2 - Formation of carbon–carbon double bonds

Summary

The formation of carbon–carbon double bonds is important in organic synthesis, not only for the obvious reason that the compound being synthesized may contain a double bond, but also because formation of the double bond allows the introduction of a wide variety of functional groups. Methods to construct alkenes are given in this chapter and some reactions that functionalize alkenes are given in Chapter 5. The formation of carbon–carbon double bonds by pericyclic reactions (such as cycloaddition reactions or sigmatropic rearrangements) are discussed in Chapter 3. Methods for the formation of alkenes in which the key step involves preparing the adjacent carbon–carbon single bond are given in Chapter 1 (see for example, Section 1.2.4).

This chapter is divided into reactions that give the alkene π-bond from substrates containing a C C single bond (typically by elimination) (Sections 2.1–2.5), or from an alkyne substrate (Section 2.6), or from two different precursors in which both the σ- and the π-bonds are formed (Sections 2.7–2.10).

β-Elimination reactions

One of the most commonly used methods for forming carbon–carbon double bonds is by β-elimination reactions of the types shown in Scheme 2.1, where X = e.g. OH, OCOR, halogen, OSO₂R, ⁺NR₃, etc. Included among these reactions are acid-catalysed dehydrations of alcohols, solvolytic and base-induced eliminations from alkyl halides or sulfonates and the Hofmann elimination from quaternary ammonium salts. They proceed by both E2 (elimination bimolecular) and E1 (elimination unimolecular) mechanisms.