Saytzev rule

Saytzev rule Regioselectivity whereby E2 elimination by a nonhindered base leads to the more stable internal alkenes. 

In Summary Alkenes are most generally made by E2 reactions. Usually, the thermodynamically more stable internal alkenes are formed faster than the terminal isomers (Saytzev rule). Bulky bases may favor the formation of the products with thermodynamically less stable (e.g., terminal) double bonds (Hofmann rule). Elimination may be stereoselective, producing greater quantities of trans isomers than their cis counterparts from racemic starting materials. It also may be stereospecific, certain haloalkane diastereomers furnishing only one of the two possible stereoisomeric alkenes. 

Elimination of haloalkanes (and other alkyl derivatives) may follow the Saytzev rule (nonbulky base, internal alkene formation) or the Hofmann rnle (bulky base, terminal alkene formation). Trans alkenes as products predominate over cis alkenes. Elimination is stereospecific, as dictated by the anti transition state. 

Preparation Haloalkane -f strong base (bulky for prim RX), E2, anti stereospecific, most stable alkene favored (Saytzev rule) except with bulky base (Hofmaim rule) (7-7,11-6)... 

Preparation Alcohol + cone H2SO4, product mixtures (Saytzev rule) (9-2, 11-7)... 

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the truns-diaxial conformation in cyclohexane derivatives (cf. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47) (48) ... 

SaytzefPs rule The German spelling of Zaitsev s rule. Also sometimes spelt Saytzev. 

The data in Table 2.2 are representative of many elimination reactions. The product distribution is found to depend on whether the substrate is neutral (with a halide or sulfonate leaving group) or positively charged (with a —N(CH3)3 or —S(CH3)2 leaving group — which become N(CH3)3 and S(CH3)2 respectively). The empirical rules linking the nature of the substrate and the product distribution are known as Saytzev s rule and Hofmann s rule. 

If elimination were to proceed by the E2 mechanism. Compound 3.1 would be expected to obey Saytzev s rule and give the more substituted alkene (Structure 3.3), whereas Compound 3.2 should give predominantly the less substituted product (Structure 3.4) according to Hofmann s rule. 

This is indeed the experimental result, and the predominant alkene formed from 3.1 and 3.2 by an El mechanism is 3.3, the more substituted alkene (that is, the more thermodynamically stable for both substrates). El reactions are generally found to obey Saytzev s rule, whatever the leaving group... 

With some substrates, elimination can occur in two directions, to give different alkene products. Saytzev s rule states that neutral substrates give predominantly the more substituted alkene, by the E2 mechanism. Hofmann s rule applies to charged substrates, such as sulfonium and ammonium salts, and these give a predominance of the less substituted alkene by the bimolecular mechanism. 

El reactions usually obey Saytzev s rule to give the more substituted alkene. Since the more stable alkene is formed, the E form predominates. 

Whenever an elimination occurs to give the more stable, more highly substituted alkene, chemists say that the elimination follows Zaitsev s rule, named for the nineteenth-century Russian chemist A. N. Zaitsev (1841—1910) who formulated it. (Zaitsevs name is also transliterated as Zaitzev, Saytzeff, Saytseff, or Saytzev.)... 

The Saytzev s Rule and Hofmann s Rule can be used to predict the orientation of elimination, and the stereochemistry is preferentially antiperiplanar. 

Figure 11-15 Potential energy diagrams for E2 reactions of 2-bromo-2-methylbutane with (A) sodium ethoxide (Saytzev ruie) and (B) potassium fert-butoxide (Hofmann rule).

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