Phosphonium salts reactivity

Phosphonium ylides are usually prepared by deprotonation of phosphonium salts. The phosphonium salts that are used most often are alkyltriphenylphosphonium halides, which can be prepared by the reaction of triphenylphosphine and an alkyl halide. The alkyl halide must be reactive toward Sw2 displacement. 

A general review of synthesis and reactivity of silica-immobilized onium catalysts, which deals mainly with phosphonium salts, is available [42]. 

The acyl radical equivalents formed by the electroreduction of acyl phosphonium salts have an interesting reactivity and the electroreduction of nonconjugated olefinic acids yields intramolecular coupling products (Scheme 28) [11, 54, 55]. 

Furthermore, phosphonium salts have been applied as catalysts in the TMSCN addition to aldehydes [118] and ketones [119]. Methyltriphenylphosphonium iodide [118] was fonnd to be a reasonably active catalyst for the addition of TMSCN to aldehydes at room temperatnre by the gronp of Plumet. In general, the yields varied between 70% and 97% in 24 h, depending on the aldehyde, applied in the reaction (Scheme 46). However, the salt did not support the addition of TMSCN to ketones, with one exception, when the highly reactive cyclobutanone was applied in the reaction [120]. 

The phosphonium salt 2 reacts similarly with enolates to give vinyl sulfides. The vinyl sulfide group can then be hydrolyzed to a ketone. The overall transformation corresponds to the reactivity of the dipolar synthon C (page 841). 

As early as 1963 Markl prepared the first representatives of this class of compounds via multi-step synthesis l,l-diphenyl-X -phosphorin 118 ( 1,1-diphenyl-phosphabenzene ) and 1,l-diphenyl-2,3-benzo-X -phosphorin i20 ( 1,1-diphenyl-phospha-naphthalene ). Neither compound could be obtained in crystalline form. Instead, treatment of the crystalline phosphonium salts iiP and 121 with aqueous alkali affords very reactive, air-sensitive yellow or orange powders 118 and 120). Acid treatment leads back to the phosphonium salts. 

In practice the low potentials permit a choice of approaches this is illustrated in Scheme 18 for a Wittig reaction in the presence of EGB wherein the desired product, 1,4-diphenylbutadiene, may be formed either from a reactive aldehyde (cinnamaldehyde) and an ylid from a benzylic phosphonium salt, or from a less reactive aldehyde and the ylid from a more acidic phosphonium salt The stereochemical course of the Wittig reaction is particularly sensitive to reaction conditions, especially cations which may be present. This point is partly made in Scheme 18 but deserves further elaboration. 

This method, sometimes called the Horner-Emmons, Wadsworth-Emmons, or Wittig-Hor-ner reaction,658 has several advantages over the use of phosphoranes.659 These ylides are more reactive than the corresponding phosphoranes, and when R is an electron-withdrawing group, these compounds often react with ketones that are inert to phosphoranes. In addition, the phosphorus product is a phosphate ester and hence soluble in water, unlike PhjPO, which makes it easy to separate it from the olefin product. Phosphonates are also cheaper than phosphonium salts and can easily be prepared by the Arbuzov reaction ... 

Most of the problems related to the phosphonium salts stereochemistry are discussed under synthesis (Section II), reactivity (Section III) and spectrometric characteristics (Section I.C.). Because of the tetrahedral geometry of phosphorus, chirality arises from the presence of four different substituents. Enantiomeric forms may be separated by... 

Apart from reactions in which anionic counterparts of phosphonium cations are essentially implicated in a phase-transfer catalysis process (polymer-supported or soluble catalysts see above), some kinds of chemical transformations in which the anion s reactivity is involved have been studied. There are two major advantages, one being experimental and the other the regenerating capability of the reagent, in monomer- or polymer-supported form. The anionic counterparts of phosphonium salts can have an influence on their own stability or structure (the formation of betaines163 and allyl-phosphonium-vinylphosphonium isomerization, for example275,278). 

The reactivity and use in organic synthesis of l,3,2,>l5-dioxaphospholanes has been presented230. Further details on various topics can be found in the following reviews Gloede (1988)344 Markovskii et a/.342, work prior to 1987 on linear equilibria between phosphoranes and phosphonium salts and Polezhaeva and Cherkasov340, results obtained prior to 1982 with cyclic derivatives. 

A unique feature of phosphaadamantane is the reactivity of the nitrogen with alkylating agents such that ammonium salts are formed preferentially to phosphonium salts. The synthesis of the methylammonium salt of PTA as well as the phosphine oxide, 0=PTA, are described below. The latter is achieved by the reaction of PTA with 30% H202. This harsh reagent is necessary because of the resistance of PTA, as well as its nitrogen analog, hexamethylenetetraamine, to mild oxidants. 

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