Ylide compounds irreversibility

N-Metalated azomethine ylides generated from a-(alkylideneamino) esters can exist as tautomeric forms of the chelated ester enolate (Scheme 11.8). On the basis of the reliable stereochemical and regiochemical selectivities described below, it is clear that the N-metalated tautomeric contributor of these azomethine ylides is important. Simple extension of the above irreversible lithiation method to a-(alkylideneamino) esters is not very effective, and cycloadditions of the resulting lithiated ylides to a,(3-unsaturated carbonyl compounds are not always clean reactions. When the a-(alkylideneamino) esters bear a less bulky methyl ester moiety, or when a,(3-unsaturated carbonyl compounds are sterically less hindered, these species suffer from nucleophihc attack by the organometalhcs, or the metalated cycloadducts undergo further condensation reactions (81-85). 

Basil et al. [518] have studied theoretically and experimentally dinuclear phosphorus ylide complexes of Au(I) and Au(II). These compounds revealed irreversible redox behavior in CV experiments. 

A two-step mechanism (Scheme 3.34) for epoxidation was proposed in which intermediate betaine A and B are obtained from the carbonyl compound and sulfonium ylides irreversibly and from aminosulfoxonium ylide reversibly (step 1). Betaine (A or B) then undergoes ring closure (step 2) irreversibly. 

A lot of progress in this area is due to the work of Trost, who introduced diphenylsulphonium cyclopropylide and phenylthiocyclopropyl lithium as extremely versatile C3-building blocks. The first reagent is easily available from the corresponding sulphonium salt by deprotonation with suitable bases (either under irreversible conditions with dimsyl sodium, or, preferably, in a reversible manner by employing potassium hydroxide in DMSO). The ylide adds to a,) -unsaturated carbonyl compounds forming... 

Because of its enhanced reactivity, diethoxyvinylidenetriphenylphosphorane adds a variety of CH acids, even those which do not react with 0x0-, thioxo- and imino-vinylidenetriphenylphosphorane. Irreversible loss of ethanol from the initially formed Michael adduct gives phosphoranes which are vinylogs of 3-keto ylides if the starting material is an acidic carbonyl compound (equation lOS). CH-acidic carbonyl compounds also may add to diethoxyvinylidenetriphenylphosphorane via the enolate oxygen, thus leading to orthoester phosphoranes which can undergo intramolecular Wittig reaction. ... 

In contrast to the synthesis of pyrazoles, carbene synthetic approaches to imidazoles are highly diversified. Photolytic reactions of 2//-azirines provide one of the well-known synthetic methods. When irradiated, these compounds undergo irreversible C—C bond cleavage to give nitrile ylides capable of reacting as 1,3-dipoles or as arylideneaminocarbenes both inter-... 

In this case, once the unstable ylide attacked the (3-side of the 6-ketone, the ylide cannot be displaced, because it is an irreversible reaction. The presence of the 4,5-epoxy ring might disturb the formation of an ideal boat form in the resulting compound 33. This may bring the 14-hydroxy group and methylene group of the... 

Dimethylsulfonium methylide and dimethylsulfoxonium methylide also differ in their reachons with a,p-unsaturated carbonyl compounds. The sulfonium ylide reacts at the carbonyl group to form an epoxide, but with the sulfoxonium ylide a cyclopropane derivative is obtained by Michael addihon to the carbon-carbon double bond. The difference is again due to the fact that the kinehcally favoured reachon of the sulfonium yhde with the carbonyl group is irreversible, whereas the corresponding reaction with the sulfoxonium yhde is reversible, allowing preferenhal formahon of the thermodynamically more stable product from the Michael addihon. For example, the cyclopropane 112 is obtained from the reaction of dimethylsulfoxonium methylide with the enone 111 (1.105). Other methods for the formahon of cyclopropanes include carbene and Simmons-Smith-type... 

In 2008, Zhang et al. succeeded in a three-component cascade reaction using achiral Ru and chiral Zr catalysis [14]. Under the influence of achiral Rh(OAc)j, oxonium ylide was generated from diazo compound 37 and alcohol 38. Consequently, this reactive intermediate was trapped by aldehyde 39 through a Lewis acid-promoted enantioselective aldol-type addition, yielding the chiral building blocks 40 with high levels of stereocontrol (Scheme 9.11). It should be noted that the presence of acidic Zr catalyst can also suppress the undesired irreversible intramolecular proton transfer of the oxonium ylide to benefit reaction pathway control. 

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