Ylids

Over twenty years ago, Wittig found that alkyl substituted phosphonium salts could be deprotonated by strong bases to yield phosphonium ylids which in turn could react with aldehydes and ketones to yield olefins and the corresponding phosphine oxide [1]. The sequence is formulated in equations 1.1-1.3 for the reaction of methyl-triphenylphosphonium halide with cyclohexanone in the presence of base to give me thylenecyclohexane. 

The Wittig reaction is commonly carried out in aprotic solvents in the presence of such strong bases as Az-butyllithium, sodium hydride or sodamide [2]. The application of phase transfer catalysis has simplified the Wittig and related reactions in a number of cases, and these examples are presented in the chapter. 

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The same disconnection can be used with simple epoxides when the sulphur ylid (A) is used a reagent for the synthon CH2. Draw a mechanism for the reaetion ... 

Notice that sulphur ylids behave quite differently from phosphorus ylids, which would of course do the Wittig reaction (ftames 41-43). 

The carbene synthon might be difficult, but since the olefin is conjugated with a carbonyl group we could try a sulphur ylid as a nucleopliilic carbene equivalent (as in frame 283). Synthesis The diene could be made by this route ... 

One example of a suitable suitable sulphur ylid is the one below (C) (Corey, Tetrahedron Letters. 1967, 2325) and yhds of this sort have been added to a,p-unsaturated ketones (Tetrahedron Letters. 1966, 3181) ... 

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

Chiefly in an hydrophobic medium, a base can extract the proton on position 2 leading to a reactive intermediate (able to give subsequent condensation) that could be an ylid (35, 36) or a carbene (37), though no dimer has ever been isolated as is the case with benzothiazolium (32, 38). Two mechanisms have been proposed for explaining the particular reactivity of thiazolium ... 

Nucleophilic reaction of an ylid, under hydrophobic conditions. 

Adducts from various quaternary salts have been isolated, in reactions with aldehydes, a-ketoaldehydes, dialkylacylphosphonates and dialkyl-phosphonates, isocyanates, isothiocyanates, and so forth (Scheme 15) (36). The ylid (11) resulting from removal of a Cj proton from 3.4-dimethyl-S-p-hydroxyethylthiazolium iodide by NEtj in DMF gives with phenylisothiocyanate the stable dipolar adduct (12) that has been identified by its NMR spectrum and reactional product, such as acid addition and thiazolidine obtention via NaBH4 reduction (Scheme 16) (35). It must be mentioned that the adduct issued from di-p-tolylcarbodiimide is separated in its halohydrogenated form. An alkaline treatment occasions an easy ring expansion into a 1,4-thiazine derivative (Scheme 17) (35). 

Stibonium Ylids and Related Compounds. In contrast to phosphoms and arsenic, only a few antimony yhds have been prepared. Until quite recendy triphenyl stibonium tetraphenylcyclopentadienyUde [15081 -36-4] C H Sb, was the only antimony yUd that had been isolated and adequately characteri2ed (192). A new method, uti1i2ing an organic copper compound as a catalyst, has resulted ia the synthesis of a number of new antimony yhds (193) ... 

SOMMELET HAUSER Ammonkjmyfld rearrangement Rearrangement of quaternary ammonium ylids to aminas by aryl trarafer. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

A. W. Johnson, Ylid Chemistry, Academic Press, New York, 1979. 

Figure 4.19 Synthesis of gold ylid complexes, including gold(II) compounds with metal-metal...

An extensive chemistry is developing of dinuclear gold(III) complexes with phosphorus ylid ligands (Figure 4.41). As mentioned in section 4.19, gold(I) compounds can undergo one- or two-electron oxidative additions,... 

Intramolecular Au-Au interactions are found in some binuclear complexes (AuX)2, where X is a chelating ligand like dithiocarbamate, phosphine ylid (R2P(CH2)2) or bidentate phosphines. Therefore, in [Au(S2CNBu2)]2 the Au-Au distance is 2.78 A (Figure 4.50). 

In neutral and alkaline media, the rate of exchange at the 3 and 6 position of 4-aminopyridazine is independent of acidity but decreases markedly when the media become more acidic. This was interpreted in terms of a rate-determining removal of the 6-proton by deuteroxide ion to give the ylid (XXIV), which reacts with deuterium oxide in a fast step. A similar result for the 3 and 6 positions of py-ridazin-4-one suggests the same mechanism. For reaction at the 5 position, the rate-acidity profile indicated reaction on the free base as did that for the 5 position of pyridazin-3-one, though the appearance of a maximum in the rate at — HQ = 0.8 was anomalous and suggested incursion of a further mechanism. 

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

Durch Einelektronen-Oxidation werden aus Phosphonium-Salzen die entsprechenden Ylide erhalten, die mit Ketonen in einer Wittig-Reaktion zu Olefinen (bis zu 95% Aus-beute) umgesetzt werden konnen6. Zuweilen treten neben dem Ylid auch die Spaltpro-dukte auf ... 

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