Phosphonium/sulfonium ylides

Sulfonium and oxosulfonium ylides or mixed phosphonium/sulfonium ylides are equally potent cr-donors for gold(i) and gold(m) centers (Scheme 32)<1 2 166 i68... 

Formulas of phosphonium/sulfonium ylide complexes should be written with onium charges at the P or S atoms and negative charges at the metal atom, but for the sake of convenience and clarity, these charges are omitted. 

A recent review on the uses of bromodimethylsulfonium bromide ([Mc2S Br]Br, BDMS) in organic synthesis opened a subject of BDMS application for the one-pot synthesis of a-haloacrylates from stabilized phosphorus ylides and aldehydes in high Z/E ratios, together with other reactions.The procedure is suggested to involve a rapid in situ formation of mixed phosphonium-sulfonium ylides followed by conversion into a-halo-phosphonium yUde, e.g. Ph3P=C(Br)COOR, and subsequent Wittig reaction. 

Arenediazonium salts also react with stabilized phosphonium, arsonium, pyridinium, and sulfonium ylides (12.111) in acetonitrile, yielding via the azo-onium salt (12.112) the azo-onium ylide (12.113, yellow to red), and in some cases the for-mazane (12.114) (Froyen and Juvvik, 1992). 

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

Functions [R2Au] are also present in the large family of gold(i) complexes of phosphonium, arsonium, and sulfonium ylides . Section 2.05.6 is dedicated to this class of complexes, where dinuclear compounds with gold in the oxidation state +2 are also common. 

Phosphonium ylides react with carbonyl compounds to give olefins whereas sulfonium ylides aflFord epoxides. In their behavior toward car-... 

The synthetic utility of a-phosphorus- and a-thio-stabilized carbanions is the subject of numerous reviews.21 Notable are additions of phosphonium ylides (237),183 sulfonium ylides (238),l84 ° oxosulfo-nium ylides (239)184 " and sulfoximine ylides (240)184,1 to electron-deficient alkenes which afford nucleophilic cyclopropanation products. In contrast, with a-(phenylthio)-stabilized carbanions, which are not acyl anion equivalents, either nucleophilic cyclopropanation or retention of the hetero substituent occurs, depending on the acceptor and reaction conditions used. For example, carbanion (241) adds to 1,1-... 

Fig. 9.1. Representative phosphonium, sulfonium, and sulfoxonium ylides— formation reactions and valence bond formulas.

When activated by anionic catalysts [potassium fluoride, cesium fluoride, tctraalkylammonium fluorides, tris(dimethylamino)sulfonium difluorotrimethylsilicate, phosphazenium, hexa-methylpiperidinium and cobaltoccnium fluorides, tetrabuiylammonium difluoro(triphenyl-silyl)silicatc. the complex tetrakis(dimethylamino)ethene/pcrfluoropropene, ammonium (and phosphonium) periluorocyclobuianc ylides], trimethyl(perfluoroalkyl)silanes will generate C-Rp bonds from carbon-halogen bonds. 

The formation of desaurins from ketones, carbon disulfide, and base 1275,1281,1282,1285-1290 believed to involve nucleophilic attack on a thioketene by the dianion of a 1,1-dimercaptoalkene, as shown for the synthesis of 572. Related syntheses involve the use of thiophosgene instead of carbon disulfide and the use of diazoalkanes or phosphonium and sulfonium ylides instead of a ketone and base. Treatment of perfluoroiso-butylene with fluoride ion and elemental sulfur in a dipolar, aprotic solvent ° °° or with sources of anionic sulfur (potassium sulfide, sodium hydrosulfide,potassium thiocyanate,sodium thiosulfate, dithiocarbamate salts, dithiophosphate salts ) give the dimer (573) of bis(trifIuoromethyl)-thioketene. Similarly, other 2,4-bis(methylene)-l,4-dithietanes are obtained by treating 2,2-dichlorovinyl ketones with anionic sulfur re-... 

Volatron, F., Eisenstein, O. Theoretical study of the reactivity of phosphonium and sulfonium ylides with carbonyl groups. J. Am. Chem. 

Unlike a phosphonium ylide, which reacts with an aldehyde or ketone to form an alkene, a sulfonium ylide reacts with an aldehyde or ketone to form an epoxide. Explain why one ylide forms an alkene, whereas the other forms an epoxide. 

Among the other developed approaches to novel P-ylides, the synthesis of a-sulfanyl-a-phosphonyl phosphonium ylides (58) in quantitative yields via the addition of two equivalents of trialkylphosphites to phosphonodithio-formates should be mentioned. The subsequent reaction of these ylides with alkyl or benzyl halides gives stabilized sulfonium ylides (59) while their heating (18-150 h, 110 °C) leads to a-sulfanyl methylene bis-phosphonates through protonation-dealkylation intramolecular reactions. The synthesis... 

Ylides are an important class of carbanion analogs, which we will encounter several times in this book. Generally, they are not anionic, but 1,2-dipolar compounds in which a carbanionic (or other anionic) center is stabilized by an adjacent cationic p-block center, where both centers have full octets of electrons. The best known ylides are phosphonium and sulfonium ylides, the following being prototypical examples ... 

The reaction is most often used for epoxide synthesis via methylene transfer. An important point concerns the difference in reactivity of sulfonium versus phosphonium ylides. The former gives three-membered rings the latter gives alkenes via the Wittig reaction. Thermodynamics is believed to account for a good deal of this difference the P+-0 bond in a phosphine oxide (BDE 544 kJ/mol) is much stronger than the S+-0 bond in DMSO (BDE for DMSO DMS + O 389 kJ/mol), which would form if the sulfonium ylide reaction resulted in an alkene. 

CH acidities of sulfonium and phosphonium salts are in the range accessible for the PTC so they can be deprotonated in liquid-liquid and liquid-solid two-phase systems. Since in the generated ylides the charges are internally neutralized the PT catalysts suppl3dng lipophilic cations are not necessary for transfer of the reacting species into the organic phase. Nevertheless in many cases of the reactions of sulfonium and phosphonium ylides, PT catalyst was used and recommended. Thus in two-phase systems with or without catalyst, an efficient synthesis of oxiranes via reactions of sulfonium ylides with aldehydes is executed (eqs. 66 and 67). 

When compared with phosphonium and sulfonium ylides, the corresponding selenonium and telluronium ylides frequently exhibit better reactivity and nucle-ophilicity, while the use of selenides and tellurides in catalytic ylide reactions is far less reported. In 2001, Metzner and coworkers reported an asymmetric epoxi-dation reaction of aldehydes using a C2-symmetric selenide 42 resembling their sulfide catalyst (Scheme 20.32). High yields (65-97%) and enantioselecti vity (76-94% ee) were obtained with a range of aromatic aldehydes and dnnamalde-hydes. However, no diastereoselectivity (trans cis = 1 1) was observed in these reactions, while the corresponding sulfide gave around 80% diastereoselectivities, which could be rationalized as the formation of a less diastereoselective early transition state with the more reactive selenonium ylides. The reactions of more electron-deficient aryl aldehydes (p-Cl and p-CFs) were less enantioselective (76% ee and 83% ee, respectively) [58]. 

Yates, B.F. Bouma, W.J. Radom, L. Detection of the Prototype Phosphonium (CH2PH3), Sulfonium (CH2SH2), and Chloronium (CH2CIH) Ylides by Neutralization-Reionization Mass Spectrometry a Theoretical Prediction. J. Am. Chem. Soc. 1984,106, 5805-5808. 

Highly Lewis basic and nucleophilic functional groups are not compatible with zinc carbenoids. The methylation or ylide formation of heteroatoms is one of the most important side reactions of these reagents. For example, amines, thioethers and phosphines readily react with the zinc reagents to generate ammonium salts", sulfonium" and phosphonium ylides" ". Terminal alkynes generally lead to a large number of by-products". ... 

With reference back to Scheme 3, the substituent G in these reactions is most commonly an onium group bearing a positive charge. Examples are sulfonium and phosphonium groups. The intermediates (7) are thus various types of ylides, and the adducts (8) are defined as being betaines. 

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