Ylide compounds generation

The sulfur ylide-mediated epoxidation of aldehydes has been thoroughly investigated [70, 71]. The chiral sulfur ylides reported by Aggarwal have been most broadly applicable, and a catalytic, asymmetric process yielding aromatic transepoxides has been developed [72]. In this process, the sulfur ylides are produced in situ from diazo compounds, generated in turn from tosylhydrazone salts (Scheme 9.15) [73],... 

Structure.—N.m.r. evidence has been presented which shows that the compounds generated by the reaction of butyl-lithium with methylphosphonium salts are not methylenephosphoranes, but lithium adducts of the ylides,14 and that they are planar18 (see Chapter 12). A carbonyl group separated by a benzene ring as in (12) has a weak stabilizing effect.16... 

Typical Procedure for Sulfur Ylide Epoxidation via In Situ Diazo Compound Generation [52] (p. 361)... 

Probably the most useful modification for the deprotonation route will be the condensation of N-substituted a-amino esters or derivatives (EWG = COOR, CN, etc.) with carbonyl compounds [Eq. (11)]. The intermediate iminium salts bear a highly basic hydroxide ion as a counteranion, which deprotonates the a. hydrogen immediately after its formation. The EWG-stabilized azomethine ylides thus generated will be smoothly trapped by the added dipolarophiles since they find no other reactive reagents in the reaction mixture. 

Use of N-substituted 2-aminonitriles in the condensation with carbonyl compounds generates cyano-stabilized azomethine ylides (87BCJ4067). The reaction of benzaldehyde with (methylamino)acetonitrile leading to ylide 84 is an example. 

Initial studies of solvent effects, on the reactions of triarylarsonium benzoylylides with p-nitrobenzaldehyde in N, A-dimethylformamide, dimethyl sulphoxide or methanol, indicated little solvent effect in these cases" ", but later studies of the more finely balanced reactions of semi-stabilized ylides have provided examples of strong influences due to the effect of different base and solvent when the ylide is generated in the presence of a carbonyl compound ". Thus, when benzyltriphenylarsonium bromide or p-chloroben-zyltriphenylarsonium bromide were treated with sodium hydride in benzene in the presence of a variety of p-substituted benzaldehydes the products were alkenes, but if sodium ethoxide in ethanol was used the isolated products were epoxides ". Likewise, when triphenylarsonium benzylylide was generated by phenyllithium in the presence of either benzaldehyde or acetaldehyde, the preponderant product was the epoxide whereas use of sodium amide as base provided mostly the alkene . Similar results were obtained when an allyltriphenylarsonium salt was deprotonated using different hexamethyldisilaz-... 

Very often phosphonium ylides are generated with organolithium compounds (in particular phenyl-, methyl-, /j-butyl- and /-butyl-lithium) as bases.- However difficulties may be attached to this method in some cases. When alkyllithium compounds are used, ligand exchange at phosphorus may occur, thus giving rise to the alternative or additional formation of a second ylide. To avoid this phenomenon in the case of triphenylphosphonium salts phenyllithium has to be used as base. Ligand exchange may also be suppressed if one uses, instead of /j-butyllithium for example, the more bulky tertiary... 

Allyl sulfides and allyl amines. Rhodium-catalyzed decomposition of ethyl diazoacetate in the presence of these allyl compounds generates products 136 and 137, respectively, derived from [2,3] rearrangement of an S- or N-ylide intermediate, besides small amounts of carbene dimers No cyclopropane and no product resulting from the ylide by [1,2] rearrangement were detected. Besides RhjfOAc) and Rhg(CO)i6, the rhodium(I) catalysts [(cod)RhCl]2 and [(CO)2RhCl]2 were found to behave similarly, but yields with the only allyl amine tested, CH =CH—CH NMe, were distinctly lower with the latter two catalysts. Reaction temperatures are higher than usually needed in rhodium-promoted diazoalkane decomposition, which is certainly due to competition between the diazo compound and the allylic hetero-... 

Arsonium ylide 27, generated under phase transfer conditions, smoothly adds to chalcones providing (trimethylsilylethynyl)cyclopropanes in excellent yields16. These compounds are obtained exclusively with the 1,3-rfs stereochemistry. Desilylation employing methanol provides the terminal cyclopropylethynes. 

The remote group of epoxyenone (184) functions as a neat intramolecular trap for the carbonyl ylide (185) generated in the 1ir,ir -excitation of this compound the product is the enol ether (186) . ... 

Muthusamy and co-workers have demonstrated [82] the reactions of the bicyclic ylide 57, generated from the diazocarbonyl compoimd 56, with symmetrical and unsymmetrical dipolarophiles. Thus, exposure of the cyclohexanone-substituted a-diazocarbonyl compound 56 to DMAD in the presence of Rh2(OAc)4 as the catalyst has furnished the cycloadduct 58 (Scheme 16). This cycloaddition was diastereoselective and, in the case of unsymmetrical dipolarophiles such as methyl methacrylate and propargyl bromide, they were regioselective and afforded oxygen heterocycles 59 and 60, respectively. The same research group has reported the 1,3-dipolar cycloaddition of the bicyclic carbonyl yUde 57 with other dipolarophiles, namely fulvenes [83]. In these tandem cycUzation-cycloaddition reactions involving fulvenes, four stereocenters and two new C-C bonds are formed in a single step. Symmetrical dipolarophiles such as macrocycHc olefins were also used for diastereoselective 1,3-dipolar cycloaddition reaction with 56 [84]. 

Rhodium-Catalyzed Reactions of Oxonium YUde Intermediates Hu et al. [158] reported the preparation of a,p-dihydroxyl and a-aUcoxy-p-aUcylamino acid derivatives through a rhodium-catalyzed three-component reaction starting from a diazo compound 108, an alcohol 109, and different electrophiles such as aldehydes 110 or imines 111. The reaction proceeds through formation of a Rh-oxonium ylide species, generated from the diazo compound 108 and the alcohol 109, which is subsequently trapped by the electrophile. Employing different Rh(II) catalysts, erythro/threo mixtures of a,p-dihydroxyl and a-alkoxy-p-alkylamino esters... 

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. 

With HMPA, Wittig reactions that give ( )-alkenes were also observed (eq 18), as was the directed selectivity of a semista-bilized arsonium ylide towards carbonyl compounds. The arsenic ylide was generated from LDA in THF or THF/HMPA solution to give exclusively epoxide (eq 19) or diene (eq 20), respectively. ... 

Scheme 7.43 Catalytic epoxide (X=0), aziridine (X=AZ-Ts or AZ-SES), and cyclopropane (X=CH-COY) formation mediated by chalcogen-ylides generated from metal carbenes using diazo compounds generated in situ from tosylhydrazones (start at lower right)...

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. 

---