Phosphonium salts elimination reactions

The submitters have shown that these reactions proceed by dehydro-chlorination of the acid chloride to the ketene, which is then trapped by reaction with the phosphorane. The resulting betaine decomposes to the allenic ester via an oxaphosphetane. In contrast, the reaction of acid chlorides with 2 equivalents of phosphoranes involves initial acylation of the phosphorane followed by proton elimination from the phosphonium salt. ... 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

It is interesting that the reaction of triphenylphosphine with a 1,2-allenyl ketone leads to the formation of a vinyl phosphonium salt 449, which upon protection of the carbonyl group would accept nucleophilic attack followed by elimination in the presence of Et3N to afford y-nudeophile substituted- ,/j-unsaturated enones 451 [197]. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

In the case of phosphonium salts, some examples of decomposition by Hofmann elimination reactions are known. The phosphonium salts which normally undergo such a decomposition possess an activated hydrogen / to the phosphorus the salts 70, with a strong electron-withdrawing Z group on the carbon / to the phosphorus (reaction 202) and the a-hydroxyalkylphosphonium salts 71 (reaction 203)... 

Studies on the influence of molar ratios between the base and the phosphonium salt have shown696,697 that the substitution reaction is the main reaction when an excess of base is used however, in contrast, the elimination reaction is clearly favoured when an insufficient amount of base is used. These results are not surprising if it is assumed that the substitution mechanism is second order with respect to the hydroxide ion concentration696,697, whereas the elimination reaction EHp is only first order. 

In another example710, the phosphonium salt 76 decomposes competitively by the two mechanisms of substitution and elimination (reaction 211). This salt behaves in a manner intermediate between salts 74 and 75 indeed, the SN(P) mechanism is not completely excluded as for the salt 74 but the EHfi mechanism is still favoured with regard to the salt 75 since the alkene formed is stabilized. 

Dehydrobromation of halogenated phosphonium salts is currently used in the synthesis of unsaturated compounds, (cf. Section II.C) provided that a labile carbanion is created in a suitable position relative to the halogen atom (reaction 261). Such bases as Et3N (for / - or (5-halogenated phosphonium salts)288 or bromide anion357 may induce this elimination. Fluorine atoms of the cationic part of 3-methyl-1,1-difluorocyclopent-2-ene-1-phosphonium hexafluorophosphate were substituted when the compound was treated with absolute methanol under mild conditions412. 

Reaction of vinyl phosphonium salts with sodium azide also yields triazoles via triazoline intermediates by elimination of triphenylphosphine (Scheme 130).418... 

For the thioacetal cleavage in the second step (ia) a classical cerium(IV)-catalysed hydrolysis gives good yields ( > 85 %) with any phosphonium salts we have tested. Subsequent elimination of triphenylphosphine by reaction of the salts 5 with triethylami-ne was always nearly quantitative. 

As the Wittig reaction forms both tt and cx-bonds, the disconnection is right across the middle of the alkene giving a choice of starting materials. So with the exo-cyclic alkene 26, very difficult to make by elimination methods, we could use formaldehyde or cyclohexanone as the carbonyl component with either phosphonium salt 25 or 28. It is a matter of personal choice whether you draw the ylid, the phosphonium salt or the alkyl halide at this stage. 

To prevent the base-catalyzed elimination of triphenylphosphine 63) 27 from the ester phosphonium salts, a frequently occurring competition reaction of ylide for-... 

A less commonly employed method of preparation is the Kirsanov reaction.3 Strictly speaking, this is the reaction of phosphorus pentachloride with benzene-sulfonamide, which can be reacted further to form benzenesulfonyliminophos-phoranes. However, the name is now used generically to cover the reactions of triphenylphosphorus dihalides and amines in the presence of a base (Scheme 5). Attack of the amine on phosphorus gives the intermediate aminophosphonium salt and HBr, elimination of another equivalent of HBr generates the IMP. If the amine is aromatic, then 2 eq. of a mild base (e.g. triethylamine) are sufficient to perform the reaction in one pot.4 However, in the case of alkylamines, it is generally necessary to isolate the phosphonium salt and treat it with sodamide to effect the deprotonation and generate the IMP.5... 

In some cases, decomposition of phosphonium salts which have a phenyl group or a negative substituent Z in a p-position to the phosphorus atom can occur via elimination reactions. According to the nature of Z, tertiary phosphines (Z = Ph, CN equation or vinylphosphonium salts (Z = OPh, OAc, OH, Br equation 16) are obtained. In the latter case, the vinylphosphonium salts cannot be isolated from the aqueous medium because they add water very easily, but trapping with methanol reveals their presence. 2 ... 

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