Reactions of Ylides

The stable dicyanomethylides 274, made directly from the triazolopyridines and tetracyanoethylene oxide (TCNEO) reacts more slowly with MEP in acetonitrile to give mainly indolizines, with cyclazines as secondary products formed 

Other heteroatom analogues such as sulfur (S) also undergo the Stevens rearrangement. 

Unstabilized sulfonium ylides such as dimethylsulfonium methylide (3.45) and stabilized sulfoxonium ylides such as dimethylsulfoxonium methylide (3.46) are the most widely used sulfur ylides. 

The sulfur ylide 3.45 on reaction with cyclohexanone gives oxirane 3.64, while the phosphorus ylide 3.65 gives the alkene 3.66. 

Trost showed that the sulfur ylide 3.68 prepared by the deprotonation of 3.67 with n-butlyllithium (n-BuLi) on reaction with benzaldehyde under the same reaction conditions gave styrene oxide 3.69 as a racemic mixture. 

Unstabilized sulfonium ylides and stabilized sulfoxonium ylides show different reactions with a,P-unsaturated carbonyl compounds the former give epoxides and the latter give cyclopropanes. The epoxide formation (i.e. 1,2-addition) is kinetically favourable while cyclopropane formation (i.e. 1,4-addition, Michael addition) is energetically favourable. 

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The two major methods of preparation are the cycloaddition of nitrile oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamines. Additional methods include reaction of /3-haloketones and hydroxylamine, the reaction of ylides with nitrile oxides by activation of alkyl nitro compounds from isoxazoline AT-oxides (methoxides, etc.) and miscellaneous syntheses (62HC(i7)i). 

A study of the photochemical reactions of some ylides of compound 1 showed the expected fragmentation to give, from compound 258 for example, 3-methyl-triazolopyridine and the products 259 and 260 postulated as derived from a carhene intermediate (00MI2). Ester 261 gives a hydrazone, and ylide 262 an indolizine. Thermal reactions of ylides are in Section IV. I. 

With a change to non-polar solvent, the reaction of ylides 269 with alkynes and alkenes changed dramatically, as shown in Scheme 10. With DM AD in toluene the ylides give pyrazolopyridines 272 in good yield (91TL4977), and with methyl propiolate (MEP) give indolizines 273 (92H(33)203). The reaction with acrylates is much less clean, but the variety of products is said to be formed from a diazene intermediate, which splits to give a diradical (93H(35)851). 

As recent results we can mention the reactions of ylides 105 in presence of PCI3, ASCI3, or SbCl3 which lead to the formation of 1 1 adducts (Scheme 31)... 

Cyclopropylphosphonium salts are produced, together with the phosphonium salts (76), in reactions of ylides with vinylsulphonium salts... 

The reaction of ylides with phosphorus(iii) halides has been extended to the ylides (Me2N) Me3 P CH2, = 1, 2, or 3. Alkylation of the resulting stabilized ylides (20) with methyl iodide took place on the tervalent phosphorus, e.g. 

Miscellaneous Reactions.- The Schlosser-Wittig reaction of ylide (209) with aldehyde (208) and treatment of the intermediate 6-oxido ylide with perchloryl fluoride has been used to construct the 13-fluoro unit (210) in a total synthesis of (+)-13-fluoroprosta-... 

However, as illustrated in Scheme 2.112, the reaction of ylide 2-490 with 4-hexen-3-one (2-491) did not lead to the expected cycloheptene, but to the cyclopropane derivative 2-493 in 98% yield by a simple addition of the ylide to 2-491 to give 2-492... 

Synthesis of a pyrimidopyrrolopyrimidine central core of alkaloid variolin B has been achieved by the reaction of ylide 226 with isocyanates. The product 227 is formed in excellent yield (Equation 26) <2000TL4777, 2002TL1005, 2003JOC489>. 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

A rather complex mixture of products is obtained from the reaction of benzyl-idenetriphenylphosphorane and CS2.48 The major product from the reaction of diphenyl disulphide with methylenetriphenylphosphorane is tris(phenylthio)-methane (44) and only a trace of the insertion product bis(phenylthio)methane is isolated.49 Presumably the salt (45) is deprotonated before it can react with the phenylthioate anion (Scheme 12). a-Thiocarbonyl-stabilized ylides (46) are obtained from the reaction of ylides with S-alkyl thiolcarboxylates.50... 

Both the reaction of yhde 162f with methyl acrylate and the reaction of ylide 162c with methyl methacrylate produce, after hydrolysis, the same optically active tran5-2-methylcyclopropanecarboxylic acid with optical purities of 43.2 and 11.9%, respectively (295). 

The most important synthetic access to acceptor-substituted carbene complexes is the reaction of ylides with electrophilic, coordinatively unsaturated transition metal complexes (Figure 4.1 see also Section 3.1.3). 

In the past 10 years, studies on the reaction of ylides generated from metal carbene complexes have witnessed tremendous progress. In particular, synthetic applications of the three most common reactions of ylides, namely [2,3]-sigmatropic rearrangement, [l,2]-shift, and 1,3-dipole cycloaddition, have gained considerable success to demonstrate... 

Semi-empirical PM3 calculations" reveal that ylide 23 is a minimum on the potential energy surface and that both steps are exothermic. The enthalpy of the reaction of ylide formation in CH3CN was estimated to be —43 kcal/ mol and the enthalpy of reaction of the second step, 1,2-hydrogen shift, was calculated to be —12.5kcal/mol. 

Only a limited number of examples have been reported. The reactivity of sulfonium ylide 98a, prepared by the reaction of thiepine 96 and dimethyl diazomalonate (Section 13.03.6.1), was examined <20060BC2218>. The reactivity of the stabilized sulfonium ylide 98a was restricted to the highly reactive Michael acceptor, tetracya-noethylene 152 (the ylide failed to react with benzaldehyde or dicyanoethylene). Reaction of ylide 98a with tetracyanoethylene 152 led to the consumption of the ylide 98a (Equation 22). Thiepine 96 was produced in the reaction and the formation of cyclopropane 153 was suggested. 

A similar reaction of ylide 200 can also be carried out under thermal conditions or in the presence of catalytic amounts of Cu(acac)2 [143]. The carbenoid reactions of iodonium ylides can also be effectively catalyzed by rhodium(II) complexes [144, 145]. The product composition in the rhodium(II) catalyzed reactions of iodonium ylides was found to be identical to that of the corresponding diazo compounds, which indicates that the mechanism of both processes is similar and involves metallocarbenes as key intermediates as it has been unequivocally established for the diazo decomposition [144]. 

Semistabilized ylides generally react with aldehydes to form mixtures of cis- and trans-oxaphosphetanes before the decomposition to the alkene starts. Therefore, stereogenic reactions of ylides of this type usually give alkene mixtures regardless of whether the work is carried out salt-free or not. 

The former Chapter Reaction of Ylides with Saturated or a,P-Unsaturated Carhonyl Compounds got rid of its three-membered ring formations and the rest strictly remodelled to lumish the new Chapter 11 Reaction of Phosphorus- or SuHur-stabilized CNucleophiles with Carhonyl Compounds Addition-induced Condensations . 

A more conventional reaction of ylides from /i-dicarbonyl precursors with some aniline tosylates furnished jV-arylated j3-dicarbonyl compounds [40],... 

To prevent the base-catalyzed elimination of triphenylphosphine 63) 27 from the ester phosphonium salts, a frequently occurring competition reaction of ylide for-... 

Reaction of Ylides with Saturated or a,/3-Unsaturated Carbonyl Compounds... 

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