Stereochemistry of E2 elimination reactions

In most cases, E2 elimination proceeds via a transition state involving the anti arrangement. Nevertheless, syn elimination is possible, and, when special structural features retard anti elimination, syn elimination becomes the dominant mode. 

Cyclohexyl systems have a very strong preference for anti elimination via conforma- 

For example, cw-4-t-butylcyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the trans isomer beeause only the cis isomer permits anti elimination from the favored ehair eonformation. 

Other eyehe systems are not so seleetive. In the decomposition of AWiV-trimethylcyclo-butylammonium hydroxide, elimination is 90% The cyclobutyl ring resists the 

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process.  

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more Elcb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethyl-ammonium salts, may favor a more Elcb-like transition state because their inductive and field effects increase the acidicity of the P protons. 

Two elements of stereochemistry enter into determining the ratio of isomeric alkenes formed in E2 reactions. First, elimination may proceed in syn or anti fashion  

Second, in many case, the product alkene may be a mixture of the cis and trans isomers. The product ratio therefore depends on these stereochemical details of the elimination. The stereochemical aspects of elimination reactions have been of interest because of the insight the data provide into the reaction mechanism. 

The occurrence of syn elimination in 3-decyl systems has been demonstrated using diastereomeric deuterium-labeled substrates. Stereospecifically labeled 5-substituted decane derivatives were prepared and subjected to appropriate elimination conditions. By comparison of the amount of deuterium in the E-and Z-isomers 

The general trend revealed by these and other data is that anti stereochemistry is normally preferred for reactions involving good leaving groups such as bromide and tosylate. With poorer leaving groups (e.g., fluoride, trimethylamine) syn 

CHAPTER 6 POLAR ADDITION Substrate Base, solvent (%) anti (%) syn Ref. 

Leaving group Per cent syn elimination E product Z product  

In this section we focus primarily on the stereochemistry of the concerted E2 mechanism. The most familiar examples are dehydrohalogenation and dehydrosul-fonylation reactions effected by strong bases. In principle, elimination can proceed with either syn or anti stereochemistry. For acyclic systems, there is a preference for anti elimination, but this can be overridden if conformational factors favor a syn elimination. The anti TS maximizes orbital overlap and avoids the eclipsing that is present in the syn TS. 

Data obtained for three different leaving groups are shown in Table 5.14. The results demonstrate that syn elimination is extensive for quaternary ammonium salts. With better leaving groups, the extent of syn elimination is small in the polar solvent DMSO but quite significant in benzene. The factors that promote syn elimination are discussed below. Table 5.15 summarizes some data on syn versus anti elimination in other acyclic systems. 

In cyclic systems, the extent of anti and syn elimination depends on ring size, among other factors. Cyclohexyl systems have a very strong preference for anti elimination via 

Pankova, M. Svoboda, and J. Zavada, Tetrahedron Lett., 2465 (1972). The analysis of the data also requires that account be taken of (a) isotope effects and (b) formation of 4-decene. The method of analysis is described in detail by J. Sicher, J. Zavada, and M. Pankova, Coll. Czech. Chem. Commun., 36, 3140(1971). 

---