Syn eliminations

Burgess Reagent (inner salt) JOC, 1973, 38, 26 occurs vis a syn elimination... 

This reaction worked best with more highly substituted diols and give predominantly syn elimination. 

A highly diastereoselective alkcnylation of c/s-4-cyclopentene-l,3>diols has been achieved with 0-protected (Z)-l-iodo-l-octen-3-ols and palladium catalyst (S. Torii, 1989). The ( )-isomers yielded 1 1 mixtures of diastcrcomcric products. The (Z)-alkenylpalladium intermediate is thought to undergo sy/i-addition to the less crowded face of the prochiral cyclopentene followed by syn-elimination of a hydropalladium intermediate. 

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

The stereospecific synthesis of an A ring synthon of la-hydroxy vitamin D has been carried out. The ( )-allcene is cyclized to give the (E -c.xo-diene 155, and the (Z)-allcene affords the (Z)-e,xo-diene 156 stereospecifically[125,126]. These results can be understood by the cis addition and syn elimination mechanism. 

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

In most cases, E2 elimination proceeds via a transition state involving the anti arrangement. Nevertheless, syn elimination is possible, and, when special structural features retard anti elimination, syn elimination becomes the dominant mode. 

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination. The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process. ... 

The occurrence of syn elimination in 5-decyl systems has been demonstrated with the use of diastereomeric deuterium-labeled substrates. Stereospecifically labeled 5-substituted decane derivatives were prepared and subjected to appropriate elimination conditions. By comparison of the amount of deuterium in the E and Z isomers of the product, it was... 

Data obtained for three different leaving groups are presented in Table 6.7. The results show that syn elimination is extensive for quatemaiy ammonium salts. With better leaving groups, the extent of syn elimination is small in the polar solvent DMSO but quite significant in benzene. The factors which promote syn elimination will be discussed shortly. 

Table 6.7. Extent of Syn Elimination as a Function of the Leaving Group in the S-Decyl System"...

The factors that determine whether syn or anti elimination predominates are still subject to investigation. One factor that is believed to be important is whether the base is free or present in an ion pair. The evidence is that an ion pair promotes syn elimination of anionic leaving groups. This effect can be explained by proposing a transition state in which the anion functions as a base and the cation assists in the departure of the leaving group. 

Steric effects also pl y a significant role. With iV-(/ ,/ -disubstituted-ethyl)-AfJ rj r-trimefliylammonium ions, syn elimination is important when the P substituents are aryl or branched. As the p groups become less bulky, the amount of syn elimination diminishes. This effect is demonstrated by the data below. ... 

The dependence on steric bulk is attributed to the steric requirements imposed by the bulky trimefliylamine leaving group. In the transition state for anti elimination, steric repulsion is increased as R and increase in size. When the repulsion is sufficiently large, the transition state for syn elimination is preferred. 

The preferential syn elimination of hydrogen fluoride from trans-l-bromo-2-fluorocyclohexane to give 1 -bromocyclohexene is achieved only when a strong base such as sodamide is used [55, 56] Potassium rcrt-butoxide causes elimination of hydrogen bromide to form 3-fluorocyclohexene [56] (equation 27)... 

Assuming that the breaking CH and CBr bonds must lie roughly in the same plane, then the HCCBr torsion angle must either be close to 0° (syn elimination) or close to 180° anti elimination). 

Step through the sequence of structures depicting rotation about the carbon-carbon bond in the two dibromoethane isomers l,2-dibromo-l,2-diphenylethane A andfi). For each, plot energy (vertical axis) vs. BrCCBr torsion angle (horizontal axis), and identify all minimum-energy structures. Which of these are reactive conformers , that is, conformers which are set up for either syn or anti elimination of HBr Which are non-reactive conformers , that is, which do not meet the requirements for elimination Do the reactive conformers correspond only to syn elimination, only to anti elimination, or are both pathways represented Which alkene would these reactive conformers lead to Are your results consistent with the observation that each isomer of the starting material gives only one alkene Explain. 

An instructive example on how stereochemical features influence the stereochemical outcome of the elimination is the pyrolysis of xanthates from erythro-and t/zrco-l,2-diphenyl-l-propanol. The erythro-dlcohoX 8 is converted into fi-methylstilbene 9 only, and the threo-dlcohoX 10 is converted into the corresponding Z-isomer 11 only. These results support the assumption of a syn-elimination process through a cyclic transition state ... 

In step 7 of fatty-acid biosynthesis (Figure 29.5), dehydration of a /3-hydroxy thioester occurs to give fm/rs-crotonyl ACP. Is the dehydration a syn elimination or an anti elimination ... 

Syn stereochemistry (Section 7.5) The opposite of anti. A syn addition reaction is one in which the two ends of the double bond react from the same side. A syn elimination is one in which the two groups leave from the same side of the molecule. 

In an effort to make productive use of the undesired C-13 epimer, 100-/ , a process was developed to convert it into the desired isomer 100. To this end, reaction of the lactone enolate derived from 100-) with phenylselenenyl bromide produces an a-selenated lactone which can subsequently be converted to a,) -unsaturated lactone 148 through oxidative syn elimination (91 % overall yield). Interestingly, when 148 is treated sequentially with lithium bis(trimethylsilyl)amide and methanol, the double bond of the unsaturated lactone is shifted, the lactone ring is cleaved, and ) ,y-unsaturated methyl ester alcohol 149 is formed in 94% yield. In light of the constitution of compound 149, we were hopeful that a hydroxyl-directed hydrogenation52 of the trisubstituted double bond might proceed diastereoselectively in the desired direction In the event, however, hydrogenation of 149 in the presence of [Ir(COD)(py)P(Cy)3](PF6)53 produces an equimolar mixture of C-13 epimers in 80 % yield. Sequential methyl ester saponification and lactonization reactions then furnish a separable 1 1 mixture of lactones 100 and 100-) (72% overall yield from 149). 

The predominating onti-diastereomers of the primary adduct form an ( (-double bond on anti elimination and a (Z)-double bond on syn elimination, the latter proceeding frequently on warming of the reaction mixture to room temperature. 

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