Diphosphaallene

Although it is not a group 14 heteroallene, arsaphosphaallene Mes As=C=PMes (+299..S ppm) over-emphasizes the point that the central carbons of heteroallenes are greatly deshielded. For reference, l-phosphaallene Mes P=C=CPh2 has a central carbon shift of +2.17.6 ppm and 1,-1-diphosphaallene Mes P=C=PMes has a shift of +276.2 ppm. 

Diphosphaallene derivatives ArP=C=PAr are peculiar compounds because of the presence of the two orthogonal carbon-phosphorus double bonds. The compounds were transformed into cation-radicals on electrochemical or chemical one-electron oxidation. As found, the unpaired electron is located on an MO constituted mainly by a p orbital of each phosphorus atom and a p orbital of the carbon atom (Chentit et al. 1997, Alberti et al. 1999). 

Consequently, the electron structures of the diphosphaallene ion radicals resemble those of the allene ion radicals, where the allenic fragment works as a bridge for conjugation (see Section 3.3.2). 

In a recent work <2005EJI2619>, an unstable phosphoniotriazaphospholide 98 was generated by [3+2] cycloaddition of diphosphaallene 97 with trimethylsilyl azide and identified by low-temperature NMR. Above — 20°C, phosphoniotriazaphospholide 98 undergoes a clean fragmentation into iminophosphane 99 and diazomethylenephos-phorane 100, which can also act as a 1,3-dipole for the diphosphacumulene 97 to afford heterocycle 101 as the final product (Scheme 7) <2005EJI2619>. 

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. 

The expected preparation of Mes P=C=0 or Mes P=C=PMes in a single step from the reaction of silylphosphide lib with carbon dioxide resulted in the isolation of phos-phino(silyloxy)phosphaethene 13 (equation 91)91. Proton abstraction from 13 with one equivalent of n-butyllilhiurn in ether at room temperature caused elimination of silyl oxide and resulted in the formation of the first stable diphosphaallene Mes P=C=PMes91. 

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 the next section we review some of the theoretical and practical details of the BOVB method. In particular we consider means by which much larger calculations may be attempted. In section 3, we present some illustrative calculations to expose the properties of BOVB wavefunctions and familiarize the reader with the BOVB description of electronic structure. This is followed by a description of some recent calculations on the pseudohalide acid HCS2N3 and a large diphosphaallene radical anion. We conclude by summarizing the strengths and weaknesses of the BOVB method as a general quantum chemical tool and suggest areas for future development. 

The formation of anions provides an interesting challenge to electronic structure methods. Not only from the point of view of providing quantitatively accurate predictions, but also from the perspective of what happens to the electronic structure when an extra electron attaches. Recent studies on the diphosphaallene system... 

The carbonyliron-assisted rearrangement of diphosphaallene Mes P=C=PMes formally involves the oxidative addition of a CH-bond of a t-butyl group across the P=C double bond to afford the dinuclear complex 81. One phosphorus atom of the 1,3-diphosphapropene-like ligand of 81 participates in a dihydrophosphaindane skeleton see Eq. (14).46... 

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). 

Insertion into the phosphorus silicon bond primarily generates a phosphaalkene, which splits off trimethylsilylthiolate and frees the phosphathioketene 1. A renewed addition of the phosphide followed by a splitting off of thiolate finally yields the diphosphaallene 2 [Eq. (54)]. 

Mixed substituted diphosphaallenes are still unknown. Attempts to react the 2,4,6-tri(isopropylphenyl)phospha-substituted diphospha-propene with f-butyllithium according to Eq. (55) yielded the dimeric compound (117). 

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>. 

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