Anti eliminations

Axial halide is in proper orientation for anti elimination with respect to axial hydrogens on adjacent carbon atoms Dehydrobromination is rapid... 

Equatorial halide is gauche to axial and equatorial hydrogens on adjacent carbon cannot undergo anti elimination in this conformation... 

The first example of a stereo electronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5 16)... 

Quaternary ammonium hydroxides un dergo elimination on being heated It is an anti elimination of the E2 type The regioselectivity of the Hofmann elimina tion IS opposite to that of the Zaitsev rule and leads to the less highly substi tuted alkene... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

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. 

Cyclohexyl systems have a very strong preference for anti elimination via conforma-... 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

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 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. 

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. 

These reactions proceed by alkoxide or fluoride attack at silicon which results in C—Si bond cleavage and elimination of the leaving group from the fi carbon. These reactions are stereospecific anti eliminations. 

S-Trimethylsilylalkyl trifluoroacetates also undergo fricile anti elimination. ... 

Stereospecific reaction (Section 7.13) Reaction in which stereoisomeric starting materials give stereoisomeric products. Terms such as syn addition, anti elimination, and inversion of eonfiguration describe stereospecific reactions. 

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. 

Examine all of the low-energy (within. 004 au or 3 kcal/ mol of the lowest-energy conformer) conformers of cis-2-methylcyclohexyl tosylate. Identify every conformer that can undergo anti elimination of OTs and H+, and predict the alkene that will be produced. What alkenes will be obtained from the cis tosylate ... 

Another interesting question concerns the rate at which each tosylate undergoes elimination. A tosylate sample contains molecules with several different conformations. The size of each conformer population depends on conformer energy, and the more reactive tosylate will probably be the one with the largest population of reactive conformers, i.e., molecules whose geometries allow anti elimination. Which tosylate, cis or trans, will have a larger population of reactive conformers Explain how you reached this conclusion. 

Elucidating the stereochemistry of reaction at prochirality centers is a powerful method for studying detailed mechanisms in biochemical reactions. As just one example, the conversion of citrate to (ds)-aconitate in the citric acid cycle has been shown to occur with loss of a pro-R hydrogen, implying that the reaction takes place by an anti elimination mechanism. That is, the OH and H groups leave from opposite sides of the molecule. 

CHAPTER 11 Reactions of Alkyl Halides Nucleophilic Substitutions anti Eliminations... 

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 ... 

Anti stereochemistry (Section 7.2) The opposite of syn. An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. 

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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