Phosphonium salts, supported

Tundo, P. and Venturello, P. Catalysis mechanism of phosphonium salts supported on silica-gel in organic-aqueous 2-phase systems, J. Am. Chem. Soc., 1981, 103, 856-861. 

Transhalogenation is also performed in the gas phase. The preferred catalysts are quaternary phosphonium salts supported on silica, alumina or glass beads which are applied in tubular reactor. At 140 °C, residence time of a few seconds is sufficient for 72-butyl bromide and -propyl chloride to equilibrate223. 

Hughes developed a polymer-bound phosphonium salt support for a solid phase Wittig-Madelung indole synthesis (Scheme 8) [21]. The method uses a commercially available polymer-bound triphenylphosphine 22. 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

Tundo, P. and Selva, M. (2005). Continuous-flow, gas phase synthesis of 1-chlorobutane (1-bromobutane) from 1-butanol and aqueous HCl (HBr) over silica-supported quaternary phosphonium salt. Green Chem., 7,464-467. 

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]. 

After these results had established the feasibility of generating and utilizing a carbohydrate phosphorane, the two systems that had been reported earlier were examined in order to determine if similar conditions would allow them to undergo the Wittig reaction. The ylide derived from phosphonium salt I condensed with both benz-aldehyde and U-chlorobenzaldehyde to produce good yields of olefinic products Villa and Vlllb. The ylide derived from phosphonium salt II also was successfully condensed with benzaldehyde, but the yield of IX was only 30 , presumably because of its extremely poor solubility even in an HMPA-THF solvent mixture. Both of these systems supported the tenet that it was possible to use unstabilized carbohydrate phosphoranes if the conditions are proper and if the g-oxygen is attached to the carbohydrate through another set of bonds. 

The dependence of kobsd on stirring speed for Br-I exchange reactions with polymer-supported crown ethers 34 and 35 has been determined under the same conditions as with polymer-supported phosphonium salts 1 and 4149). Reaction conditions were 90 °C, 0.02 molar equiv of 100-200 mesh catalyst, 16-17% RS, 2% CL, 20 mmol of 1-bromooctane, 200 mmol of KI, 20 ml of toluene, and 30 ml of water. Reaction rates with 34 and 35 increased with increased stirring speed up to 400 rpm, and were constant above that value. This result resembles that with polymer-supported onium ion catalysts and indicates that mass transfer as a limiting factor can be removed in experiments carried out at stirring speeds of 500-600 rpm, whatever kind of polymer-supported phase transfer catalyst is used. 

Complexation constants of crown ethers and cryptands for alkali metal salts depend on the cavity sizes of the macrocycles 152,153). ln phase transfer nucleophilic reactions catalyzed by polymer-supported crown ethers and cryptands, rates may vary with the alkali cation. When a catalyst 41 with an 18-membered ring was used for Br-I exchange reactions, rates decreased with a change in salt from KI to Nal, whereas catalyst 40 bearing a 15-membered ring gave the opposite effect (Table 10)l49). A similar rate difference was observed for cyanide displacement reactions with polymer-supported cryptands in which the size of the cavity was varied 141). Polymer-supported phosphonium salt 4, as expected, gave no cation dependence of rates (Table 10). 

The most straightforward way to obtain polymeric phosphonium salts involves introducing the phosphonio groups on to a suitable polymeric structure, for example by reacting tertiary phosphines with a poly(chloromethylstyrene) (reaction 99). The polymeric phosphonium salts obtained in this way are mostly used as polymer-supported phase-transfer catalysts for nucleophilic substitutions reactions under triphase conditions. 

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). 

This reaction is only of limited synthetic utility, and has mainly been used to verify that metallations had indeed taken place [1-3], or for the regioselective introduction of deuterium or tritium into a molecule. The solvolysis of silanes, organogermanium compounds, and phosphonium salts to yield alkanes with simultaneous cleavage from the support is discussed in Section 3.16. 

Vinylpolystyrene, a useful intermediate for the preparation of various functionalized supports for solid-phase synthesis [7,57-59], has been prepared by the polymerization of divinylbenzene [7], by Wittig reaction of a Merrifield resin derived phosphonium salt with formaldehyde [59-62], or, most conveniently, by treatment of Merrifield resin with trimethylsulfonium iodide and a base [63] (Figure 5.7). 

A selection of commercially available phosphonium salts suitable for the activation of carboxylic acids in the presence of amines is sketched in Figure 13.5. Phosphonium salts such as those shown in Figure 13.5 do not react with amines, and are well suited for preparing amides on insoluble supports, with either the amine or the acid linked to the support (Table 13.5). Solutions of these reagents in dry DMF are quite stable and can be used even after standing at room temperature for several days [84]. BOP, one of the first phosphonium salts used for peptide synthesis [85], leads to the formation of mutagenic HMPA, and should therefore be replaced by the less hazardous PyBOP [86],... 

Hughes I, Application of polymer-bound phosphonium salts as traceless supports for solid phase synthesis, Tetrahedron Lett., 37 7595-7598, 1996. 

Uronium/guanidinium salt mediated cyclizations may also be performed on solid supports, However, in contrast to the case involving phosphonium salts... 

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