Conformational Control of E2 Elimination

Base-promoted E2 elimination involves simultaneous loss of and X from neighboring carbons. Applying this rule to 2-methylcyclohexyl tosylate suggests that two different products might form, but the actual situation is more complicated. One tosylate isomer gives only one of the two possible alkenes, while the other gives both. 

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  

Analyze the low-energy conformers of trans-2-methylcyclohexyl tosylate in the same way. What alkenes will be obtained from the trans 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. 

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Abstract This chapter emphasises on the important aspects of steric and stereo-electronic effects and their control on the conformational and reactivity profiles. The conformational effects in ethane, butane, cyclohexane, variously substituted cyclohexanes, and cis- and tra/ ,v-decalin systems allow a thorough understanding. Application of these effects to E2 and ElcB reactions followed by anomeric effect and mutarotation is discussed. The conformational effects in acetal-forming processes and their reactivity profile, carbonyl oxygen exchange in esters, and hydrolysis of orthoesters have been discussed. The application of anomeric effect in 1,4-elimination reactions, including the preservation of the geometry of the newly created double bond, is elaborated. Finally, a brief discussion on the conformational profile of thioacetals and azaacetals is presented. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

The removal of a molecule of a hydrogen halide from an alkyl halide to yield an alkene is effected under strongly basic conditions, e.g. a concentrated alcoholic solution of sodium or potassium hydroxide or alkoxide. This overall reaction has been submitted to most rigorous mechanistic studies. Most of the factors (temperature, nature of base, structure of substrate, solvent, etc.) which control product composition have been evaluated. It thus appears that under the conditions noted above, an E2 process, in which the participating sites adopt an ann -periplanar conformation leading to an anti-elimination process, is generally favoured. 

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