Transylidation phosphonium ylides

Since the pioneering work of H. J. Bestmann and coworkers in the 1960s, acylation of phosphonium ylides is a well known process for the preparation of P-ox-ophosphonium yhdes. The classical way (a) using acylating agents such as acyl chlorides and in situ transylidation is still useful (Scheme 4) [16,17]. 

In several instances, transylidation through PhI=NTs leads to stable new ylides. For example, upon reaction with anisole, triphenylphosphine, and DM SO the corresponding sulphonium and phosphonium ylides were formed [51]. Further, the same ylide has been used for the preparation of iodine-carbon ylides by exchange with fi-diketones. The mildness of the method makes it suitable for high purity ylides, especially when they are thermally labile [52]. 

Triphenylarsonium ethylide. trans-Epoxides can be prepared stereoselec-tively via arsonium ylides under special conditions (10,445). A simpler method involves transylidation of a phosphonium ylide to an arsonium ylide (equation I).2... 

A number of triphenylbismuthonium diacylylides and also the diphenylsulphonylylide undergo transylidation when treated with dimethyl sulphide in the presence of copper(I) chloride in benzene at room temperature, giving the corresponding dimethylsulphonium ylides no such reaction took place with diphenyl sulphide Some of these bismuthonium ylides reacted with triphenylarsine or triphenylphosphine and were converted thereby into arsonium or phosphonium ylides they did not react with triphenylstibine L The conversion of the diacetylylide into the 4,4-dimethyl-2,6-dioxocyclohexylide as mentioned in Section IX.A should also be noted. 

Deprotonation of a phosphonium salt by an ylide is a transylidation reaction,- which is of importance especially in those cases where ylides are reacted with electrophiles (see Section 1.6.1.3), but may also be applied to isolated phosphonium salts. For an unequivocal reaction the two involved ylides must differ sufficiently with respect to their base strength (as for example in equation 14). 

If the reaction provides a salt whose acidity is greater than that of the starting phosphorane precursor (for example in the case of X = acyl), a second mole of the starting ylide reacts with the intermediate phosphonium salt in a transylidation reaction cf equation 14) giving rise to the formation of an a-substituted ylide and the conjugated salt of the original ylide. If the proton of the phosphonium salt generated in the first step is not sufficiently acidic for this proton transfer to occur, the phosphonium salt formed in the first step can be isolated and has to be deprotonated by an external base. 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

Two moles of methylenetriphenylphosphorane are reacted with one mole of a dihalogen compound. In the first step methylenetriphenylphosphorane is C-alkylated yielding an to-halophosphonium salt, which subsequently reacts with a second mole of methylenetriphenylphosphorane forming a transylidation equilibrium with methyltriphenylphosphonium halide and an w-halogenated ylide. This compound undergoes intramolecular C-alkylation, in the course of which the resulting exocyclic phosphonium salt... 

Absolute Configuration.— Bestmann has described a new method for the determination of the absolute configuration of carboxylic acids. When the optically active acid chloride (16) reacts with two equivalents of the racemic phosphorane (17) to give the phosphonium salt (18), partial kinetic resolution is observed. The second equivalent of now enantiomerically enriched phosphorane then reacts with the salt (18) in a process of transylidation, giving the optically active phosphonium chloride (19), which is precipitated, and the diastereoisomeric acyl ylide (20), which remains dissolved. The absolute configurations of the chlorides (19) have been proven by chemical correlation with the corresponding bromides, whose absolute configurations are known. 

Ketoesters.—A synthesis of a-branched /5-ketoesters is provided by electrolysis of the phosphonium chloride (76), which is formed on reaction of an acid chloride with an alkoxycarbonylalkylidenephosphorane if excess ylide is present, transylidation of the salt (76) occurs, leading to formation of the allenic ester (77), as shown in Scheme 24. 

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