Phosphonium salts analysis

In a series of papers published throughout the 1980s, Colin Poole and his co-workers investigated the solvation properties of a wide range of alkylammonium and, to a lesser extent, phosphonium salts. Parameters such as McReynolds phase constants were calculated by using the ionic liquids as stationary phases for gas chromatography and analysis of the retention of a variety of probe compounds. However, these analyses were found to be unsatisfactory and were abandoned in favour of an analysis that used Abraham s solvation parameter model [5]. 

This material may be converted directly to a phosphonium salt 1.40 g. (0.0054 mole) of the crude iodide is dissolved in 20 ml. of benzene, and 1.42 g, (0.0054 mole) of triphenylphosphine [Phosphine, triphenyl-] is added. On standing, 2.5 g. (77%) of the triphenylphosphonium salt precipitates as a colorless 1 1 complex with benzene, m.p. 135-137°. Recrystallization from methanol-benzene raises the melting point to 140-142°. Analysis calculated for C28H29PI CeH6 C, 68.23 H, 5.39. Found C, 68.15 H, 5.28. 

During formation of the addition compounds 12 and 16, no free formaldehyde accumulates. We assume that the liberated formaldehyde immediately reacts with tris-hydroxymethyl-phosphine, forming the quaternary tetrakis-hydroxymethyl-phosphonium salt 13. The addition compounds 12 and 16 are relatively unstable, but can be purified for analysis. Intermediates 12 and 16 can also be employed in the synthesis of symmetrical or unsymmetrical phosphamethin-cyanines. For example, Klapproth synthesized 18 in 60% yield by condensing 16 with 77. 

Archer et al.206 surveyed all phosphonium salts known in 1981 in a correlation analysis of the positioning of cation and anion. Exclusive face orientation of the anion with respect to the cation centre was found, i.e. the anion was positioned along the direction of approach leading to the apical position in a trigonal bipyramidal intermediate. The positioning of the anion did not correlate with nucleophilic attack at the phosphorus centre but did correlate with a-hydrogen abstraction (ylide formation)206. 

Phosphonium salts, which are generally stable in the form of iodide and tetraphenyl-borate crystalline solids, are frequently purified by recrystallization. Where this is not possible, thick-layer chromatography (0.2-1 mm Kieselguhr, silica) may afford a satisfactory method of purification12,13. In this manner, analysis using secondary ion mass spectrometry (SIMS) has a detection limit 10-50 ng per spot14. 

When unsaturated ketones are used as reactants, the C=C bond is preferentially reduced. Most of the complexes are transformed and thus deactivated after their first catalytic run. The phosphane (tppts) of the complexes underwent reactions with the organic products, giving phosphonium salts, which are responsible for the deactivation. The analysis of the aqueous phases shows that the recycling would be difficult in many cases. 

Samaan used reduction at a mercury cathode to convert the phosphonium salt 33 to the phosphine (34) (79PS89). This gave the opposite stereoisomer from that available as the major product (35) from base hydrolysis followed by silane reduction. The stereochemistry of each compound was established by NMR analysis. 

A possible correlation between the strucmre of the phosphonium salt and polymerization control was investigated. Based on kinetic analysis and NMR data , it was proposed that a fast equilibrium is established between the propagating enolate (46) and a dormant ylide (47) in the polymerization initiated by Ph3CPPh4 (equations 44 and 45). 

These derivatives were described as uronium salts by analogy with phosphonium salts, but X-ray analysis demonstrated that HBTU, HATU, and HAPyU crystahize as aminium salts (guanidinium A-oxides) rather than uronium salts.t NMR studies of HAPyU in solution show the same structure as that observed in the crystahine form.P This discovery changes the nomenclature of these compounds, but their acronyms are unchanged, and they are described in the literature as uronium or aminium salts without discrimination. HBTU should be reformulated as A-[(l//-benzotriazol-l-yl)(dimethylamino)methylene]-A-methyl-methanaminium hexafluorophosphate A-oxide, HATU as A-[(dimethylamino)(l//-l,2,3-triazolo[4,5-b]pyridin-l-yl)methylene]-A-methylmethanaminium hexafluorophosphate A-oxide, and HAPyU as 0-(7-azabenzotriazol-l-yl)-A,A,A, A -bis(tetramethylene)uronium hexafluorophosphate. 

Structural analysis of 29 reveals an essentially planar PCPCP skeleton with (E),(E)-conformation. Phosphenium salts 29 are protonated at an ylidic earbon atom with the concomitant re-association of the halide to the central phosphorus atom. They are oxidised at the central phosphorus atom, by halogen or ortho-quinones, giving phosphonium salts, or by elemental sulfur or selenium to... 

Another problem associated with the use of a phosphine on a commercial scale is its conversion, due to the presence of a small amount of oxygen in the reaction zone, to the corresponding phosphine oxide, which will not act effectively as a ligand. The use of a phosphonium salt can minimize this type of conversion. Although the mechanism of the action of the phosphonium salt has not been made clear, it is considered, from the fact that aryl groups should be present, that a very rapid equilibrium with the corresponding phosphine may partially occur. Upon analysis of the actual reaction mixture, however, no trivalent phosphine is detected either in the reaction solution or in the palladium complex. 

Pyrazol-3-ones 97a were heated in tetrahydrofuran with ketenylidenetriphenyl-phosphorane to afford the corresponding pyrazole-3-one phosphonium salts 119b-f in 44—87% yield. An X-ray single crystal analysis of 119f is presented (00JCS(P1)1723) (Scheme 38). 

Further analysis of (18) and (19) incorporates many points from earlier chapters. The phosphonium salt (19) must be protected and will be derived from alcohol (20) via a halide. This is the same skeleton as ketone (21) (Chapter 1). 

The preparation and implementation in peptide synthesis of analogues of phosphonium salts such as HBTU (27) and HATU (28) bearing a positive carbon atom in place of the phosphonium residue have also been reported (Fig. 7). Although they were initially assigned a uronium-type structure [29,31,70,71], presumably by analogy to the corresponding phosphonium salts, it has been determined by X-ray analysis that A -[(l/f-benzotriazol-l-yl)(dimethylamino)methylene]-iV-methylmethanaminium hexafluorophos-phate iV-oxide (HBTU), iV-[(dimethylamino)-lH-l,2,3-triazolo[4,5-fojpyridino-1 -ylmethylene] -N-methylmethanaminium hexafluorophosphate... 

Analysis of FTIR spectra of the ionomer(Figures 3 and 4) was also informative about the structure. It was reported previously that IR absorption bands at 1893 cm" and 1905 cm"l were characteristics of 4-methylstyrene (of > 50) and 4-bromomethyl styrene (of Exxpro elastomer, Figure 5). Such absorption bands were not found in the FTIR spectra of the above ionomers. However, new absorption peaks, characteristics of the quaternary phosphonium salts, at 1580, 1100, 100, 670-730 cm" were observed. The presence of both phosphorous and boron in the above ionomers was also confirmed in terms of the elemental analysis data, collected from the inductively coupled plasma/atomic emission spectroscopy(ICP/AES) technique. These results were in good agreement with the expected convertion of the respective phosphonium salts. 

In catalytic Pd(0) reactions, this phosphonium salt was treated with a Pd(ll) source, base, and substrates to form an active catalyst for Sonogashira, Suzuki, and Heck coupling chemistry (Eq. 42, Eq. 43, Eq. 44). The reactions used 0.5 mol% of the Pd catalyst, >99.9% of which was recovered based on Pd analysis of the filtrate of a nanofiltration using UV-visible and total reflection X-ray fluorescence analysis. The spectroscopic analyses reportedly could detect as little as 0.05% of the 0.01 mmol of starting Pd catalyst in the leachate. The membrane used in this chemistry was a solvent-resistant nanofiltration membrane consisting of a porous poly(acrylonitrile) layer and a dense surface layer of poly(dimethylsiloxane). This membrane worked through nine cycles in the... 

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