Tetraalkylphosphonium salts

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

ESR data obtained on y-irradiation of tetraalkylphosphonium salts in solution (96% sulphuric acid) at 77 K differ markedly from those for the trialkylphosphonium ions44. The latter irradiation produces the expected phosphabetaine radical cation and, for example, tetrabutylphosphonium iodide has features assigned previously to I2 45. On the other hand, PH was not detected44,46. Trialkylphosphonium salts, such as trimethyl and triethyl compounds, produce radicals whose ESR spectra are characteristic of the radical cations of R3P +. The central region of the ESR spectra is dominated by features from... 

A fundamental route for the preparation of simple tetraalkylphosphonium salts is the reaction of a tertiary phosphine with a haloalkane or other substrate upon which a simple nucleophilic substitution reaction can occur. (In comparing phosphorus nucleophiles with the corresponding nitrogen-centered nucleophiles, it must be remembered that the phosphorus is significantly more nucleophilic than is the nitrogen. For example, while triphenylamine is devoid of nucleophilic character in reaction with ordinary haloalkanes, triphenylphosphine exhibits high reactivity.) Reactivity of the phosphorus in such nucleophilic substitution reactions, as with other types of nucleophiles, decreases with increasing substitution about the electrophilic site of the substrate. 

While tetraalkylphosphonium salts may be generated by the reaction of elemental phosphorus (white or red see Section 3.2) with haloalkanes, the reaction is complicated by the concomitant formation of trialkylphosphine dihalides. The resultant mixtme is generally converted to the corresponding trialkylphosphine oxide by treatment with ethanol under... 

While triphenylalkylphosphonium salts as ylide precursors unambiguously lead to alkylidenetriphe-nylphosphoranes, in the case of tetraalkylphosphonium salts ylide formation may take place principally at different a-positions relative to the phosphorus. " Furthermore an intramolecular transylidation process causes the ligands at the phosphorus in methylenetrialkylphosphoranes to exist alternately as alkyl or as alkylidene substituents, thus giving rise to the formation of isomeric ylides. 

Protic and solvated ionic liquids in this subgroup have been discussed in Section 11.4. The lowest-melting tetraalkylphosphonium salt, crystallographically characterised, is [P q lo lo... 

PT catalysts include tetraalkylammonium and tetraalkylphosphonium salts, crown ethers (which also sequester cations and break up ion pairs), and PEGs. 

Quaternary ammonium salts (quats) are an economically advantageous class of industrial compounds. They have surface-active properties, possess anti-microbial activity and are known to be bioactive (Boethling Lynch, 1992 Juergensen et al., 2000). Contrary, reports regarding low melting tetraalkylphosphonium salts were relatively rare in the literature during the last decades (Bradaric et al., 2003). 

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