Phosphonium salts, conversion with bases

Cleavage reactions are best carried out in aqueous solution. In aprotic solvents, electrogenerated bases lead to the conversion of onium salts to the ylids which are not reducible [49]. The sequence of reactions shown in Scheme 5.2 shows that the bond cleavage process for phosphonium salts proceeds with retention of configuration around the phosphorus atom [50]. Retention of configuration at arsenic is also observed [51]. This electrochemical process is a route to asymmetric trisub-stituted phosphorus and arsenic centres. 

Under these reaction conditions, the use of ammonium and related onium salts with nucleophilic anions has been found effective at converting the HCl/SnCU-initiated, uncontrolled polymerizations into controlled/ living processes [105], Similar results are reported for TiCl4-based polymerizations [174,175], Effective salts include tetraalkylammonium and phosphonium salts R4N+Y and R4P+Y (Y = I, Br, Cl, CH3COO R = CH3, C2Hs, 71-C4H9, etc.). As added nucleophiles do in nonpolar solvents, the added salts retard the polymerization, narrow the MWD of the polymers, and render their M values directly proportional to conversion and close to the calculated values (one living chain per initiator molecule). 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also spelled ylide and called a phosphorane) to give an alkene. The conversion of a carbonyl compound to an alkene with a phosphorus ylid is called the Wittig reaction. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from a triaryl phosphine and an alkyl halide (10-31) ... 

It is not clear whether enolization is avoided under the lithium-free, high-concentration conditions, or whether it occurs reversibly enough to permit eventual conversion of the ketone to the alkene. However, the most successful procedures involve alkoxide bases (159, 168-170) or require the presence of excess phosphonium salt (171). Proton exchange and reversible enolate formation are likely under these conditions, and aldol condensation pathways would also be reversible when potassium or sodium bases are used. Thus, excellent yields of alkenes are possible with the most hindered substrates, provided that other pathways for irreversible enolate decomposition are not available. 

Butenoate-phosphoranes 275 were prepared by the same route. The less stabilized ylides 275fj i were isolated as phosphonium salts after quenching with trifluoroacetic acid, but conversion to TFA salts was carried out also on 275a d in order to protect the phosphorane double bond during ozonolysis to 272. The free bases 272a d,f,i cyclized to the corresponding penems 276 upon mild heating in benzene. 

A mixture of 2-(phenoxymethyl)oxirane, S-phenyl thiobenzoate, and a little Bu4NBr in dry DMF stirred at 110° for 25 h - 2-benzoyloxy-3-(phenylthio)propyl phenyl ether. Y 71% (82.4% conversion). F.e. and with 18-crown-6-metal complexes s. T. lizawa et al., Bull. Chem. Soc. Japan 62, 597-8 (1989) with a polymer-based phosphonium salt cf. Chem. Letters 1988, 1605-8. 

Protection and Deprotection.—N-Protected a-amino-acids are readily esterified by methanol or ethanol in 60—80% yield after reaction with an enamine (e.g. from isobutyraldehyde and piperidine) and t-butyl isocyanate. Such amino-acids can also be esterified efficiently with alkyl halides under phase-transfer conditions with no racemization. Direct esterification of a-amino-acids with ethyl toluene-p-sulphonate in boiling ethanol gives a-amino-acid ethyl esters in 80—90% yield as the sulphonate salts. The protection of acid functions by formation of the 2-chloro-(or bromo-)ethyl esters has been discussed. These derivatives survive exposure to both moderately acidic and basic conditions and are removable by conversion into the iodoethyl analogues followed by zinc reduction. Alternatively, they may be converted into hydrophilic ammonium or phosphonium salts which exhibit enhanced acid stability but which are cleaved by very dilute base. Yet another method for the removal of such groups using supernucleophilic Co phthalocyanin anions has been reviewed. Further routes to 2,2,2-trichloroethyl esters have been described, one of which employs an activated ester intermediate and is suited to acid-labile substrates. 

Ylide and Related Reactions. AUyloxymethylenetriphenyl-phosphonium chlorides yield aUyl vinyl ethers by conversion into ylides with r-BuOK and treatment with aryl and eno-lizable acyclic aldehydes and ketones. Q , Unsaturated nitriles are obtained with high Z-stereoselectivity when diphenyl-cyanomethylenephosphonate is converted into its potassium salt by treatment with r-BuOK in THF, followed by addition of aldehydes with bulky substituents. o -Fluoro-Q, -unsaturated thia-zolines are obtained with slight l -stereoselectivity when mixtures of fluorophosphonomethylthioamides, or the corresponding thia-zolines (eq 94), and aldehydes are treated with t-BuOK in THF at 0 °C higher E-selectivity is usually observed when BuLi in THF is used as the base at —78 °C. ... 

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