E2C reactions

For convenience, we will refer to this class of reactions as E2C reactions, though the actual mechanism is in dispute. 

Effect of solvent on El vs. E2 vs. ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB mechanisms will be aided by increasing polarity of solvent and by increasing ionic strength. With certain substrates, polar aprotic solvents promote elimination with weak bases (the E2C reaction). 

Abstraction of the /3 proton in E2C reactions has a low isotope effect. For example, Reaction 7.36 has a kHlkD of only 2.391 This is consistent with the iso-... 

Orientation of double bonds If the double bond can be oriented toward either of two carbons in an E2 reaction, the product depends on where the transition state of the particular reaction lies in the spectrum. Since all E2 transition states have some double-bond character, the relative stability of the possible double bonds will always be of some importance to product determination. In an E2C reaction the double bond is apparently so highly developed at the transition state that the relative olefin stability is the controlling factor in deciding the product. In an E2H reaction, however, the relative acidity of the two kinds of... 

E2C reactions give entirely anti elimination. This fact seems to be universal, and the need for anti elimination is even more important than formation of the most stable product.109, 110 Thus, for example, 50 with N(Bu)4C1 gives >99.9 percent 51, whereas the other diastereomer, 52, gives >99.9 percent 53.111 Of course, 51 is the more stable olefin. 

Effect on El versus E2 versus ElcB. In the El mechanism, an external base is generally not required The solvent acts as the base. Hence, when external bases are added, the mechanism is shifted toward E2. Stronger bases and higher base concentrations cause the mechanism to move toward the ElcB end of the El-E2-ElcB spectmm."° However, weak bases in polar aprotic solvents can also be effective in elimination reactions with certain substrates (the E2C reaction). Normal E2 elimination has been accomplished with the following bases " HjO, NR3, OH, OAc, OR, OAr, NHj, COa ,... 

LiAlH4, I , CN , and organic bases. However, the only bases of preparative importance in the normal E2 reaction are OH, OR, and NH2, usually in the conjugate acid as solvent, and certain amines. Weak bases effective in the E2C reaction are Cl , Br E, OAc, and RS. These bases are often used in the form of their R4N salts. 

The results for the cyclohexyl compounds (18b-d) parallel those reported for E2C reaction of cyclohexyl halides and tosylates with I, Br and PhS over 30 years ago it has been argued that for (18b) reaction occurs preferentially via the conformer which has the thianthrenium oxy group in the axial position, and consequently leads to a product distribution similar to that for (18c) rather than (18d). 

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