=0, S, NR, (0R)2] are versatile C2-building blocks. They can act either as C-nudeophiles-only in a manner typical of phosphorus ylides, or as cumulenes undergoing [2 + n -cycloadditions with other cumulenes such as CO2, COS, RNCO etc. Most prominent is their tandem addition-Wittig alkenation of hydroxy- or amino-substituted carbonyl compounds. With aptly chosen reaction partners all these pathways may lead to heterocyclic products. Some recent applications of these methods to the syntheses of azetidines, five-membered lactams, lactones, tetramates, tetronates and pyrroles as well as to six-membered quinolones and to macrolides are delineated. 

Keywords Phosphorus ylides Phosphacumulenes Tetronates Tetramates... 

Hydroxyisoindole-, isoindoline- and indane-l,3-diones have been reacted with both stabilized alkylidenephosphoranes (e.g., acyl-methylenetriphenylphosphoranes) and also active ylides such as phosphacumulenes. ... 

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