Ylides, reactions

Modification of functional groups incorporated in a heterocycle is possible via ylide reactions. The 5-methylisoxazole (578) on reaction with n-butyllithium and methanesulfenyl... 

The intermolecular version of the above described reaction has also been reported [92]. In the first example the reaction of a rhodium catalyst carbonyl ylide with maleimide was studied. However, only low enantioselectivities of up to 20% ee were obtained [92]. In a more recent report Hashimoto et al. were able to induce high enantioselectivities in the intermolecular carbonyl ylide reaction of the... 

Oxidation 205-213 /J-Oximinosulphoxides, chiral 336 Oxiranes 169 reactions of 305 synthesis of 639 2-Oxo-l,2,3-oxathiazolidines 71 /1-Oxosulphones, synthesis of 169 Oxosulphonium ylides, reactions of 219 /3-Oxosulphoxides reduction of 347-349 synthesis of 337-340... 

Keywords Cycloprop anation Insertion Ylide reactions Asymmetric catalysis Synthesis... 

Among other olefins prepared in conventional ylide reactions with carbonyls are (68), (69), (70), and (71). ... 

Betaine Reaction A ylide Reaction B Reaction C Reaction D ... 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

As shown in Figure 4.1, the initial step of the conversion of an ylide into a carbene complex is an electrophilic attack at the ylide. Reactions of this type will, therefore, occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex L M. Complexes which efficiently catalyze the decomposition of even weakly nucleophilic ylides can easily react with other nucleophiles also, such as amines or thiols. This has to be taken into account if reactions with substrates containing such strongly nucleophilic functional groups are to be performed. 

Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with allyl... 

In a similar approach, Garner et al. (78) made use of silicon-based tethers between ylide and dipolarophile during their program of research into the application of azomethine ylides in the total asymmetric synthesis of complex natural products. In order to form advanced synthetic intermediates of type 248 during the asymmetric synthesis of bioxalomycins (249), an intramolecular azomethine ylide reaction from aziridine ylide precursors was deemed the best strategy (Scheme 3.84). Under photochemically induced ylide formation and subsequent cycloaddition, the desired endo-re products 250 were formed exclusively. However, due to unacceptably low synthetic yields, this approach was abandoned in favor of a longer tether (Scheme 3.85). 

A -Cyano- and A -(p-toluenesulfonyl)-A -(trimethylsilylmethyl)-5-methylisothio-ureas 269a and 269b have also been utilized as synthetic equivalents of azomethine ylides. ° Reaction of 269a and 269b with aromatic aldehydes and aryl ketones, in the presence of CsF, gives 2-iminooxazolines 270a-e in modest-to-good yield. These 2-iminooxazolines apparently are stable to isolation and do not isomerize to 2-aminooxazolines (Scheme 8.76). 

Stereoselective oxonium ylide reaction, in particular the asymmetric catalysis, has been a problem of considerable challenge in this field. Since the first report by McKervey and co-workers in the asymmetric induction in metal carbene-mediated ylide formation/[2,3]-sigmatropic rearrangement in 1992, there have been efforts being directed... 

The asymmetric catalysis has not been well explored in the reaction of a metal carbene complex-generated ammonium ylide. The ammonium ylide reaction is assumed to proceed through a free ylide rather than a metal... 

Sulfonium compounds undergo elimination similar to that of their ammonium counterparts (7-6 and 7-7) in scope and mechanism. The decomposition by heat of sulfonium hydroxides has been known for many years.218 The ylide reaction was discovered more recently.219 Neither is important synthetically. 

Phosphonium ylides can also be used to prepare from substituted ethynylphosphonium371, 1,3-propenylidenebisphosphonium salts or, more curiously, 1,1-propenylidenebisphos-phonium ylides (reaction 98). 

The parent of the diazo compounds, diazomethane, CH2=N=N, has been mentioned before in connection with ylide reactions for ring enlargement (Section 16-4A) and the preparation of methyl esters from acids (Table 18-7). It is one of the most versatile and useful reagents in organic chemistry, despite the fact that it is highly toxic, dangerously explosive, and cannot be stored without decomposition. 

Keywords chalcone, benzylidene aniline, trimethyloxosulfonium iodide, ylide reaction, cyclopropane, aziridine... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

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