Epoxidations betaine intermediate

The proposed betaine intermediates can be formed, in a completely different manner, by nucleophilic substitution by a phosphine on an epoxide (10-50) ... 

Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature. A sharp distinction between the two ylides emerges in their reactions with a, ( -unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition and gives cyclopropanes (compare Entries 5 and 6 in Scheme 2.21). It appears that the reason for the difference lies in the relative rates of the two reactions available to the betaine intermediate (a) reversal to starting materials, or (b) intramolecular nucleophilic displacement.284 Presumably both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place. 

Betaine intermediates result from the addition of pyridines to such systems as a,(3-unsaturated esters, quinones and epoxides. The use of acetylenic esters in such reactions has been reviewed (63AHC(l)i43). 

Stoichiometric sulfur ylide epoxidation was first reported by A.W. Johnson [23] in 1958, and subsequently the method of Corey and Chaykovsky has found widespread use [24-26]. The first enantioselective epoxidations using stoichiometric amounts of ylide were reported in 1968 [27, 28]. In another early example, Hiyama et al. used a chiral phase-transfer catalyst (20 mol%) and stoichiometric amounts of Corey s ylide to effect asymmetric epoxidation of benzaldehyde in moderate to good enantiomeric excess (ee) of 67 to 89% [29]. Here, we will focus on epoxidations using catalytic amounts of ylide [30-32]. A general mechanism for sulfur ylide epoxidation is shown in Scheme 10.2, whereby an attack by the ylide on a carbonyl group yields a betaine intermediate which collapses to yield... 

The reaction of sulfur ylides with aldehydes and ketones was first reported by Johnson in 1961, but is for some reason better known as the Corey-Chaykovsky reaction. The reaction between sulfur ylides and an electrophilic carbon atom of a C=0 gives a betaine intermediate. The favoured reaction path is therefore an internal Sn2 process which furnishes an epoxide with regeneration of the sulfide (Scheme 3.28). 

In 1955 Wittig et al. found that triphenylphosphine could induce the deoxygenation of epoxides at 200 Mechanistically, this process probably involves anti opening of the epoxide followed by syn elimination of triphenylphosphine oxide from a betaine intermediate. Accordingly, the reaction proceeds with inversion of stereochemistry, which means that franj-ejjoxides give ci5-alkenes. The systems of bis(di-methylamino)phosphorous acid/butyllithium, o and lithium diphenylphosphide have been examined... 

Reaction of the sulfur ylide with the carbonyl group of aldehydes, ketones, or enones forms a betaine intermediate, which decomposes by intramolecular displacement of Me2S by the oxyanion to yield the corresponding epoxide. 

It was found, as reported, that the reaction of 2-iodo ethyltriphenylphosphonium iodide 20 with 18 afforded epoxide 25 as a mixture of isomers in addition to the desired 4 in a 1 1 ratio. Alternative approaches were investigated in an attempt to minimize this major byproduct, but they were unsuccessful. For example, employing a method described by Shen (where the initially formed betaine intermediate was deprotonated with a second equivalent of base and then iodinated) produced des-iodo olefin 23. Utilizing Hanessian s phosphonates in this process also resulted in only des-iodo olefin 23. 

Diels-Alder reactions, 133, 135 epoxidation, 69-72, 516 grafting on polyethylene, 462 hydroformylation, 44 hydrogenation, 41, 42 isomerization catalysts, 133, 484 isomerization during polymerizations, 484 isomerization kinetics, 484 isopropyl alcohol radical reaction, 207 MA copolymerization, 532, 534, 541 Michael reactions, 63-66 nitrone adducts, 224, 225 olefin copolymerization, 288 olefin ene reactions, 162 phenanthrene adducts, 181 plasticizers use, 14 production—synthesis, 14, 78-81 radical copolymerization, 270, 275-277, 307, 315, 317, 333, 345, 365, 379 radical polymerization, 239, 264, 287 reaction with allyl alcohol, 46 reaction with sodium bisulfite, 53 styrene copolymerization, 365, 483 tetraalkyl methylenediphosphonate adduct, 66 transesterification, 46 /7-xylylene copolymerization, 359 dialkyl stannyl, PVC stabilizer, 275 diaryl, synthesis from MA, 80 pyridinium, betaine intermediate, 216... 

Polyfluoroalkyl- andperfluoroalkyl-substituted CO and CN multiple bonds as dipolarophiles. Dmzo alkanes are well known to react with carbonyl compounds, usually under very mild conditions, to give oxiranes and ketones The reaction has been interpreted as a nucleophilic attack of the diazo alkane on the carbonyl group to yield diazonium betaines or 1,2,3 oxadiazol 2 ines as reaction intermediates, which generally are too unstable to be isolated Aromatic diazo compounds react readily with partially fluorinated and perfluorinated ketones to give l,3,4-oxadiazol-3-ines m high yield At 25 °C and above, the aryloxa-diazolines lose nitrogen to give epoxides [111]... 

It was amply emphasized above that the formation of homologous ketones is the principal competing reaction in this condensa-tJoor4M,e To explain this observation it has become customary to invoke a transient intermediate zwitterion, termed by Arndt and Eistert 4 5 a diazomum betaine which can collapse into an epoxide with attendant nitrogen expulsion, or can undergo rearrangement to (me or two possible oarbonyl compounds. The process may be represented schematically as shown in Eq. (284). 

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. 

A two-step mechanism (Scheme 3.34) for epoxidation was proposed in which intermediate betaine A and B are obtained from the carbonyl compound and sulfonium ylides irreversibly and from aminosulfoxonium ylide reversibly (step 1). Betaine (A or B) then undergoes ring closure (step 2) irreversibly. 

Sulphonium ylids also react as nucleophiles towards carbonyl groups but the intermediate sulphonium betaines decompose to give oxiranes (epoxides) fioo] through an internal... 

The use of sulfur in place of the phosphorus brings about a different mode of decomposition of the intermediate betaine. Two sulfur ylides, dimethylsulfonium methylide (Scheme 3.49a) and dimethylsulfoxonium methylide (Scheme 3.49b), have been used. Both ylides react with ketones to give epoxides, but the stereochemistry may differ. 

The betaine 73 can sometimes be isolated. As shown in 16-46, intermediate 73 can also go to the epoxide. The evidence for this mechanism is summarized in the review by Gutsche. ° Note that this mechanism is essentially the same as in the apparent insertions of oxygen (18-19) and nitrogen (18-16) into ketones. 

Formation of an epoxide must, however, involve an intermediate betaine which reacts further by intramolecular displacement of an arsine. 

A mechanism has been proposed for the formation of epoxide 25 in which intermediate betaine A plays an important role. This ring closes to the corresponding epoxyphosphonium salt B with the elimination of iodine (Figure 14). Phosphonium salt B can then eliminate triphenylphosphine oxide after an aqueous or methanolic work-up. 

Aldehydes and ketones react with diazomethane with evolution of nitrogen. Diazonium betaines are assumed to be intermediates, and in them electron-attracting groups R such as trichloromethyl or / -nitrophenyl favor intramolecular substitution which affords epoxides in other cases, however, the removal of nitrogen is coupled with 1,2-shifts of hydrogen or alkyl or aryl groups ... 

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