Synthesis of phosphonium salts

Using a domestic oven it was shown that reaction of triphenylphosphine with an organic halide is very rapid under the action of microwave irradiation. Reaction times were reduced to a few minutes only, compared with 30 min to 14 days when conventional heating is used [84]. 

Nucleophilic substitutions of benzyl chloride or benzyltrimethylammonium chloride as electrophiles with Ph3P or Bu3P as nucleophiles have been performed with accurate control of the power and temperature by use of a monomode reactor. The results were carefully compared under similar conditions with microwave or A activation [85] (Eq. (30) and Tab. 3.11). 

Although the microwave effect is not appreciable at 150 °C, it becomes clearly apparent when the temperature is decreased to 100 °C. When delineating microwave effects, careful attention needs to be paid to the temperature level. If this is too high, the microwave effect will be masked and the temperature has to be minimized in order to start from a low yield under the action of A and therefore have the possibility of observing microwave enhancements. 

This conclusion is in agreement with the kinetic results from Radoiu et al. [86] obtained for the transformation of 2- and 4-tbutyl phenols in the liquid phase in the presence of montmorillonite KSF as catalyst either under the action of MW or A (Eq. (31) and Tab. 3.12). 

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Synthesis of Phosphonium Salts by Metal-Catalyzed Addition Reaction... 

Redox systems involving R2P and R2PX substituted benzenes and biphenyls have been studied by cyclic voltammetry and ESR spectrometry. The successive addition of one electron has been postulated25. The formation of a free radical on one-electron reduction has been confirmed by the observation of an ESR signal showing splittings from 31P. The synthesis of phosphonium salts in nearly quantitative yields from phosphane has also been established. 

If an Sn2 reaction has neutral starting materials and an ionic product, then a polar solvent is better. A good choice is DMF, a polar aprotic solvent often used for the synthesis of phosphonium salts by the S 2 reaction. 

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... 

In a related synthesis of phosphonium salts, Pd2(dba)3 catalyzed addition of alkenes and the acid Tf2NH to PPhs to yield anti-Markovnikov adducts (Scheme 61) [108]. By analogy to related chemistry with alkynes [109], a plausible mechanism involves protonation of Pd, alkene insertion, and P-C reductive elimination. 

Scheme 61 Proposed mechanism for Pd-catalyzed synthesis of phosphonium salts from PPhs and an alkene...

Balema VP, Wiench JW, Pruski M, Pecharsky VK. Solvent-free mechanochemical synthesis of phosphonium salts. Chem Commun 2002 37 724-5. 

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... 

SCHEME 3.39 Synthesis of phosphonium salt 172 in Corey s synthesis of leukotriene B, (157) [90],... 

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