Transition state eliminations

Ab initio methods using the 6-31 + G basis sets have been used in a theoretical study of competing, S n2 and El reactions of NCCH2CH2CI with HO and HS in the gas phase. The antiperiplanar elimination transition state, which is favoured over those for n2 and El(gauche) reactions, is more lcB-like than that for the slower El anti) reaction of ethyl chloride. 

Fig. 2. Calculated /i-elimination transition state for Me2AlEt, and elimination product...

This difference would be reflected in AH and in reaction rates only if the elimination transition state strongly resembled the products. To the extent that the elimination transition state resembles the starting complex cis-ML RR, one would expect relative rates to be largely a function of the energies of the bonds being broken. There is then no reason to expect elimination of R-H from the mixed species to be faster than elimination of both R-R from ML4R2 and H-H from ML4H2. 

Ab initio and density functional calculations of the thermal syn elimination transition states for E reaction of organic amine oxide, sulfoxide, and phosphoxide have confirmed the expected planar geometry and known order of reactivity.71... 

It is interesting to point out that the o-OH group in 2-phenylethyl chloride produced the cyclic benzodihydrofuran in 87% yield, while the o-CH30 derivative gave the latter in about 25% yield164. This difference was reasonably ascribed to the more favored participation ability of the elimination transition state, thus producing more of the corresponding cyclic product. 

The substituent effect data indicates that the charge polarity in the vinyl ether transition states is less than in the corresponding ester elimination transition states. The activation energies (corrected) naturally parallel the rate coefficient variations and show a systematic decrease of 2.3 to 2.4 kcal.mole for successive methyl substitutions for hydrogen at the a-carbon. Corrected for gauche destabilizations of the ether ground states, this indicates a transition state charge stabilization by methyl of about 1.6 kcal per CH3. Ester stabilizations have previously been estimated to be about 3.3 kcal per CH3. 

For xanthogenate and acetate elimination from cyclic systems, the regioselectivity is dependent upon the accessibility of the alternative syn elimination transition states. The c/i-2-phenyl acetate (123 X = OAc) and xanthogenate (123 X = OCSSMe) both eliminate exclusively away from the phenyl substituent to give 3-phenylcyclohexene (124 equation 55), whereas the corresponding /ran5-2-phenyl com-... 

The syn-elimination transition states are very much destabilized relative to the anti-elimination transition. In the anti-elimination the conformation that leads to the tram product is more stable that the cis by approximately 1 kcal/mol and the tram product is formed to a greater extent. 

Fig. 6.9 Chugaev elimination transition state of xan-thate ester generated using the AMI method...

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