Phosphacumulenes

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. " ... 

During the synthesis of diphosphiranes from the symmetrical diphosphene by cyclopropanation, whatever the nature of the carbenoids used (diazo derivatives, carbenes or halogenocarbenes), in all cases the formation as byproducts of the phosphaalkenes and the unstable phosphinidene was observed (5-20%). So, for isopropylidene carbene, the formation of the phosphacumulene, more stable than the parent diphosphirane can reach 30% <91TL3687>. 

Dehalogenation Reactions. Since Uthium naphthalenide is a particularly effective initiator for halogen-metal exchange, it has found widespread use for the conversion of dihaUdes to unsaturated species. Thus, 1,2-dichlorodisilanes have been converted to silenes (eq 8) and diphosphiranes to phosphacumulenes. In a related field, silicon cages have been constracted from trichloro-disilanes. 

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... 

Fig. 1 Phosphacumulenes (triphenylphosphoranylidene)ketene (2), sodium cyanomethyl-enetriphenylphosphorane (3), and (diethoxyvinylidene)triphenylphosphorane (4)...

Ylidic compounds also include phosphacumulenes and cyclic derivatives such as (6.447) (Section 6.21). 

Further work has also appeared on the synthesis and reactivity of three-coordinate, pentacovalent (a X ) phosphorus compounds. New examples include the extended phosphacumulene (19 " " and the phosphavinylidene(oxo)phosphorane (197), a diphosphaallene featuring both and -phosphorus atoms." The involvement of a X -species as intermediates in phosphorylation reactions" and in nucleophilic attack at phosphoryl centres" has also received further attention. 

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. ... 

Phosphacumulenes can be subdivided into compounds with uneven numbers of carbon atoms in the polyene chain (4) and compounds with even number of carbon atoms (5). 

Azomethines do not react with phosphacumulenes. An exception is the reaction of N-4-nitrobenzylidene-4-nitroaniline, which reacts with Ph3P=C=C=NPh across its C=C bond... 

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