Ylide compounds generation mechanisms

In 1994 Aggarwal et al. were the first who reported a catalytic cycle in which sulfur ylides were generated in the presence of carbonyl compounds by metal-catalyzed decomposition of diazo compounds [204]. The proposed mechanism is shown in Scheme 7.58. 

The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [1194], presumably formed by the mechanism sketched in Figure 4.10. 

Addition of ethyl chlorocinnamate to 216 in the presence of base resulted in spontaneous cyclization to a mixture of products (Scheme 2). Ylides could not be detected during the reaction and an alternative mechanism was suggested.177178 In situ generation of ylides 208 (R = H) in the presence of base and reaction with fluoro- and nitrovinyl compounds gave the corresponding 2,3-disubstituted products 217 (R = F,CF3)179 and 218,180 whereas methyl... 

Carbenoid generation of nitrogen ylides represents a useful alternative to the widely employed base-promoted methodology.49 The reaction of aliphatic diazo compounds with tertiary amines was first investigated by Bamford and Stevens in 1952.50 The formation of a-benzyl-a-dimethyl-aminofluorene (99) from the reaction of diazofluorene (97) with ben-zyldimethylamine is consistent with a mechanism involving the generation of ammonium ylide 98 which then undergoes a [l,2]-benzyl shift. 

Fig. 17.13. Mechanism of the Swern oxidation of alcohols. The actual reagent is an "activated DMSO" (compound B or D), which reacts with an alcohol with formation of A or C, respectively. Dissociation leads to the sulfonium salt E, which is then converted into the sulfonium ylide F after NEt3 addition and raising the temperature from -60 to -45 °C. /3-Elimination via a cyclic transition state generates the carbonyl compound and dimethyl sulfide from F.

The reactions of carbonyl compounds with benzyltrialkylstibonium ylides have been investigated (Scheme 9).39 The products are either benzyl alcohols (52) or mixtures of alkenes and epoxides depending on the base used to generate the ylide. A mechanism is suggested for the formation of (52). 

A more serious complication for the betaine mechanism arose when it was found that oxaphosphetanes are more stable than betaines. The earliest evidence was encountered by Ramirez et al. (17), who found that certain phosphines react with two equivalents of hexafluoroacetone to give 1,3,2-dioxaphospholane derivatives 23 (Scheme 6). These compounds rearrange into unusually stable oxaphosphetanes 26 via fragmentation to 24, followed by a proton shift to generate an intermediate ylide 25 (17). Shortly thereafter, Vedejs and Snoble (18) used P nuclear magnetic resonance (NMR) methods to show that more typical Wittig reactions of nonstabilized ylides PhjP CHR also produce oxaphosphetanes and that these intermediates can be easily observed at temperatures below 0°. Since betaines did not accumulate in any of the experiments, their conversion to oxaphosphetanes could not be rate... 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

Despite the great synthetic utihty of diazocarbonyl compounds in the generation of carbonyl ylide intermediates, definitive mechanistic studies on the metal-catalyzed cycloaddition of carbonyl yhdes are scarce. Among the various metal catalysts, dirhodium(II) catalysts are the most effective and versatile for diazo decomposition. Because of the rapid catalytic tmnovers of these reactions, structural information about the intermediates is difficult to obtain. A reasonable mechanism can be rationahzed on the basis of product distribution, and especially on the basis of enantioselective outcome of various carbonyl yhde reactions [55-63]. 

In an approach to the stemofoUne class of alkaloids, the Martin group discovered an unusual set of conditions for generating azomethine yHdes. Oxidation of compound 186 under Swem conditions afforded a 5 1 mixture of 189 and 190 in 69% yield (Scheme 38) (2011TL2048). The formation of these two molecules can be easily rationalized via an intramolecular 1,3-dipolar cycloaddition of dipole 188, but the mechanism through which the azomethine ylide is formed under Swem conditions is not well understood. The authors proposed that the oxidized product 187a derived from 186 reacted with one of the electrophifrc species formed under the reaction... 

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