Ei elimination

As in the El-E2-ElcB mechanistic spectrum, Bredt s rule applies and if a double bond is present, a conjugated system will be preferred, if sterically possible. Apart from these considerations, the following statements can be made for Ei eliminations ... 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal Ei elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to cw-2-methylthiane 1-oxide by an intramolecular cyclization mechanism <75TL2235>. 

The EiCB-Ei elimination spectrum Several investigators suggested that the spectrum of E2 transition states ranges from one similar to that of EjcB elimination, in which C—H bond breaking has proceeded considerably further... 

Elimination reactions (cf. 982, Section 3.2.3.12.1.2). If salts like 1005 are heated in the absence of nucleophile, Ei elimination occurs thus, the pentacyclic triflates (1005 R = primary alkyl) decompose at 150C. The sec-alkyl analogues form alkenes even at 20 C. 

Cases of Ei eliminations are common in pyrolysis. Most of these reactions occur with double or triple bond formation. Several examples are given below. 

Besides E, elimination, in some cases, an Ei elimination mechanism can be followed, and the more stable olefin is formed. Instead of Hofmann s rule, Zaitsev s rule is obeyed (the double bond goes mainly toward the most highly substituted carbon). In some reactions the direction of elimination is determined by the need to minimize steric interactions, sometimes even when the steric hindrance appears only during the transition state. Also an E2 mechanism may be followed. It should be remembered that an E2 reaction requires a proton acceptor and occurs as follows ... 

EI eliminations of alkyl halides are rarely useful for synthetic purposes because they give mixtures of substitution and elimination products. Explain why the sulfuric acid-catalyzed dehydration of cyclohexanol gives a good yield of cyclohexene even though the reaction goes ty an El mechanism. (Hint What are the nucleophiles in the reaction mixture What products are formed if these nucleophiles attack the carbocation What further reactions can these substitution products undergo )... 

Figure 2 Effect of relative permittivity ( r) on the solution degradation rate constant for trecetilide fumarate, a compound undergoing SNi substitution and Ei elimination through the same carbonium ion intermediate.

For Ei eliminations, the starting materials adopt a syn-periplanar conformation (Section 6.2.1)... 

Hammick reaction 705 Hinsberg synthesis of amines 54 Hofmann elimination, eis elimination 415... 

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