Trans-stereospecificity

Olefins also undergo the Ritter reaction with nitriles in the presence of diphenyl diselenide, ammonium persulfate, and trifluoromethanesulfonic acid to produce oxazolines.When cyanamide is used, 2-aminooxazolines are obtained. The active electrophilic agent is phenylselenyl sulfate formed by oxidation of diphenylselenide with ammonium persulfate. The reaction is trans-stereospecific. 

In systems without steric constraint, trans stereospecificity is observed. This aspect has been aptly demonstrated in the addition of iodine azide to 1-methylcyclobutene.10... 

The work of Lown and coworkers122 on iodonium nitrates has been reviewed.1 Addition of these reagents proceeds in a trans stereospecific manner, but with mediocre regiospecificity. The Lown group has also studied BrNC>3 and found it to be a more regiospecific reagent.123... 

A detailed study of the reaction of (-butyl mercaptide anion with K-region epoxides, derived from carcinogenic and related polycyclic hydrocarbons, was carried out in aqueous dioxane and in THF.147 In the former solvent system, 1,2-trans stereospecific addition took place, whereas addition-dehydration was observed in THF. 

The addition of sulphenyl halides is in each case trans stereospecific... 

Use of resolved PhCHMeNHMe as conq>lexing agent and nucleophile in this reaction causes asymmetric induction in the amino acetate, with de 20-60% alternatively, resolved PhCHMeNMe2 can be used as complexing agent, followed by an achiral amine as nucleophile, but ee of only 3-12% was achieved.Related transformations, but this time trans stereospecific, can be achieved using mercury activation in these cases the mercury itself is the oxidant (Scheme 55). 

Oxytelluration of olefins with PhTeBrs or Ph2Te2-Br2 in aqueous THF generates (P-hydroxyalkyl)aryltelluiium dihalides. The reaction is tran -stereospecific in the case of cw-2-butene and cis- and trans-4-octenes and regiospecific in the cases of all terminal olefins. Their reactions with reducing agents in aqueous sodium hydroxide generate the Markovnikov hydration products. 

Dialkyl(alkylthio)sulfonium salts add across carbon-carbon double and triple bonds in a trans-stereospecific manner (37). Episulfonium salts are conveniently prepared by treatment of olefins with methyl(bismethylthio)sulfonium... 

More recently, these concepts have been extended to include a range of elegant amidoselenation processes. Alkenes react with benzeneselenenyl chloride and nitriles to yield -amido phenyl selenides in excellent yield.Oxidative elimination of the latter provides the corresponding ally lie amides (Scheme 60). Since the reaction proceeds via an episelenonium intermediate, the amido selenides are formed with trans stereospecificity. However, when applied to nonsymmetrical alkenes (equation 50) it only proceeds with regioselectivity. 

Therefore, it was proposed that there were two distinct mechanisms One involves a higher-energy bromonium ion that is trans stereospecific. The other is similar to that proposed for bromination of maleic acid in acetic acid, which involves a 1,4 addition shown below, rather than a 1,2 addition. 

Recall from Section 7 13 that a stereospecific reaction is one in which each stereoiso mer of a particular starting material yields a different stereoisomeric form of the reaction product In the ex amples shown the product from Diels-Alder cycloaddi tion of 1 3 butadiene to as cinnamic acid is a stereo isomer of the product from trans cinnamic acid Each product although chiral is formed as a racemic mixture... 

The Diels-Alder reaction is stereospecific Substituents that are cis in the dienophile remain cis in the product substituents that are trans in the dienophile remain trans m the product... 

The reaction has been extended to include carbanions generated from phosphonates. This is often referred to as the Horner-Wittig or Homer-Emmons reaction. The Horner-Emmons reaction has a number of advantages over the conventional Wittig reaction. It occurs with a wider variety of aldehydes and ketones under relatively mild conditions as a result of the higher nucleophilicity of the phosphonate carbanions. The separation of the olefinic product is easier due to the aqueous solubility of the phosphate by-product, and the phosphonates are readily available from the Arbusov reaction. Furthermore, although the reaction itself is not stereospecific, the majority favor the formation of the trans olefin and many produce the trans isomer as the sole product. 

Thermal decomposition of unsubstituted 3,4,5,6-tetrahydropyridazine at 439 °C in the gas phase proceeds 55% via tetramethylene and 45% via a stereospecific alkene forming pathway. The thermal decomposition of labelled c/s-3,4,5,6-tetrahydropyridazine-3,4- f2 affords cfs-ethylene-l,2- f2, trans-ethylene-l,2-if2, c/s-cyclobutane-l,2- f2 and trans-cyclo-butane-1,2- /2 (Scheme 57) (79JA3663, 80JA3863). 

The methiodide of 2,5-dihydrothiophene (239) is transformed in high yield to Z)-l-(methylthio)buta-l,3-diene (240) on treatment with alkali (81AJC1017). The thermal cheletropic extrusion of sulfur dioxide from both cis and trans isomers of 2,5-dihy-drothiophene 1,1-dioxides is highly stereospecific. For example, c/5-2,5-dimethyl-2,5-dihydrothiophene 1,1-dioxide (241) yields ( , )-hexa-2,4-diene (242) and sulfur dioxide (75JA3666, 75JA3673). 

N- Aminoaziridines have been converted to alkenes by reaction with a variety of oxidizing agents (70JA1784). Usually, the deamination reaction is stereospecific. The oxidation of l-amino-2,3-diphenylaziridines with manganese dioxide, however, was not stereospecific. The trans compound gives entirely frans-stilbene, whereas the cfs-aziridine forms a mixture of 85% trans- and 15% c -aikene. cw-Stilbene is not isomerized to trans under the reaction conditions, and the results are explained in terms of an azamine intermediate which can isomerize through a tautomeric equilibrium. 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

When ethyl trifluoroacetylacetate is treated with an allylic alkoxide, tran-sesterification is followed by ester enolate Claisen rearrangement m a process that on decarboxylation yields stereospecifically the tnfluoromethyl ketone product [22] (equation 19)... 

Stannanes also add across double bonds offiuonnated olefins in a free radical reaction Trimethylstannane undergoes stereospecific addition to hexafluorocyclo-butene to afford trans 1,2,3,3,4,4 hexafluoro 1 (trimethylstannyl)cyclobutane [5] (equation 1)... 

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