Electron Acceptors Electrophiles, a-Synthons

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

The electrophilicity of C = C double bonds conjugated with electron withdrawing groupings leads to a -synthons. This is an important example of the vinyiogous principle  

Whenever functional groups are connected with a C=C double bond, their reactivity is often relayed through that double bond. Analogous rules can be applied to the corresponding cyclopropane derivatives. 

A few examples of reactivity transfer through more than three carbon atoms are also known (a -synthon N. Miyaura, 1975, 1976). 

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