Substitution, vinyl element effect

This review deals with the replacement of substituents in the vinylic position hy anionic or neutral nucleophiles. Its division according to mechanistic routes suffers from the fact that for many systems there is a strong connection and mutual intercalation between several routes, but we will try to show the similarities in the behaviour of different systems and to discuss the various criteria which have been used for differentiation between the mechanistic pathways. Some topics, e.g. the stereochemistry and the element effect, are discussed in greater detail than others, especially when the data could be collected in convenient tables. No attempt has been made to cover all the synthetically used vinylic substitution reactions of which reviews are available, e.g. on /3-chloro-vinyl ketones (Kochetkov, 1952, 1961 Kochetkov et al., 1961 Pohland and Benson, 1966), fluoro-olefins (Chambers and Mobbs, 1965) or tetracyanoethylene (Cairns et al., 1958 Cairns and McKusick, 1961). 

First, the process may not involve a nucleophilic attack on the vinylic carbon. This fact is well recognized in vinylic substitution (2, 3), and a typical example is the substitution of ( )- and (Z)-(3-halovinyl sulfones (3 and 4 X = Cl or Br) by PhS- and MeO- in MeOH (4-6). Both reactions of both substrates are of a second order and give retention of configuration, and the element effects kBJka are 2.3 (E) and 2.2 (Z) with PhS- and 0.84 (E) with MeO-, values that are consistent with rate-determining nucleophilic attack on the vinylic carbon (2). However, for the Z isomer, kBJka with MeO- is 185, and because a-hydrogen exchange is rapid under the substitution conditions, the reaction of the Z-bromide probably proceeds via elimina-... 

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