Ammonium and Phosphonium Halides

Ammonium and Phosphonium Halides.— The classical ND work of Levy and Peterson on the ammonium halides in their various forms has been supplemented by work, based on fuller data, on NHaBr in three of its polymorphic forms, leading to analyses of the vibrations, and disorder, of the NH + ion. 

In the ammonium halides the formation of N-H X bonds is a dominant theme in the various structures. In the phosphonium halides, PHj+OBr/I)- the formation of hydrogen bonds is, as it were, avoided. Sequeira and Hamilton reported a ND study of PH4+I- in 1968 and now we have a report on the isomorphous PH4+Br. These structures resemble the low-temperature (cubic) forms of NH4+CI-, with the difference that the tetrahedral PH4- - ion is rotated through 90° about one twofold axis of the tetrahedron, which distorts the structure and debases the symmetry to the tetragonal system. The P-H bonds arc now directed (approximately) towards more distant bromide ions with P Br = 4.03 and H Br = 2.6S A, which can represent only a very weak hydrogen bond. There is a much closer contact, P Br = 3.47 A, with no hydrogen involved. The situation is reminiscent of that in the densest forms of ice (see p. 212). 

The other hydrogen is in more debatable territory. The authors describe this as perhaps a case of incipient bifurcation . We must recognize that even an unloved hydrogen atom has to be somewhere in space. 

A brief note on ND work with D2Co(CN)j at 77 K reports a hydrogen bond between cyanide ions with N D N = 2.60 A. 

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Quaternary ammonium and phosphonium halides were used as the phase transfer catalysts. For effective coupling, high-shear mixing and high concentrations of polymer and base were used. 

Oxetane undergoes ring-opening polymerization under the action of MAD in conjunction with onium salts, including quaternary ammonium and phosphonium halides, giving a narrow MWD polyether (Scheme 6.173) [221]. Use of MesAl in place of MAD resulted in no polymerization. The aluminum ate complex seemed to be an initiator, which underwent a trigger reaction involving halide transfer to the aluminum-oxetane complex. 

Abdallah, D.J., R.E. Bachman, J. Perlstein, and R.G. Weiss. 1999. Crystal structures of symmetrical tetra-n-alkyl ammonium and phosphonium halides. Dissection of competing interactions leading to biradial and tetraradial shapes. J Phys Chem 103 9269. 

Partial substitution of halides on bismuth predictably gives alkoxy-bismuth halides, and salts of bismuth anions are obtained by reaction of the alkoxide with an ammonium or phosphonium halide (28). Two isostructural derivatives [Bi2Cl4(thf)2(ju,-OAr)2] (Ar = C6H3Me2-2,6 and C6H2Me3-2,4,6) are composed of dimeric units residing on a C2 axis 8. The bismuth is in a five-coordinate distorted square pyramidal geometry in each case. The apical sites are occupied by terminal... 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

Reactivity follows the sequence RI > RBr > RC1, and SnL > SnBr2 > SnCl2. The uncatalysed reaction of methyl iodide with stannous iodide at 160 °C has been reported,72 but usually a catalyst is necessary. These catalysts are similar to those which are used for the direct reactions, and include dialkyl sulphides, amines, ammonium and phosphonium salts, and copper(II). The trialkylstibines appear to be particularly effective and the compounds RSnX3, R = Ci to Cis, have been prepared by this method. A similar reaction with the a,co-dihalides, X(CH2) X, n = 4-5, provides access to functionally substituted tetraorganotin compounds, but the aryl halides are unreactive. 

Halide salts of quaternary ammonium and phosphonium cations, and those derived from heterocyclic bases (both aromatic and alicyclic), have long been known (Table 11.14). Most are relatively high melting (and would not be considered ionic liquids), though the structural characteristics of quaternary ammonium halides have been studied by those with interests in plastic crystals and so-called rotator phases [7]. Many deconpose at or near their melting temperatures, a characteristic that would limit interest in them as reaction media [667]. Indeed, it is probably as a result of the view, widely held in the past [668], that such materials may be intrinsically unstable that the onset of the recent massive interest in ionic liquids was delayed. 

In 2014, Kim et al. reported the synthesis of alkoxo-bridged dinuclear aluminium complexes obtained via reaction of trimethylaluminium with aliphatic amino-ethanol derivatives, as shown in Scheme 18.49. This class of complex displaying pendant -CH2CH20Me or —CHaCHaNMca arms was highly active in the cycloaddition of carbon dioxide and epoxides in the presence of an ammonium or phosphonium halide as... 

Full details are now available of a study of polystyrene-supported quaternary ammonium and phosphonium catalysts (80) for triphase catalysis (TC) in a liquid-solid-liquid mode. Long spacer chains between the quaternary centre and the polymer backbone were found to be unnecessary for TC activity, and phosphonium catalysts were more effective than ammonium thus (80 n = 3, A = P) compared favourably with soluble octadecyltributylphosphonium bromide as a PT catalyst for reactions such as halide to halide interconversions. A test reaction, that of potassium acetate with 1-bromo-octane, has been used to examine the TC activity of silica in the solid-solid-liquid mode. Silica impregnated with a cationic surfactant appears to be as effective as alumina (4,163), whereas silica bearing covalently bound quaternary ammonium groups exhibits increased effectiveness. 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

Tetrasubstituted phosphonium halides are just as effective as their ammonium counterparts. A combination of tetraphenylphosphonium bromide and either 18-crown-6 or polyethylene glycol dimethyl ether with spray-dried potassium fluoride converts 4-chlorobenzaldehyde to 4-fluorobenzaldehyde in 74% yield [67] In addition, the halogen of a primary alkyl chloride or bromide is easily displaced by fluorine in aqueous saturated potassium fluoride and a catalytic amount of hexadecyltributylphosphonium bromide [68] (Table 7 Procedure 4, p 194)... 

Several sol-gel entrapped catalysts are likely to soon find commercial applications. A variety of transition metal catalysts physically entrapped in silica matrices as ion pairs generated from the metal halides and quaternary ammonium or phosphonium salts developed in the mid-1990s by Avnir and Blum resulted in truly heterogeneous, stable and... 

Halide displacement by thioanions such as thioalkoxides, thiocyanate and sulfite, can be readily carried out under PTC conditions in activated arenes. The reaction of nitro- and dinitro-aryl halides with thiophenol and NaOH in the presence of an ammonium or phosphonium salt catalyst proceeds readily in toluene at room temperature to give the phenyl aryl sulfides in nearly quantitative yield.189 Excellent yields of thiocyanoarenes were obtained from the reaction of the 2,4-dinitrohalobenzenes with KSCN in toluene at 90 C with a tetraalkyl onium salt PTC catalyst.190... 

Aryl tellurium trihalides accept halide ions from tropylium bromide, ammonium halides, phosphonium halides, arsonium halides, sulfonium chlorides, telluronium chlorides, and diphenyl iodonium chloride to form aryltetrahalotellurates(lV)1-5. 

Solvent free methods have also impacted on the preparation of other alternative reaction media. Namely, a range of ionic liquids (ILs) was prepared (including imidazolium, pyridinium and phosphonium salts) through halidetrapping anion metathesis reactions (Figure 2.17). The alkyl halide by-product was easily removed by vacuum or distillation and the products were obtained quantitatively in high purity. In addition to being solvent free, this route is more atom economic than the usual route to room temperature ionic liquids (RTILs) as it does not use silver(i), alkali metal or ammonium salts which are normally used in an anion metathesis reaction. 

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