Applications of Phosphonium Salts

Phosphonium salts are used as a source of ylids for alkene synthesis. Other applications include insecticides and fungicides [11]. Important applications of THPC are as flame retardants for textiles and paper, the improvement in uptake of colour and shrinkage resistance of wool, and the production of organic polymers by condensation with phenols or amines. Phosphonium salts also find 

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In this chapter, specific paragraphs are not devoted to the zwitterionic structures, such as betaines, or to the applications of phosphonium salts in organic synthesis, but both topics are mentioned incidentally. 

Analc ous relationships are found in the reactions of the phosphonium salts with ketones such as benzophenone. With the application of phosphonium salt XXVIa, 31% of the Schiff base XXX could be isolated, whUe withXXVIb only 4% of the desired product XXXI could be obtained. Finally, with 0-carbonylmethylene phosphonium chloride, which contains an unprotected carbonyl group, no further reaction occurs. 

In the past two years, many other applications of phosphonium salts as catalysts or intermediates of chemical reactions have also been described. Some of the most relevant ones are summarized below. 

The alkaline hydrolysis of phosphonium salts has been particularly studied from a theoretical point of view as a model for nucleophilic substitution reactions on tetracoor-dinated phosphorus. However, it also has interesting applications, since it corresponds... 

Phosphonium salts can be dealkylated by treatment with alkoxides to yield alkanes. Although the hydrolytic cleavage of phosphonium salts in solution has been well investigated, the solid-phase variant of this reaction has not yet found broad application. One example, in which traceless linking was based on the alkoxide-induced dealkylation of a resin-bound phosphonium salt, is given in Table 3.48 (Entry 1). 

The method of greatest applicability is the reaction of alkali metal phosphides with dihalo-alkanes, -alkenes, etc. (equation 4). Generally this reaction proceeds well and in high yield excess dihaloalkane should be avoided as this leads to the production of phosphonium salts which can contaminate the product. 

Typically, nonstabilized ylides are utilized for the synthesis of (Z)-alkenes. In 1986, Schlosser published a paper summarizing the factors that enhance (Z)-selectivity. Salt effects have historically been defined as the response to the presence of soluble lithium salts. Any soluble salt will compromise the (Z)-selectivity of the reaction, and typically this issue has been resolved by the use of sodium amide or sodium or potassium hexamethyldisilazane (NaHMDS or KHMDS) as the base. Solvent effects are also vital to the stereoselectivity. In general, ethereal solvents such as THF, diethyl ether, DME and t-butyl methyl ether are the solvents of choice." In cases where competitive enolate fomnation is problematic, toluene may be utilized. Protic solvents, such as alcohols, as well as DMSO, should be avoided in attempts to maximize (Z)-selectivity. Finally, the dropwise addition of the carbonyl to the ylide should be carried out at low temperature (-78 C). Recent applications of phosphonium ylides in natural product synthesis have been extensively reviewed by Maryanoff and Reitz. 

In contrast to this situation, few applications have been reported pertaining to the use of phosphonium salt ILs, several of which are now readily available in industrially relevant quantities [8]. 

The above example illustrates how useful reactivity inherent in phosphonium salt ILs in comparison to nitrogen-based systems can be unraveled through reaction screening. We have engaged in a more systematic attempt to evaluate potential unique applications for phosphonium salt ILs. Our consideration of the nature and reactivity of phosphonium-based salts as catalysts or media for organic reactions, which differs or is not possible with nitrogen-based systems, led us to speculate that they might function as Lewis acids [15]. It was envisioned that due to the... 

In conclusion, it has been shown that the investigation of phosphonium salt ILs as solvents and catalysts can reveal advantages in comparison to nitrogen-based systems that may be rationally conceived or that may be unpredictable beforehand. It was demonstrated that an active, stable, and recyclable palladium species is generated upon dissolution of Pd2(dba)3-CHCl3 in phosphonium salt ILs. This recyclable catalyst offers advantages in both Suzuki and Heck coupling processes and will no doubt find applications in other paUadium-mediated processes as well. 

The unique reactivity of phosphonium salt ILs both as recyclable, solvent-free media for metal-catalyzed reactions and as mild Lewis acids in catalytic processes represents fertile groimd for research that is still in its infancy. The discovery and development of many new applications of this newer subset of ionic liquids will continue at an accelerated pace over the next few years. 

Reactive ylides, in apolar solvents under salt-free conditions, preferentially form olefins with the (Z)-configuration (see Section C.l). To optimize this effect, various techniques have been developed for preparing salt-free ylide solutions [21-25]. The phosphonium salt is deprotonated, for example with sodium amide in THF or liquid ammonia [23], with sodium hexamethyldisilazane in an ether solvent such as THF [21], or with potassium r-butoxide in THF or toluene [22], with the addition of crown ether if appropriate [24]. A particularly elegant application of the salt-free Wittig reaction is the instant-ylide technique [25]. 

For the synthesis of carotenoids labelled in the central part, the C15 + C10 + C15 scheme is used. The syntheses of the labelled Cio-central parts are described in Section B.2. The C15-parts are used in the form of phosphonium salts. For nearly all the end groups of carotenoids, the syntheses of the corresponding Cis-phosphonium salts have been reported [58-61]. Many of these Ci5-phosphonium salts can be prepared in labelled form by application of the reactions discussed in Section B.l. In this Section the preparation of the Cis-phosphonium salts that are used in the synthesis of labelled p,P-carotene (3), astaxanthin (403) and spheroidene (97) is now described. 

The particular features of phosphonium salts were exploited for a number of synthetic applications in 2014. Phosphonium chloride salts found applications as chlorine source and as modifiers for homogeneous catalyst systems. As an example, Muller, Rosenthal and co-workers reported the study of a chromium-based catalyst for the selective tri-merization of ethylene. A phosphonium precursor of the type i cyclo-(PR2CH2CH(OH) )2][Br]2) was used for the preparation of iron(n) complexes containing unsymmetrical P-N-P pincer ligands (Scheme 5). The group of Prof. Morris tested these compounds as catalysts for the asymmetric hydrogenation of ketones and imines. ... 

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