Ylide intermediate

NH2)50s(2,3-T -L)], where L = furan, pyrrole, and thiophene. Although neither the furan nor thiophene complexes react with maleic anhydride over a period of 10 days, the pyrrole complex (15) reacts rapidly at room temperature and 101.3 kPa to form a mixture of endo (17) and exo (16) complexes. An a2omethine ylide intermediate was postulated as the key intermediate through which maleic anhydride added to the 2- and 5-positions of the coordinated pyrrole ring. 

Deacylations are known. C-Acyl groups in 1,3,4-thiadiazoles are cleaved by sodium ethoxide in ethanol (68AHC(9)165). Imidazole-2-carbaldehyde behaves similarly, yielding imidazole and ethyl formate this reaction involves an ylide intermediate. 3-Acylisoxazoles (405) are attacked by nucleophiles in a reaction which involves ring opening (79AHC(25)147). 

The evidence obtained clearly indicates that the above photorearrangements proceed by a mechanism involving a nitrile ylide intermediate since cycloadducts could be isolated when the irradiations were carried out in the presence of trapping agents. Intramolecular cycloaddition of the nitrile ylide followed by a 1,3-sigmatropic hydrogen shift of the initially formed five-membered ring readily accounts for the formation of the final product. 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

Figure 4.88 Flow patterns, visualized by formation of the colored ylide intermediate, allowing one to judge on mixing efficiency [13],...

Compounds in which a carbonyl or other nucleophilic functional group is close to a carbenoid carbon can react to give ylide intermediate.221 One example is the formation of carbonyl ylides that go on to react by 1,3-dipolar addition. Both intramolecular and intermolecular cycloadditions have been observed. 

Following the failure of attempts to synthesize the appropriate phosphonium ylide intermediates,... 

A mechanistic interpretation for the formation of 35 is depicted in Scheme 5. Deprotonation of an allylic proton yields ylide intermediate 36. This then adds to methyl acrylate to give intermediate 37, which cyclizes to construct a cyclopropane ring together with the fission of the S-C bond to afford the final adduct 35. 

Treatment of the cycloadduct 83a with sodium methoxide afforded a pyrrole derivative 86. It was proposed that the key step involved an amino group migration to the ylide carbon of amino sulfonium ylide intermediate 84 as depicted in Scheme 11 <1999TL95>. 

The dominant role of the traditional copper catalysts, generally used under heterogeneous conditions, has not been challenged as yet. Only a few reports shed light on the efficiency of alternative catalysts. Copper(II) triflate allows high-yield intramolecular cyclopropanation of y,8-unsaturated diazoketone 182160) it is superior to CuS04 (53 % yield 192 ) or Rh2(OAc)4160). The solvent is crucial for an efficient conversion If the reaction is carried out in ether, the solvent competes with the double bond for the electrophilic metal carbene to give 184, presumably via an oxonium ylide intermediate. 

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

The homologation of selenoesters 379 with diazomethane in the presence of Cu or Cul to give a-selenoketones is thought not to involve a carbenoid pathway and an Se-ylide intermediate but rather a tetrahedral species resulting from nucleophilic attack of CH2N2 at the carbonyl carbon atom. The role of the catalyst is seen in facilitating nucleophilic attack at C=0 by complexation at the selenium atom. 

Hynninen and coworkers <99JCS(PT1)2403> used a similar approach to prepare phytochlorin-C6o diad 38 (Scheme 11). The protocol employed the pyrolysis of the natural chlorophyll a molecule 35, followed by transesterification and demetallation to furnish derivative 36. Subsequent oxidation of 36 with OsCU and NaI04 has allowed the synthesis of the formyl derivative 37, which was further used as precursor of the azomethinic ylide intermediate in the 1,3-DC reaction with Cm leading to the formation of diad 38. Photochemical studies revealed that this diad underwent a fast intramolecular photoinduced electron transfer in polar solvents such a benzonitrile <99JACS9378>. 

The aza-[2,3] Wittig rearrangement of aziridines is an excellent method for the synthesis of substituted piperidines. The analogous reaction of an epoxide has recently been examined <06TL7281>. Reaction of divinyl epoxide 48 with /-butyl diazo acetate provides the ylide intermediate 49, which then undergoes the [2,3] Wittig rearrangement to 50, Several catalysts were examined as catalysts for the formation of 49. It is noteworthy that the copper catalyst performed much better than the more widely used rhodium catalysts. 

Reduced fused azepines (e.g. 40) have been used in a new ring expansion strategy to afford fused hexahydroazoninoindoles (e.g. 41) from reaction with methyl propiolate in methanol to give the ylide intermediate A which then ring expanded via the methanol stabilised intermediate B to give 41 <06T1239>. 

Microwave irradiation in solvent-free conditions induces the cleavage of the 2,3-bond of 2-aroyl-aziridines 135 to give an azomethine ylide intermediate, which subsequently undergoes cycloadditions to a multiple bond and leads to oxazolidine, imidazoline, naphthooxazole and pyrroline derivatives 136 in good yields (Scheme 9.41) [32b], Reactions were performed at atmospheric pressure in an Erlenmeyer flask placed in a commercial domestic oven. The reactions were complete in 10-15 min while the conventional method requires 18-20 h. 

Isatin derivatives and proline react to give stereospecific formation of an azo-methine ylide intermediate via the decarboxylation route. The resulting 1,3-dipole undergoes a cycloaddition reaction with N-substituted maleimide (6), as a dipolaro-... 

It is assumed that the mechanism proceeds via activation of the imine by the ruthenium catalyst (structure 169), followed by reaction with ethyl diazoacetate to generate a metal-bound ylide intermediate. Intramolecular ruthenium- assisted attack of the carbanion 170 onto the iminium ion provides the corresponding aziridine with moderate to high // selectivity. Imines bearing electron-donating groups (R2) showed significant rate enhancement. 

The yields of CO production in the reaction of BrFC with substituted benzaldehy-des are dependent on the electronic effects of para substituents. Carbonyl ylide intermediates were trapped with dimethyl acetylenedicarboxylate. Sterically hindered tetraphenylcyclopentadienone gave a high yield of CO and no ylide could be trapped in this case. 

Wittig rearrangement of benzylic quaternary ammonium salts upon treatment with alkali metal amides via the ammonium ylide intermediates. 

It has been postulated that in the hydrazine solution (pH 8) the N-amino-4,6-dimethylpyrimidinium ion is partly deprotonated into its conjugate base, the resonance-stabilized ylide 31, so that in fact in this solution two equilibrated active species, 30 and 31, are present (62TL387 63AG604).The formation of an ylide intermediate was experimentally supported by the isolation of the dimer 2,4,7,9-tetramethyldipyrimido[l,2-fe r,2 -c]hexahydrotetrazine (32), which is formed when the pyrimidinium salt 30 is treated with liquid ammonia (Scheme III. 19). 

The rate-pD profile for the 2-position could be accounted for by parallel rate-determining proton abstraction from the conjugate acid of imidazole by OD and by DgO leading to an ylide intermediate (Scheme 2). These pathways together with OD -catalyzed proton abstraction from the neutral imidazole molecule accounted for the 4(5)-... 

Rates of detritiation of [2- H]-benzimidazole (10) and the corresponding 1-methyl compound have been measured over a pH range at 85° and the bell-shaped pH-rate profile (Fig. 1) accounted for in terms of rate-determining attack by OH on the benzimidazole cation with the formation of an ylide intermediate. Because of the ionization of the N-H group at high pH, the development of negative charge a to the... 

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