Ylide, Selenium

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

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. 

A number of benzo- or dibenzo-fused seven membered phosphorus heterocyclic systems have also been studied. These include the benzo-fused oxa-bridged phosphaalkene 76 prepared by thermolysis of 2,3-diphenylindenone 23-epoxide (as a source of the carbonyl ylide 1,3-dipole intermediate) in the presence of /-butylphosphaalkyne. This bridged phosphaalkene is unusually stable even without inert gas blanketing . Reaction of 76 with sulfur or grey selenium stereoselectively affords the thia- or selenaphosphiranes 77 (X = S, Se respectively). <00T6259>... 

In all the above examples, the synthesis of nitronates (5) is rather chemose-lective. In any case, data on the formation of their structural isomers, viz, the corresponding nitrocyclopropanes (13), are lacking. However, the synthesis of five-membered nitronates (5) with the use of sulfur or selenium ylides is not chemoselective (see Scheme 3.16). 

Nitronates (5) can be synthesized by the Michael addition of in situ generated sulfur or selenium ylides (12) (58-60, 62, 63) to conjugated nitroalkenes... 

The selenium ylides (140) with dimethyl acetylenedicarboxylate afford moderate yields of tetrasubstituted furans (Scheme 31) (73KGS1447). 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

CuCl-catalyzed decomposition of iodonium ylides prepared from /3-keto esters and diacetoxyiodobenzene, has been developed (equation 151)331. 1-Methylbenzvalene is obtained in a good yield by treating a mixture of lithium cyclopentadienide and 1,1-dichloroethane with butyllithium332. The tandem cyclization substitution in l-selenyl-5-hexenyllithiums derived from corresponding selenacetals via selenium/lithium exchange produces bicyclo[3.1.0]hexane derivatives333. 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Phosphorus—selenium bonds, alkyne additions, 10, 782 Phosphorus—sulfur bonds, alkyne additions, 10, 781 Phosphorus ylides with gold(I), 2, 275 with trinuclear Os clusters, 6, 740 Phosphoylides, actinide complexes, 4, 203 Photoacoustic calorimetry, in thermochemistry, 1, 612-613 Photoactive molecular devices, and -[RefCO) ,] fragment, 5, 886... 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

The ability of carbanions to react with elemental selenium can be advantageously used for the synthesis of selenocarbonyl compounds. For example, sulfur ylides 201 (E = +SMe2) have been reacted with elemental selenium to generate the corresponding selenocarbonyl compounds 202 (Scheme 59).374,375 But Staudinger selenylation also has been applied to the synthesis of selenoketones 202 from phosphorus ylides 201 (E = +PPh3), which have been trapped by dienes in hetero-Diels-Alder reactions.376-383... 

Krief, A. In The Chemistry of Organic Selenium and Tellurium Compounds. Synthesis of Selenium and Tellurium Ylides and Carbanions Application to Organic Synthesis Patai, S., Ed. Wiley, 1987 Vol 2, chap 17, p 684. 

An unusual four-component reaction between a phosphorus ylide (2equiv), elemental selenium, and tetracya-noethylene (TCNE) produced a 3,4-dicyanoselenophene derivative <1995TL8813>. 

Sulfur ylides are most frequently employed in this methodology. The cyclopropanations of Michael acceptors with arsenic, selenium and tellurium ylides " have also been... 

Reaction of the dimethylselenium oxide adduct378 with benzoylfluoroacetone 379 gives the selenium ylide 380. ... 

Other heteroatom analogues such as oxonium, selenium and iodonium ylides also undergo [2,3] -sigmatropic rearrangements. 

Also, the colorless selena-A -phosphinidene 8b has been obtained by addition of selenium to a phosphoms ylide (Scheme 13) in THF at room temperature <2002JA9706, 2003BCJ471>. 

Krief A (1987) Synthesis of selenium and tellurium ylides and carbanions application... 

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