Salts phosphonium

1 Preparation. - The tritylphosphonium salts (285) have been prepared by treatment of triarylphosphines with a trityl tetra(fluoroaryl)borate reagent. An X-ray study of the salt from tris-(p-anisyl)phosphine showed a very long phos-phorus-trityl bond, ca. 1.93 A, attributable to steric repulsion, and also considerable distortion from the expected tetrahedral geometry at phosphorus. A 

C-C coupling reaction between an arylalkynylstannane with p-bromophenyl-triphenylphosphonium bromide. A stereoselective synthesis of ( )- and (Z)-allylphosphonium salts is afforded by the palladium-catalysed addition of triphenylphosphine to allenes, in the presence of an acid, the stereochemical 

1 Preparation.- Tetra-isopropylphosphonium salts are difficult to prepare by direct quaternization of tri-isopropylphosphine. However, a route to these compounds is provided by quaternization of the phosphine with iodoethane, followed by generation of the 

Arylphosphonium salts are formed in the uncatalysed reactions of triphenylphosphine with -bromophenyl ketones at 280°C. 201 Cyclisation of the ylides (125) with a,6-unsaturated ketones leads 

A variety of products have been isolated in earlier studies of the phosphorus trichloride-tris(dimethylamino)phosphine-aluminium chloride system, and a reinvestigation has now shown that the nature of these products is dependent on the sequence in which the reagents are allowed to react with each other. The most interesting new compounds obtained from this system in dichloro- 

2 Reactions of Phosphonium Salts.- Interest continues in the effects of solvent on the rate of alkaline decomposition of phosphonium salts. It has now been shown that, in the respective reactions of hydroxide ion and methoxide ion with tetraphenyl-phosphonium bromide in mixtures of DMSO and methanol, the rates of the reactions increase as the proportion of the dipolar aprotic 

A number of studies of thermal decomposition of phosphonium salts have been reported. The hydrazonovinylphosphonium salts 

1 Preparation- The role of 02p- and N2p- through space hyper-valent interactions involving a developing phosphonium centre in quaternization reactions has been reviewed, together with their role in other aspects of phosphonium salt chemistry. 

Quantitative yields of alkylphosphonium salts are obtained in the reactions of primary alcohols with triphenylphosphine in 48% 

Conventional quaternization procedures have been used in the preparation of both cyclic and acyclic salts, e.g., (101) and (102), derived from 1,1 -bis(diphenyIphosphinomethyl)ethene. 

Careful deprotonation of the latter using an ylide has given the 1,3-bis(phosphonio)propenide salt (103), a useful model for the free allyl anion.Direct quaternization has also been used in the synthesis of the heteroarylmethylphosphonium salts (104), and phosphonium salts bound to a polyoxetane polyether. 

Heterocyclic phosphonium salts, e.g., (105), have been prepared by the reactions of urea-bridged P,P-dipho8phines with methyl triflate. - - Two ehantiomeric iron carbonyl complexes of the butadienylphosphonium salt (106) have been obtained by the trapping of a complexed organic cation with triphenylphosphine.  

Electrochemically-promoted reversible interconversion of alkyltriphenyl-phosphonium salts and the related ylides has been shown to occur in the presence of benzophenone oxime O-methyl ether as a mediator, providing an example of electrochromism. Nucleophilic addition to vinylphosphonium salts has again been widely used as a means of generating ylides, and for the synthesis of heterocyclic systems New developments include the catalysis of addition of 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored.  

The chemistry of phosphinidene and phosphenium systems continues to be an active area. The electronic configurations of vinylnitrene and vinylphosphinidene have been compared in a theoretical study, which predicts that both have triplet ground states. A triplet ground state is also found for phenylphosphinidine, whose properties are very similar to those of methylphosphinidene. A theoretical consideration of factors affecting the singlet-triplet energy separation in phosphinidenes has concluded that the singlet state is favoured by substituents 

Further progress has been reported in the chemistry of CT X, -p -bonded systems. Full details of such systems stabilised by intramolecular coordination, as in, e.g., 334, have been described. The kinetically stable system 335 has been prepared and its solid state structure determined . The P-halobis(imino)-CT -phosphoranes 336 have also been prepared, and detailed NMR studies of bis(imino) phosphoranes reported . Quin s group has continued studies of the generation and characterisation of reactive a X -systems, e.g., 337 . Methods for the generation of monomeric metaphosphate esters in solution have been investigated. A theoretical approach has been used to probe the mechanism of the reaction between phosphanylnitrenes 338 and boranes. The thiophosphonic anhydride 339 behaves as a source of the dithioxophosphorane 

trappable with suitable dienes. Thus, e.g., on heating 339 with norborana-diene at 80 °C, the 1,2-thiaphosphetane 341 is formed.  

Infrared, Raman, x-ray and nentron diffraction studies have established the existence of tetrahedral PH cations in salts snch as PH4I. Phosphonium salts containing the tetrahedral PH4 cation (4.151) are generally less stable than the corresponding NHJ salts and dissociate more easily. The chloride and bromide easily form gases at room temperature and only the iodide, PH4I, mp= 18.5°C (vp = 50 mm at 20°C, and snblimes at 62°C), is crystalline, but it is decomposed by water to yield PH3. The PH cation is stable in solutions of strong acids such as H3O BFj and MeOH BF3, but the perchlorate is very explosive (above). 

Phosphonium halides are produced by direct union of phosphine and hydrogen halide (4.139) or acid. A convenient preparation is from diphosphorus tetraiodide and white phosphorus (4.152). 

Phosphonium salts of the PH cation have few uses but their derivatives are important (Chapter 6.9). Cations probably exist, for example, P5H2 with structures similar to certain known iodides 

The phosphide anion, PHj, is obtained from phosphine by reaction with an amide. 

Triphenylphosphine-oxide, -sulfide, and l,2-bis(diphenylphosphinoyl)ethane have found use as reagents for the selective liquid-liquid extraction of silver(I) and mercury(II) from their binary mixtures with other di- and trivalent metal ions.- 

1 Preparation. - A range of 1,3-dithianylphosphonium salts (218) has been prepared in the course of further studies of sulfur lone pair anomeric effects in these systems.Conventional quatemization reactions have been used in the synthesis of the salt (219) and a range of polymer-supported phosphonium salts (220). A new efficient route to salts of the type (221) has been developed. The of -azolylalkylphosphonium salts (222) are readily accessible from the reactions of the corresponding a -bromoalkylphosphonium salts and azoles. Routes to vinylphosphonium salts, e.g., (223), continue to be explored, and their reactivity utilised in the synthesis of phosphonium salts bearing heterocyclic substituents, e.g., (224). The betaine (225) has been 

2 Reactions. - The equilibrium acidities (pKha) of six p-substituted benzyltriphenylphosphonium salts, and also those of related allylphosphonium salts, have been determined, together with the homolytic bond dissociation enthalpies of the acidic C-H bonds. A study of the data available in the Cambridge Structural Database reveals that tetraphenylphosphonium cations in crystals associate through phenyl-phenyl non-bonded interactions which are attractive, concerted, and widespread in nature. An attractive force of 60-85 kJmoP has been calculated.  

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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... 

A variation of the Madelung cyclization involves installing a functional group at the o-methyl group which can facilitate cyclization. For example, a triphenylphosphonio substituent converts the reaction into an intramolecular Wittig condensation. The required phosphonium salts can be prepared by starting with o-nitrobenzyl chloride or bromide[9]. The method has been applied to preparation of 2-alkyl and 2-arylindoles as well as to several 2-alkenylindoles. Tabic 3.2 provides examples. 

Tetrakis(hydroxymethyl)phosphonium Salts. The reaction of formaldehyde (qv) and phosphine in aqueous hydrochloric or sulfuric acid yields tetrakis-(hydroxymethyl)phosphonium chloride [124-62-1/, Albright Wilson s Retardol C, or the sulfate [55566-30-8] (Retardol S), (C4H 2C4P)2SO [55566-30-8]. 

LRC-100Finish. The use of LRC-100 flame retardant for 50/50 polyester cotton blends has been reported (144). It is a condensation product of tetrakis(hydroxymethyl)-phosphonium salt (THP salt) and A/A7,A7 -trimethylphosphoramide [6326-72-3] (TMPA). The precondensate is prepared by heating the THP salt and TMPA in a 2.3-to-l.0-mole ratio for one hour at 60—65°C. It is appUed in conjunction with urea and trimethylolmelamine in a pad-dry-cure oxidation wash procedure. Phosphoms contents of 3.5—4.0% are needed to enable blends to pass the FF 3-71 Test. 

Phosphonium Salt—Urea Precondensate. A combination approach for producing flame-retardant cotton-synthetic blends has been developed based on the use of a phosphonium salt—urea precondensate (145). The precondensate is appUed to the blend fabric from aqueous solution. The fabric is dried, cured with ammonia gas, and then oxidized. This forms a flame-resistant polymer on and in the cotton fibers of the component. The synthetic component is then treated with either a cycUc phosphonate ester such as Antiblaze 19/ 19T, or hexabromocyclododecane. The result is a blended textile with good flame resistance. Another patent has appeared in which various modifications of the original process have been claimed (146). Although a few finishers have begun to use this process on blended textiles, it is too early to judge its impact on the industry. 

Halex rates can also be increased by phase-transfer catalysts (PTC) with widely varying stmctures quaternary ammonium salts (51—53) 18-crown-6-ether (54) pytidinium salts (55) quaternary phosphonium salts (56) and poly(ethylene glycol)s (57). Catalytic quantities of cesium duoride also enhance Halex reactions (58). 

Textile Flame Retardants. The first known commercial appHcation for phosphine derivatives was as a durable textile flame retardant for cotton and cotton—polyester blends. The compounds are tetrakis(hydroxymethyl)phosphonium salts (10) which are prepared by the acid-cataly2ed addition of phosphine to formaldehyde. The reaction proceeds ia two stages. Initially, the iatermediate tris(hydroxymethyl)phosphine [2767-80-8] is formed. 

This compound is unstable, particularly at alkaline pH, and decomposes to release hydrogen. It is not isolated but reacts i formaldehyde and a mineral acid, for example hydrogen chloride [7647-01-17, to form the phosphonium salt. 

Phosphonium salts are readily prepared by the reaction of tertiary phosphines with alkyl or henzylic haHdes, eg, the reaction of tributylphosphine [998-40-3] with 1-chlorobutane [109-69-3] to produce tetrabutylphosphonium chloride [2304-30-5]. 

Kinetics are slow and many hours are requited for a 95% conversion of the reactants. In the case of the subject compound, there is evidence that the reaction is autocatalytic but only when approximately 30% conversion to the product has occurred (19). Reaction kinetics are heavily dependent on the species of halogen ia the alkyl haHde and decrease ia the order I >Br >C1. Tetrabutylphosphonium chloride exhibits a high solubiHty ia a variety of solvents, for example, >80% ia water, >70% ia 2-propanol, and >50% ia toluene at 25°C. Its analogues show similar properties. One of the latest appHcations for this phosphonium salt is the manufacture of readily dyeable polyester yams (20,21). 

In addition to tetrabutylphosphonium chloride, typical phosphonium salts that can be produced include tetraoctylphosphonium bromide [23906-97-0], tetrabutylphosphonium acetate [17786-43-5] (monoacetic acid), and tetrabutylphosphonium bromide [3115-68-2]. Inmost cases, these compounds can be prepared with alternative counterions. 

Pure tetrahedral coordination probably occurs only ia species where there are four identical groups and no steric distortions. Both PCU and PBr" 4, present ia soHd phosphoms haUdes, appear to have poiat symmetry. Other species, eg, H PO and POCl, have only slightly distorted tetrahedra. Similar geometries occur ia salts, esters, and other derivatives of phosphoric, phosphonic, and phosphinic acids as well as phosphine oxides and phosphonium salts. 

One of the most useful reactions in forming a P—C bond is the Michaehs-Arbusov reaction, which is a characteristic reaction of tricoordinate phosphoms compounds containing an alkoxy group (22). Alkylation of the electron pair is followed by rearrangement of the initial phosphonium salt. 

The reaction proceeds through an intermediate phosphonium salt which can be isolated in some instances. The Michaehs-Arbusov reaction is especially useful for converting trialkyl phosphites, (RO) P, to alkylphosphonic esters, and to esters of phosphonocarboxyhc acids. 

Preparation and Properties of Organophosphines. AUphatic phosphines can be gases, volatile Hquids, or oils. Aromatic phosphines frequentiy are crystalline, although many are oils. Some physical properties are Hsted in Table 14. The most characteristic chemical properties of phosphines include their susceptabiUty to oxidation and their nucleophilicity. The most common derivatives of the phosphines include halophosphines, phosphine oxides, metal complexes of phosphines, and phosphonium salts. Phosphines are also raw materials in the preparation of derivatives, ie, derivatives of the isomers phosphinic acid, HP(OH)2, and phosphonous acid, H2P(=0)0H. 

The addition of alkyl haUdes to phosphines is analogous to the reactions with amines. Because primary phosphonium salts are highly dissociated, the reaction proceeds to the tertiary or quartemary salts. 

The addition of P—H bonds across a carbonyl function leads to the formation of a-hydroxy-substituted phosphines. The reaction is acid-cataly2ed and appears to be quite general with complete reaction of each P—H bond if linear aUphatic aldehydes are used. Steric considerations may limit the product to primary or secondary phosphines. In the case of formaldehyde, the quaternary phosphonium salt [124-64-1] is obtained. 

Phosphonium salts may also be prepared by the addition of tertiary phosphines to carbonyl compounds or olefins (97). 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Other Accelerators. Amine isophthalate and thiazolidine thione, which are used as alternatives to thioureas for cross-linking polychloroprene (Neoprene) and other chlorine-containing polymers, are also used as accelerators. A few free amines are used as accelerators of sulfur vulcanization these have high molecular weight to minimize volatility and workplace exposure. Several amines and amine salts are used to speed up the dimercapto thiadiazole cure of chlorinated polyethylene and polyacrylates. Phosphonium salts are used as accelerators for the bisphenol cure of fluorocarbon mbbers. 

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

In the BASF synthesis, a Wittig reaction between two moles of phosphonium salt (vitamin A intermediate (24)) and C q dialdehyde (48) is the important synthetic step (9,28,29). Thermal isomerization affords all /ra/ j -P-carotene (Fig. 11). In an alternative preparation by Roche, vitamin A process streams can be used and in this scheme, retinol is carefully oxidized to retinal, and a second portion is converted to the C2Q phosphonium salt (49). These two halves are united using standard Wittig chemistry (8) (Fig. 12). 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

For practical appHcation in mixtures of water—organic solvent, only ammonium and phosphonium salts containing 15 or more C atoms are sufficiently lipophilic. In empirical catalyst comparisons crown ethers (hexaoxacyclooctodecanes) (1)—(3) were often as effective as the best onium salts. 

Benzyltriethylammonium chloride [56-37-1] is the most widely used catalyst under strongly basic conditions. Methyltrioctylammonium chloride [5137-55-3] (Ahquat 336, Adogen 464) is probably the least expensive catalyst. Others of high activity and moderate price are tetra- -butylammonium chloride [1112-67-0] bromide [1643-19-2] hydrogen sulfate [32503-27-8], tetra- -butylphosphonium chloride [2304-30-5], and other phosphonium salts of a similar number of C atoms. Many other onium salts can also be utilized. 

The benzylation of a wide variety of aHphatic, aromatic, and heterocycHc amines has been reported. Benzyl chloride is converted into mono-, di-, and tribenzyl amines by reaction with ammonia. Benzylaniline [103-32-2] results from the reaction of benzyl chloride with aniline. Reaction with tertiary amines yields quaternary ammonium salts with trialkylpbospbines, quaternary phosphonium salts and with sulfides, sulfonium salts are formed. 

The isoxazoles (585) were formed regioselectively from the (dioxoalkyl)phosphonium salts (584) with hydroxylamine hydrochloride, the direction of cyclization being different from that of the nonphosphorus-containing 1,3-dioxo compound (see Chapter 4.16). Aqueous sodium hydroxide converted (585) into the isoxazole (586) and triphenylphosphine oxide. Treatment of (585) with n-butyllithium and an aldehyde gave the alkene (587). With hydrazine or phenylhydrazine analogous pyrazoles were formed (80CB2852). 

Alkyl(or 3-aryl)-5-methylisoxazoles (306) were prepared by the regiospecific reaction of phosphonium salts (304) with hydroxylamine, followed by the treatment of the resulting isoxazole-containing phosphonium salts (305) with aqueous sodium hydroxide (80CB2852). 

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