REACTIVITY OF PHOSPHONIUM SALTS

H. Freyschlag, H. Grassner, A. Niirrenbach, H. Pommer, W. Reif, W. Samecki, Formation and Reactivity of Phosphonium Salts in the Vitamin A Series, Angew. Chem. Int. Ed. Engl. 1965, 4, 287-291. 

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. 

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. 

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). 

Some stabilized ylides are too stable to be very reactive. In this case phosphonates are used instead of phosphonium salts. For the Horner-Wadsworth-Emmons-reaction see Chapter 2. 

The resistance of cellulose to microbial attack when it was treated with resins was attributed to chemical bonds formed between the fiber and the resin this was ascertained by treating cotton with a variety of phosphonium salt-resin compositions having different degrees of cross-linking and homopolymerization (127). Some of the more recent approaches for producing antimicrobial fibers include the use of reactive dyes (128) and mixtures of zirconyl and copper salts (15) on cotton and the bromination of jute (129). 

Other anions (X = Br , I , HO , CH3O ) also exhibit enhanced nucleophilic reactivity in crown-ether mediated reactions of K X [357, 358], Montanari et al. [653] compared the effects of phosphonium salts, coronands, and cryptands on the Sn2 reaction of -octylmethanesulfonate with various nucleophiles in chlorobenzene. 

Comparison of these data (Table V) show, quite clearly, how much more reactive the phosphonium salts are in the resin system than compounds such as substituted imidazoles and organo-zinc salts (e.g., zinc stearate and octoate). The only accelerator that shows substantially more potency than the quaternary phosphonium salts is uranyl nitrate. However, this compound is not a latent accelerator in the sense that storage lifetimes are only of the order of a few days. 

The extra stabilisation makes the ylid rather unreactive and phosphonate esters 91 are often used instead of phosphonium salts in these reactions. Treatment with a base (NaH or RO is often used, BuLi will certainly not do) gives an inherently more reactive enolate anion 92 rather than an ylid. These Horner-Wadsworth-Emmons reagents (H WE as we shall call them, though they go under many other names) react with ketones as well as aldehydes and the product is normally the E-alkcnc16 93. 

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... 

The formation of guanidino side products is not critical for the stepwise solid-phase synthesis of standard peptides, in which a preactivation of reactive amino acids is conducted. However, if activation in situ is carried out or activation of hindered amino acids, protected peptides, or a carboxylic moiety before a cycli-zation step is performed, the use of phosphonium salts may be more efficient. For slow activation reactions, the addition of more coupling reagent (e.g., PyAOP) during the course of the reaction is advisable because the coupling derivative is hydrolyzed after a few minutes. 

It is worth noting that a higher anionic reactivity is produced by increasing steric hindrance around the cation. This can be achieved by increasing the length of alkyl chains or by shortening the heteroatom-carbon bond using ammonium instead of phosphonium salts. 

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... 

While the synthesis of phosphonium salts from activated alkenes is readily achieved by the addition of triphenylphosphine and related substrates, unactivated alkenes are significantly less reactive in this type of reaction. To overcome this issue and provide a pathway to phosphonium salts from substrates such as 1-hexene, Grubbs recently reported the formal hydrophosphonation of these sluggish substrates (Scheme 4.32 and Example 4.26) [84]. The key substrate for the addition was the triphenylphosphonium salt [HPPh3][BF ]. The reaction was found to be promoted by both classic radical initiators such as AIBN and ACN. Additionally, UV light was found to be effective and was determined to be the... 

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