Phosphorus Cations

Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions). A. H. Cowley and R. A. Kemp, Chem. Rev., 1985,85, 367 (100). 

Cationic low-coordinated n-bonded phosphorus compoimds add Lewis donors, such as amines or phosphines. In contrast to the trigonal bipyramide formed in donor-addition to the silico-nium cation, the phosphorus cations add donors in a perpendicular fashion, depending on the nature of the n-bonds toward phosphorus. According to quantum chemical calculations the various cations reveal different stabilities and hence a strong variation in donor addition abilities. 

Finally, Lacour and coworkers reported the synthesis of hexacoordinated phosphorus cation 62 [100]. Tropolone, BINOL and PCI5 react in CH2Cl2at reflux to generate in one step a novel hexacoordinated phosphorus cation. 

A modification of GSR has been reported by Classon and co-workers.190 The idea remains the same create a covalently bound phosphorus cation and displace with a nucleophile—in this case, a halogen. Both bromine and iodine have been used.190 Three different systems were evaluated (1) chlorodiphenylphophine, iodine-bromine, and imidazole (2) p-(dimethylamino)phenyldiphenylphosphine, iodine-bromine, and imidazole or (3) polymer-bound triphenylphosphine, iodine-bromine, and imidazole. The last two were found to be very similar to just triphenylphosphine itself, and displayed reactivity inferior to the first system. The polymer-bound reagent does allow for easier removal of triphenylphosphine oxide produced in the course of the reaction. As with the original procedure, and consistent with a Sn2 mechanism, inversion of configuration occurred. Again, as with the original method vicinal diols were readily converted into alkenes.191 This... 

Reaction of PCI3 or P4 with 2 or 3 equiv of complex 58 afforded triphosphete 59 (37%) along with chlorophosphorane 60. The synthesis of the first 10-electron phosphorus cation 25 (39%) was achieved by reacting the compound 59 with ylide 60 in the presence of NaBPln (Scheme 5) <1996AGE2242, 1996AGE2618, 1997JOM(529)151>. 

Aluminum cation % Phosphorus cation Oxygen atom Lanthanum cation Calcium cation... 

An example of the alternative cationic species is provided by [Ph2PCH2NMe3]+I . Baird and coworkers developed this theme in preparing the all-phosphorus cationic phosphoniumphosphines [Ph2P(CH2) PMe3]X (n = 2,3, 6, 10 X = NO3, Cl, PFe), to which they gave the trivial names II-, III-, VI-, and X-phosphos. These ligands form water-soluble transition metal complexes such as [(nbd)RhCl(II-phosphos)] [PFg]... 

In a variation on the same theme, a novel C2-symmetric hexacoordinated phosphorus cation (191) was synthesized from tropolone (190), R-BINOL and PCI5 and shown to be an efficient NMR shift reagent for chiral anionic phosphate (e.g. BINPHAT, Figure 2) and borate anions. 

Keywords Phosphinomethanides / Spirocyclic E Compounds / Phosphorus Cation(P )... 

The Mitsunobu reaction has been used previously to prepare 5 -0-acylnucle-osides and nucleoside 5 -phosphates [111, 112]. With purine nucleosides, the approach failed (< 1 % yields) in the preparation of 5 -phosphates, the main product being N3,5"-cyclonucleosides resulting from an intramolecular nucleophilic attack by a purine ring nitrogen atom on the 5 -carbon atom. The predominant formation of the purine cyclonucleosides was attributed to electrostatic interactions between the phosphorus cation and the purine base which brought the reaction sites (5 and 3-N) close enough to favor cyclization [113]. 

Table 25.2.1 BDEs in Phosphorus-Cation Clusters/Complexes...

The most interesting results have been obtained using cationic Lewis aeids. Indeed, in 1989 we showed that addition of trimethylsilyl triflate to the carbene 17 led to the so-called methylenephosphonium cation 36, which was the first example of a stable trieoordinated phosphorus cation. [53] This compound features a short phosphorus-carbon double bond (1.62 A) and trigonal planar phosphorus and carbon centers, as expeeted for a eompound isovalent and isoeleetronie with olefins however, the double bond is dramatically twisted (60°). 

Phosphorus cations, even up to P g, have been observed as unstable species in the mass spectrometer [72] (Chapter 13.4). Since NJ has been characterised [73] (4.36a), P5+ (4.36b) may exist, although a pyramidal structure is predicted (6.937,4.37a). 

Large phosphorus cations such as those in (4.37b-d) have not yet been characterised, but such cations may have sufficient stability to exist as spectroscopic species. However, 4-linked pyramidal P atoms appear to exist, at least formally, in compounds such as (4.38) [74] and (6.937) moreover, such species may occur as intermediates in some reactions [74,75]. Some complex Bi + cage-like cations are also known [47]. 

The hydrogen atoms on the carbon attached to the resulting phosphorus cation are somewhat acidic because they are adjacent to a positive charge, a significant electron-withdrawing group. Thus, treatment of the phosphonium salt with a strong base, such as butyUithium in THE or sodium hydride in DMSO, removes one of these protons and produces the yUde. 

An important modification (often called the Horner-Wadsworth-Emmons reaction) of fhe Wiffig reaction makes use of phosphonate esters, RP0(0R )2- It is highly stereoselective for the formation of E-alkenes.The reaction and mechanism are depicted below for the preparation of (E)-stilbene. Instead of using a phosphorus cation to stabilize the negative charge, as in the phosphonium ylide above, a phosphonate ester group is used to stabilize an adjacent carbanion. 

Weigand, J.J., Burford, N., Davidson, R.J., Cameron, S. and Seelheim, R, New synthetic procedures to catena-phosphorus cations Preparation and dissociation of the first cyclo-phosphino-halophosphonium salts, J. Am. Chem. Soc. 131 (49), 17943-17953 (2009). 

Stabilized relative to the neutral bases (actually the former show even larger saturation effects than those discussed in section 2.3.2 for the latter). On the other hand, Staley and Beauchamp find that, relative to phosphonium ions, phosphorus cation ion radicals are stabilized by methyl groups by roughly only 35 percent as much as the phosphonium ions are stabilized relative to their neutral bases (series 6). It is also to be noted that, relative to the neutral bases, methyl groups stabilize phosphonium ions by about 1.7 times as much as ammonium ions [50]. Finally, as noted previously, methyl, ethyl, i-Pr, and t-butyl groups stabilize ammonium ions relative to free base by successive increments of 2.7,2.3, and 2.0 kcal mol There is, however, very little difference between these substituents in their stabilizing effects on nitrogen cation radicals values of... 

Since this type of delocalization is hindered by poor orbital overlap of phosphorus, methyl groups stabilize phosphorus cation radicals... 

---