Phosphirenium salts

FIGURE 9. JP c in phosphirenium salts and ortho-donor substituted arylphosphonium salts... 

Phosphirenium salts are instantaneously opened on hydrolysis, affording the corresponding phosphine oxides28 (reaction 238). 

Another approach to the synthesis of phosphirenes by formation of two P-C bonds involves the electrophilic addition of halo- and dihalophosphine-AlCls complexes with alkynes, yielding phosphirenium salts. In some cases, the use of thallium(l) triflate instead of AICI3 improves the method <1997JOM(529)189>. In the normal case, the chlorophosphirenium salts are not isolated but rather immediately reduced with tributylphosphine to furnish the phosphirenes (Scheme 12) <19960M1597>. 

By analogy with the synthetic method used to prepare the first phosphirenium salts C [18], we initially tried to prepare a diphosphirenium salt of type 2 P by [2+1]-cycloaddition reactions of various phosphenium salts to ferf-butylphos-phaalkyne. However, all our attempts failed (Scheme 5). 

Having in hand the diphosphirenium salt 2 Pa, we investigated the possibility of synthesizing a diphosphirenylium salt of type 2P. However, we had to keep in mind that, in contrast with the phosphirenium salts C, which are known to be stable [18, 23], the phosphirenylium salts H have only been observed spectroscopically in S02 solutions at -78°C [24a,b], or isolated when complexed in an q3-fashion to an electron-rich Ni° center [24 c]. The difficulty in isolating species such as H is essentially due to the very high electrophilicity of the phosphorus atom. In addition, the desired diphosphirenylium 2P suffers from the presence of either a weak P=P or P=C double bond [14] (Fig. 2). 

Fig. 2. Phosphirenium salt C, phosphirenylium salt H, and resonance forms of the diphos phirenylium salt 2P...

Structural data available from experimental [22a] as well as theoretical [22c, 41] studies demonstrate that diphosphirenium salts 2 P also benefit from a certain amount of aromatic stabilization. However, the resonance energy of o4-phosphorus-containing systems is smaller. From a quantitative point of view, the electron delocalization in phosphirenium salts C has been calculated to be half that of the corresponding phosphirenylium salts H [42]. 

As a general rule, tetra- and pentacoordinated derivatives of phosphirenes display a much higher stability than the corresponding derivatives of the phosphirane ring. Some kind of electronic stabilization is probably at work in phosphirenium salts (see the structural data of one such salt <88CC462 , which can either be synthesized directly (see below) or from tervalent phosphirenes (Equation (2)) <85JA4700>. Methyl triflate in chloroform at — 50°C has also been used successfully... 

Finally, mention should be made of the ring-chain tautomerism observed in some phosphirenium salts (Scheme 9 and Equation (34)) <90TL819, 93TL1331). In the first case, the existence of the phosphenium salt was demonstrated through its trapping by a conjugated diene. 

A study of the reactivity of l-chloro-lH-phosphirenes 342 with nucleophiles has shown that the chlorine is easily replaced. Ab initio calculations suggest that IH-phosphirenes invert their configuration at phosphorus by a rotation of the PX group above the C2 moiety, rather than by the more usual trigonal inversion pathway involving a C2v-transition state. Related calculations on the aromaticity of IH-phosphirenium cations 343 have shown that the a ABMO associated with the P-X bond acts like an empty p-orbital. A facile route to the phosphirenium salts 344 is afforded by the reaction of the phosphiranium salt 345 with alkynes. The P -azaphosphirene system 346 has been obtained from the reaction of an iminophosphene with terminal alkynes. ... 

The first examples of phosphirenium salts (129) have been prepared from the reactions of alkynes with the aluminium trichloride adducts of dichlorophos-phines. The reactions of phosphines with fluorotrihalogenomethanes lead to the first examples of fluorine-containing phosphoranium salts (130). The related nitrogen-bridged salt (131) is formed in the reactions of lithium bis(diphenyl-phosphino)amide with iodomethane. ... 

Hexyne reacts with phosphorus trichloride to give the phosphirenium salt 402 by insertion of a second molecule of the alkyne into the phosphorus-chlorine bond of the intermeazdiate 401 ... 

Although parent phosphirenes (Table 6.21) are not known, a number of derivatives have been well characterised. Typical of these are 1,2,3 triphenylphosphirene (6.889a) and the tetraphenylphosphi-renium cation (6.889b) (Figure 6.23). Phosphirenium salts do not exhibit aromatic behaviour. 

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