Trans-elimination

The coupling of 1,8-diiodonaphthalene (25) with acenaphthylene (26) affords acenaphth[l,2-a]acenaphthylene (27). It should be noted that the reaction involves unusual trans elimination of H—Pd—1[32], This tetrasubstituted double bond in 11 reacts further with iodobenzene to give the [4, 3, 3]propellane 28 in 72%. This unusual reaction may be accelerated by strain activation, although it took 14 days[33]. 

No j3-bromo- or iodoalkyl complexes have yet been isolated. The reaction of vicinal dibromides or diiodides with [Co (CN)j] , [Co(CN)5H], or a Co(I)-DMG complex merely gives the olefin 32, 75,105,109,161), though kinetic evidence was obtained for the intermediate formation of the j8-bromo complex in the reaction of [Co"(CN)5] with a,j8-dibromopropionate and a,/3-dibromosuccinate (75). It is interesting that the pentacyanide produced is the bromo or iodo, and not the aquo, complex 32, 75), which suggests that the decomposition may involve a cis rather than a trans elimination of Co—X. The /3-chloroethyl complex can be prepared by tbe reaction of [Co(CN)5H]3- with CICH2CH2I 105). 

The second reaction is a stereospecifically trans elimination (E2). The marked proton in (25) cannot be lost as it is aia to the leaving group (I). Trans elimination (29) is possible only to give (26),... 

It was shown that the fragmentation of 143 occurs stereospecifically in the sense of a trans-elimination erythro-145 gives the olefin (E)-144 while threo-745 furnishes (Z)-14495K... 

Direct experimental evidence for the trans elimination in E2 reactions has been obtained from deuterated -2-bromobutane (the deuterium atom occupies position 3). This compound on dehydrobromination forms trans and cis butene-2 in the ratio of 6 1... 

The Peterson reaction utilizes a metal derivative of chloromethyltrimethylsilane and the yields are better than the Wittig reaction. The intermediate P-hydroxysilanes can undergo either cis or trans elimination depending on reagent choice. 

Much evidence supports the conclusion that the elimination of the group HX from alkyl halides by bases is a trans elimination reaction. This means that the atoms H and X leave from the opposite site of the incipient double bond. It is mostly explained by assuming that the electrons which are left by the leaving proton and which will form the double bond prefer to attack the leaving group X from the rear (50). The transition state for the elimination, if it is concerted, is most stable if H, X, and the carbon atoms 1 and 2 lie on one plane, which in most molecules is best realized in the trans position (51). ... 

In order to determine whether trans elimination may occur also in the removal of elements of water from alcohols, the dehydration of menthol... 

For the neomenthol to undergo trans elimination it would be necessary for the original chair conformation with the hydroxyl in the more stable equatorial position (55, 56) to flip to another chair conformation with hydroxyl in the axial position. 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

The kinetic data obtained by Kochloefl et al. (62) can be interpreted by a trans elimination reaction (Table I). 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

In order to explain the formation of nortricyclene from 2-ea o-norbornanol, it is necessary to assume a back side attack at the hydrogen attached to carbon 6. The general mechanism here is similar to the trans elimination reaction as discussed under menthol, 1,4-cyclohexanediol, and bornanols. 

The dehydration to 1-butene proceeds most probably via a trans elimination reaction. The formation of 2-butenes, which were the primary products of reaction, can best be explained by a removal of hydrogen from a y-carbon atom, as was indicated in the case of menthols ... 

The equal distribution of at C(l) and C(3) in a-methylstyrene is best explained by a trans elimination reaction of the elements of water from 2-phenyl-l-propanol-l-C, followed by a rapid equilibration of a-methylstyrene produced through the formation of the highly stabilized tertiary carbonium ion ... 

The elimination of the elements of water from 2-butanol can best be explained by trans elimination reaction involving the anchimeric assistance of hydrogen. The assistance of neighboring hydrogen in reactions in solution has been repeatedly observed 96, 97). The elimination reaction can be presented as follows ... 

There is a strong parallel between elimination reactions in solution and the dehydration of alcohols over alumina. The trans elimination reactions and the anchimeric assistance of alcohols over aluminas suggest that the dehydration must occur within either the submicroscopical pores, or crevices, or channels of the aluminas. The aluminas therefore must surround the alcohol molecules providing acid sites to act as proton donors or electron acceptors and basic sites to act as proton acceptors or electron donors. For that reason the aluminas seem to act as solvating agents and therefore may be considered as a pseudosolvents for dehydration reactions. 

The procedure below is the most convenient and quick way to prepare ethoxyethyne [24). Yields are usually excellent provided that the Z-isomer is the major component in the mixture of isomers of the bromovinyl ether. The E-isomer remains unchanged under the conditions of the elimination (rule of trans-elimination). The bromovinyl ether can be prepared in good yields starting from ethyl vinyl ether. 

The oxidation probably involves a mercurated complex through the electron pair on nitrogen followed by a concerted removal of a proton from the a-carbon and then cleavage of the mercury-nitrogen bond. The suggestion that a trans elimination occurs is reinforced by the evidence that yohimbine can be hydrogenated by mercuric acetate, whereas reserpine methyl reserpate, deserpine, or pseudoyohimbine does not react [122]. 

Figure 10. Possible trans elimination of acetyl groups...

Deoxygenation of epoxidesEpoxides are converted to alkenes with retention of configuration by treatment with this combination in hexane at 0 -> 25°. The reaction involves anti-opening of the epoxide to an iodohydrin, which undergoes trans-elimination to an alkcnc. 

The addition-elimination reaction of olefins presents a dichotomy of mechanistic results in that many of the reactions are consistent with either a cis addition, cis elimination or trans addition, trans elimination, and in others, either a cis addition followed by a trans elimination or vice versa. Furthermore, in some cases predominately Markovnikov addition is observed, and in others anti-Markovnikov is observed. 

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