1.3- Diphosphaallenes

Reaction of bis(trimethylsilyl)phosphines with carbon dioxide afforded adducts and addition-silatropy products instead of phosphaketene or 1,3-diphosphaallene (R—P=C=P—R ) (equation 33, compare with equations 72 and 91)52. Similarly, reaction of 3 with carbon disulfide resulted in the addition-silatropy product (equation 34, cf. equations 73 and 94)53. Successful preparations of phosphaallenes utilizing alkali metal silylphosphides bearing bulky substituents are described in Section V. 

In the reaction of 9 with diphenylketene, the major product was a 1 2 adduct (equation 71, cf. equation 88)49. Phosphaketene was formed in the reaction of 9 with carbon dioxide (equation 72) however, 1,3-diphosphaallene was not obtained under these conditions52. Reaction of 9 with carbon disulfide gave the addition-silatropy product Mes P=C(SSiMe3)2 (equation 73)85 but neither Mes P=C=S nor Mes P=C=PMes was obtained. 

Therefore, the addition-silatropy-elimination reaction of RP(SiR3)2 (elimination of hexaalkyldisiloxane) is not a suitable method for the preparation of 1-phosphaallenes and 1,3-diphosphaallenes, probably because spontaneous elimination of hexamethyldisil-oxane from an intermediate R(Me3Si)P—C(OSiMe3)=X is slower than the second addition reaction or the silatropy reaction. On the other hand, the reaction of alkali metal silylphos-phides seems to be promising for the introduction of phosphorus-carbon double bonds and is exemplified in Section V. 

X-ray structure analysis of the 1,3-diphosphaallene was carried out by Karsch and coworkers118, who prepared the diphosphaallene according to equation 92119. The diphosphaallene was also prepared independently by Appel and coworkers by reaction of silylphosphide 11a and phosphaketene Mes P=C=0 (equation 93)90. Appel and Knoll mentioned the reaction of the silylphosphide 11a with carbon disulfide8a and an intermediary formation of phosphathioketene Mes P=C=S was postulated in this reaction (equation 94). 

In boiling toluene, the 1,3-diphosphaallene complex Mes P[W(CO)5]= C=PMes undergoes a hydrogen migration from carbon to phosphorus to afford the tetrahydro-l-phosphanaphthalene complex 86 see Eq. (17).46... 

The preparation of 1,3-diphosphaallenes can be attained starting with different 1,3-diphosphapropenes, which can be transferred to the carbodiphosphane via cleavage of siloxane or silanolate, or via HX abstraction (114-116) (Scheme 13). 

Another way to synthesize 1,3-diphosphaallenes is via a reaction between lithium trimethylsilylphosphide and carbon disulfide at 0°C [Eq. (53)] (118). 

The elimination of lithium silanolate is a method for the synthesis of 1,3-diphosphaallenes and can also be used for the preparation of 1-phosphaallenes (121) [Eq. (57a)]. Additionally, these compounds... 

Its structure as determined by an X-ray investigation is shown in Fig 21. It may be understood as a dimer of the assumed phosphathio-ketene intermediate. The cycloaddition of the phosphathioketene corresponds to the behavior of unsubstituted carbaketenes (146) and so is different from that of the phosphaketenes described earlier, while thioketenes dimerize to 1,3-dithietanes (147,148). An asymmetric retro ring cleavage can be initiated if l-thia-3-phosphetane is irradiated by a mercury lamp generating carbon disulfide and the 1,3-diphosphaallene [Eq. (78)] (117, p. 33). 

Diphosphiranes were also obtained in the reaction of dichlorocarbene with sterically protected 1,3-diphosphaallene in 34% yield (Equation (4)) <91CC124>. The reaction may proceed via a phos-... 

With em-dihalogenodiphosphiranes, the reaction gives quantitative yields of 1,3-diphosphaallene probably involving the same mechanism as Scheme 8 and an unstable allylic anion intermediate <92JOM(436)169>. 

Like their behavior toward the organolithium derivatives or the Grignard reagents, the gem-dihalogenodiphosphiranes (2d-e) or their photochemically opened isomers react with the anionic metal transition complexes leading to 1,3-diphosphaallene in quantitative yield <93JOM(453)77>. 

Diphosphaallene 54 and 1,3-arsaphosphaallene 56 were prepared from phosphanylidene carbenoid 1 and the corresponding halogen-containing phosphine and arsine, respectively (Scheme 26) [55,56]. 

Photolysis of 5 is a convenient synthetic method to generate the 1.3-diphosphaallenes 6, which undergo dimerization across two P=C bonds to give the cyclodimer 7 ". ... 

Sterically hindered 1,3-diphosphaallenes, such as 6, are air- and moisture-stable and can be purified by column chromatography. Less hindered 1,3-diphosphaallenes are isolated as the head-to-tail dimers. 

Phosphaalkenes, phosphorus-carbon double-bond compounds, tvhich are usually unstable, can be synthesized by means of a Peterson-type reaction, the so-called phospha-Peterson reaction [415, 416]. One synthetic route to phosphaalkene 254 is by treatment of a lithium silylphosphide 253 bearing a bulky aryl substituent tvith an aldehyde, tvith elimination of a lithium silanolate (Scheme 2.155) [417, 418]. This reaction is found to be selective because of steric repulsion. Some ketones are also amenable to the phospha-Peterson reactions [419-422]. Phosphorus-containing cumulative double-bond compounds such as 1-phosphaallenes 255, 1,3-azaphosphaallenes 256, and 1,3-diphosphaallene 257 are also synthesized by vrray... 

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