Betaines epoxides from

It has now been shown that arch-betaines derived from RCHO and ArCHO form exclusively // arc.v-cpoxidcs without evidence of crossover in the presence of a more reactive aldehyde 82 in contrast, the frarc.v-cpoxidc formed exclusively by syrc-betaines derived from ArCHO are found to incorporate only a more reactive aldehyde if present. syrc-Bctaincs from RCHO form mixtures of cis- and trans-epoxides, with and without incorporation of a more reactive aldehyde if present. It has therefore been concluded that the high trans selectivity observed in epoxidation with aromatic aldehydes is a result of irreversible formation of arch-betaines and reversible formation of syn-betaine. The lower selectivity in the case of aliphatic aldehydes is a consequence of only partial reversibility in the formation of syn-betaine. 

Stereoselective epoxidation can be realized through either substrate-controlled (e.g. 35 —> 36) or reagent-controlled approaches. A classic example is the epoxidation of 4-t-butylcyclohexanone. When sulfonium ylide 2 was utilized, the more reactive ylide irreversibly attacked the carbonyl from the axial direction to offer predominantly epoxide 37. When the less reactive sulfoxonium ylide 1 was used, the nucleophilic addition to the carbonyl was reversible, giving rise to the thermodynamically more stable, equatorially coupled betaine, which subsequently eliminated to deliver epoxide 38. Thus, stereoselective epoxidation was achieved from different mechanistic pathways taken by different sulfur ylides. In another case, reaction of aldehyde 38 with sulfonium ylide 2 only gave moderate stereoselectivity (41 40 = 1.5/1), whereas employment of sulfoxonium ylide 1 led to a ratio of 41 40 = 13/1. The best stereoselectivity was accomplished using aminosulfoxonium ylide 25, leading to a ratio of 41 40 = 30/1. For ketone 42, a complete reversal of stereochemistry was observed when it was treated with sulfoxonium ylide 1 and sulfonium ylide 2, respectively. ... 

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). 

The final example is slightly different from the previous protocols in two ways. First, the IMP is generated by Sn2 displacement of a benzotriazole moiety. The IMP then opens an epoxide, which generates a betaine with an extra carbon atom between phosphorus and the negatively charged oxygen. Collapse of this betaine results in the formation of an aziridine as opposed to a carbon-nitrogen double bond. 

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. 

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... 

If the assumption that the severity of the steric interactions between the R and the Ph3P+-OC- substitutents impacts the extent of regioclosure is valid, then a substituent more sterically-demanding than methyl should exert a profound directive influence on the course of the regioclosure of betaines F and G to produce enantio-enriched epoxides. This expectation was realized from the results of the reaction between (S)-(+)-benzylethane-l,2-diol [(5)-3] and DTPP where epoxide 6 is obtained with 94%... 

In order to account for the origin of the enantioselectivity and diastereoselectivity of benzylidene transfer, it is necessary know whether the sulfur ylide reactions are under kinetic or thermodynamic control. From cross-over experiments it was found that the addition of benzylsulfonium ylide to aldehydes was remarkably finely balanced (Scheme 9) [28]. The trans-epoxide was derived directly from irreversible formation of the anti-betaine 4 and the cis-epoxide was derived from partial reversible formation of the syn-betaine 5. The higher transselectivity observed in reactions with aromatic aldehydes compared to aliphatic aldehydes was due to greater reversibility in the formation of the syn-betaine. 

As the trans epoxides are derived from irreversible formation of anti-hetaines, the observed enantioselectivity must arise from different activation energies associated with the possible transition states leading to betaine formation. The number of transition states which need to be considered can be reduced by gaining information on the structure of the yUde. 

Nucleophilic attack by the triphenylphosphine opens the epoxide, producing a betaine, 1. Proton abstraction from bisphenol A yields the phenoxide anion, 2. The phenoxide reacts with the electrophilic carbon attached to the positive phosphorus, 3, regenerating the catalyst. When the phenol is exhausted, the betaine can decompose into a terminal olefin and triphenylphosphine oxide (the final step in the Wittig... 

The role of the triphenylphosphine is similar in the rubber-modification reaction. In this case however the betaine abstracts a proton from the carboxylic acid. The carboxylate anion attacks the electrophilic carbon attached to the phosphorus forming an ester linkage and regenerating the catalyst. The rubber is terminated on both ends by epoxides and can later be crosslinked into the epoxy matrix during the coating and curing phases. This sequence is shown in Reaction Scheme 5. 

Transannular interaction between the 4-substituent and the intermediate betaine formed between 4-phenylsulphonylcyclohexanone and diazomethane in methanolic KOH is inferred from the greater relative proportion of epoxides accompanying ring-enlarged ketones in the reaction mixture the 4-phenylthio-analogue forms epoxides in only small amounts. ... 

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... 

---