Saytzeff selectivity

Figure 4.22 demonstrates related base effects towards an a-bromo carboxylic ester, with the regioselectivities being almost perfect as compared to the ones in Figures 4.21. Quinoline eliminates HBr from a-bromo isovaleric acid ester with Saytzeff selectivity. In contrast, KO-tert-Bu effects Hofmann-selective HBr elimination. The selectivity of the ferf-BuO reaction... 

Fig. 4. 35. Explanation for the particularly high Saytzeff selectivity of the El elimination from tert-amyl alcohol and its derivatives.

Fig. 4.21. Steric base effects on the Saytzeff/Hofmann selectivity of an E2 elimination. The small base EtO can attack the H atoms in both positions / to the leaving group, i.e., it does not matter whether the H atom is bound to a primary or secondary C atom. The regioselectivity therefore results only from product development control the thermodynamically more stable.

Fig. 4.23. leaving-group effect on the Saytzeff/Hofmann selectivity of an E2 elimination. Poor, positively charged leaving groups react preferentially to give the Hofmann product via an E2 elimination with El(J) character. 

When used for the dehydration of alcohols most acidic oxides result in the formation of alkenes with Saytzeff orientation, i.e. an E2 mechanism is operative. This means that secondary 2-alkanols are transformed to 2-alkenes. A characteristic catalyst achieving such selectivity is alumina treated with alkali metal cations, ammonia, or organic bases [2], Such treatment is necessary because pure, strongly acidic alumina induces skeletal rearrangements and double bond migration. The importance of choosing a catalyst with the appropriate characteristics is illustrated in Scheme 2 [17,18]. 

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