Saytzeff products

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 effect is relatively small and both products are formed, usually with one predominating. The more-substituted Saytzeff product typically predominates when the leaving group is small, e.g. halide. On the other hand, when there is a large leaving group present, e.g. quaternary ammonium, then steric effects... 

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. 

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 importance of the base in determining the nature of the transition state and thereby the product can be seen from Table 7.13. When Reaction 7.38 is carried out with KO-/Bu in /-butanol (E2H conditions), 76.9 percent Hofmann olefin is obtained. However, when the same reaction is carried out with +N(Bu)4 Br- (E2C conditions), 97.3 percent Saytzeff product is obtained. The... 

Fig. 4.7. Regioselectivity of the elimination of H/Het from Rsec—Het. When C/5 has fewer alkyl substituents than CI3 the constitutionally isomeric products are Hofmann product (A) and Saytzeff product (B).

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. 

Figure 4.21 shows how the bulkiness of the base influences the regioselectivity of the elimination of HBr from tert-amyl bromide. The stericahy undemanding base EtO can react with the H atoms in all -positions to the leaving group, irrespective of whether the H atom is bound to a primary or a secondary C atom. Thus, regioselectivity derives from product development control alone the thermodynamically more stable Saytzeff product is formed preferentially, though not exclusively. 

Fig. 4. 22. Saytzeff product is produced preferentially although not exclusively. The reaction of the sterically hindered base tBuO is directed preferentially toward the more readily accessible H atoms at the primary C atom. Therefore, it mainly provides the Hofmann product.

The reaction from Figure 4.11 proves that a selenoxide pyrolysis is a sy -elimination. The cyclohexylphenyl selenoxide shown reacts regioselectively to produce the less stable Hofmann product (D). The Saytzeff product (E) is not produced at all, although it is more stable than D. This observation shows that the transition state C of an anti-... 

The direction of elimination from esters has been extensively studied (DePuy and King, 1960). In general, where special structural features are absent, olefin with the smallest number of alkyl substituents is most abundant. Thus from s-butyl acetate 60% of butene-1 is produced, while from t-pentyl acetate, 2-methylbutene-l occurs to the extent of 75%. In distinction to the halides, which, as has been seen, give the Saytzeff product, the esters give predominately the Hofmann product. This may in part be due to the fact that the carboxylic acid which is found along with the olefin, is not capable of bringing about isomerization, as can the hydrogen halide in the case of the alkyl halides. 

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