Saytzeff -alkenes

We do not have the same strict stereochemical requirements as in the E2 mechanism, and isomeric alkenes may well be produced. If several hydrogens are available for elimination, then the preferred product formed is the more-substituted Saytzeff alkene. 

There must be a stereochemical requirement in this elimination. If the Saytzeff alkene is not produced because the methyl group is trans to the leaving group, then the H and the leaving group must be trans and the elimination must be anti—the characteristic stereochemistry of E2 elimination. This evidence differentiates between the two possibilities in part (a). 

Dehydration. Cu(OTf)2 (0.1 equiv.) is an effective catalyst for dehydration of primary, secondary, or tertiary alcohols and of diols at 25°. The alcohol can be used neat or as a suspension in decalin or heptane. Yields are generally higher than those obtained with H2S04 or POCl3/pyridine. Saytzeff (E)-alkenes are formed predominantly. 

If there is possibility of formation of more than one alkene due to the availability of more than one 3-hydrogen atoms, usually one alkene is formed as the major produet. These form part of a pattern first observed by Russian ehemlst, Alexander Zaitsev (also pronounced as Saytzeff) who In 1875 formulated a rule which can be summarised as in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. Thus, 2-bromopentane gives pent-2-ene as the major product. 

It is a general observation that, where different alkene products can arise through E2 elimination, the more-substituted alkene predominates. 2-Menthene contains a double bond with two alkyl substituents, whereas the double bond in 3-menthene has three substituents. The more-substituted alkene is termed the Saytzeff product the less-substituted alkene is termed the Hofmann product. We recommend you disregard the proper names, and think of the products in terms of more-substituted alkene and less-substituted alkene . 

This elimination involves a small leaving group, so the more-substituted alkene predominates. However, E and Z isomers of this Saytzeff product are produced, and in unequal amounts. That the major product is the -alkene can be rationalized in terms of minimizing steric repulsion during the transition state. 

In dehydration and dehydrohalogenation the preferential order for removal of an H is 3°>2°> 1 (Saytzeff rule). We can say the poor get poorer. This order obtains because the more R s on the C==C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is... 

This E2 elimination (Table 7-3) give the less substituted alkene (Hofmann product) rather than the more substituted alkene (Saytzeff product Section 6.3). 

The reaction has been extensively used for the determination of the structure of naturally occurring bases (e.g. the alkaloids). However it has rather limited preparative value, even though the elimination reaction occurs without any rearrangement of the carbon skeleton, and the regioisomer which predominates in the product is the less highly substituted alkene (Hofmann rule contrast the Saytzeff rule). Such alkenes are now more usually prepared by other procedures noted below. 

Saytzeff rule states that the major product of elimination will be the most substituted alkene. 

Formation of Regioisomeric Alkenes by /3-Elimination Saytzeff and Hofmann Product(s)... 

If the /1-elimination of H/Het from R —Het can, in principle, afford regioisomeric alkenes whose C=C double bonds (Figure 4.7) contain a different number of alkyl substituents, they are differentiated as Hofmann and Saytzeff products the Hofmann product is the alkene with the less alkylated double bond, and the Saytzeff product is the alkene with the more alkylated double bond. Because C=C double bonds are stabilized by alkyl substituents, a Hofmann product is, in general, less stable than its Saytzeff isomer. Accordingly, eliminations of H/Het from Rv(,f —Het, which exhibit product development control, furnish a Saytzeff product with some regioselectivity. 

When the carbenium ion intermediate of an El elimination can be deprotonated to give two regioisomeric alkenes, generally both of them are produced. If these alkene isomers are Saytzeff and Hofmann products, the first one is produced preferentially because of product development control. This is illustrated in Figure 4.34 by the El elimination from ferf-amyl... 

The elimination in Figure 4.36 supports the idea that the alkenes initially formed from tertiary alcohols under El conditions can be reprotonated. The Saytzeff and the Hofmann products shown there can be protonated to provide the tertiary carbenium ion through which they were formed and also to a different tertiary carbenium ion. The consequence of this is that in the major product obtained after the final deprotonation, the C=C double bond is no longer located at the C atom that carried the OH group in the starting alcohol, but is moved one center away. 

Saytzeff s rule) An elimination usually gives the most substituted alkene product. Zaitsev s mle does not always apply, but when it does, the reaction is said to give Zaitsev orientation, (p. 263)... 

And the most stable alkene (most substituted) is the usual product (Saytzeff rule). 

The Saytzeff rule (or Zaitsev s rule or Saytsev s rule named after Alexander Mikhailovich Zaitsev) implies that base (sterically unhindered) induced eliminations will lead predominantly to the alkene in which the double bond is more highly substituted. However, if the base, for example, is potassium f-butoxide [(CH3)3COK], the bulkiness prohibits the base from pulling the proton off of the most substituted carbon. In such cases, the less substituted alkene, i.e. Hofmann elimination, is preferred. 

Hofmann elimination loss of a proton leading preferentially to the alkene carrying the smaller number of alkyl groups (cf Saytzeff elimination below)... 

Alkenes may be obtained by elimination reactions from alkyl haUdes, alcohols, sulfonates or amines. The substitution pattern of the alkene and the stereospecificity of these methods depend quite subtly on the structure of the individual substrate. If the leaving group occupies an unsymmetrical position in a compound, one of two isomeric alkenes can be formed (Scheme 3.1). Elimination to give an alkene bearing the greatest number of alkyl groups is known as the Saytzeff elimination and is commonly found with alkyl halides. When the elimination gives preferentially the less-substituted alkene, it is known as the Hofmann elimination This substitution pattern is commonly foimd with the elimination of alkylammonium salts. In practice, mixtures of alkenes are often obtained. 

Each step of this E, elimination reaction is reversible and thus the reaction is driven to completion by removing one of the products, the alkene. In these reactions several alkenes can be produced. The Saytzeff rule states that the more substituted alkene is the more stable and thus the one formed in larger amount. And the trans isomer is more stable than the cis isomer. With this information it should be possible to deduce which peaks on the gas chromatogram correspond to a given alkene and to predict the ratios of the products. 

So far, we have spoken only of E2 elimination. In El elimination., orientation is determined in the second step conversion of carbonium ion to alkene. As we might expect, orientation is essentially the same regardless of what leaving group has departed earlier in the formation of the carbonium ion. Orientation is strongly Saytzeff, reflecting much alkene character in the transition state. 

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