Zaitsev elimination

E2 thermodynamic elimination to give the more substituted olefin. 

Hofmann elimination, on the other hand, furnishes the least highly substituted olefins. 

Book of Abstracts, 214th ACS National Meeting, Las Vegas, NV, September 7-11,(1997). 

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How can you explain the fact that ham- -bromo-2-methylcyclohexaneyields the non-Zaitsev elimination product 3-methylcvclohexene on treatment with base ... 

Zaitsev Elimination Form alkene with more substitution... 

But-2-ene will be formed in preference to but-l-ene, as the secondary carbonium ion is significantly more stable than the primary carbonium ion, i.e. the former is the thermodynamic product. Thus, in El reactions, Zaitsev elimination is preferred. However, it does not apply if the predicted product would suffer steric strain. 

In ElcB reactions, there is usually little ambiguity as to the regiochemistry of the resultant compound, because normally there is only one site from which an acidic hydrogen may be removed as a proton. Thus, in El reactions, Zaitsev elimination is preferred. Both alkyl or aryl groups at the a-carbon favour the El pathway. Further, generally, the more branching in the carbon chain, the more elimination occurs, because of the increased importance of alleviating steric strain. 

When X=Br. The elimination reaction forms predominantly the most highly substituted alkene in a Saytzev (or Zaitsev) elimination. The H-C bond is broken at the same time as the C-Br bond, and the transition state resembles the alkene product. As a result, the more highly substituted alkene is formed faster, because this is the more stable alkene product (see Section 6.1). 

The Zaitsev elimination, first reported in 1875 by the Russian chemist Alexander Zaitsev (1841-1910), was published in Justus Liebigs Annalen der Chemie and the German spelling of his name, Saytzeff, was used.1 Several historical accounts of Alexander Zaitsev have been reported.2 The olefin forming E2 elimination has been reviewed.3... 

An example of the Zaitsev elimination is the formation bromopentane. A mixture of pentene isomers are produced.4,5... 

The Zaitsev elimination reaction is commonly used in organic synthesis and since it has been known for over 130 years many examples are known in the literature. 

In 1994, Ito et al. utilized a Zaitsev elimination in the synthesis of 28.20 The reaction utilized the base KOH to accomplish a double elimination of bromide from 27 to form diene 28. 

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