Nucleophilic additions to multiple bonds

Isotope effects and the large negative entropies of activation for the pyrolyses make it appear probable that the transition states for Reactions 7.46 and 7.47 are 62 and 63, respectively. Substituent effects, however, indicate that the transition states do have some polar character. 

When the electron density of a carbon-carbon bond is reduced by strongly electron-withdrawing substituents, nucleophilic attack at one of the vinylic or acetylenic carbons may occur. Electron withdrawal may be either by induction or by resonance. Examples of nucleophilic addition are shown in Equations 7.49-7.53. 

In general, the mechanisms of nucleophilic additions to double bonds have not been as much studied or systemized as those of electrophilic addition. Reactions 7.51 and 7.52 are examples of the very useful Michael condensation, in which a carbanion adds to an a,/ -unsaturated carbonyl or nitrile compound. The usefulness of these reactions arises from the fact that the number of ways of building longer carbon chains from smaller ones is limited. 

The mechanism of the Michael condensation is not actually a 1,2-addition as implied in Equations 7.51 and 7.52, but rather a 1,4-addition as shown in Equation 7.54. Protonation occurs first on the oxygen atom because 64b contributes more to the overall structure of the anion than 64a. The stereochemistry of 1,2-addition in the Michael condensation is therefore irrevelant to the mechanism of the condensation.124 Other nucleophilic additions to alkenes125 and alkynes126 go either syn or anti depending on the particular reaction. 

The rate-determining step in nucleophilic additions is usually nucleophilic attack on the multiple bond.127 For example, the entropy of activation of a Michael condensation is always a large, negative quantity. This arises from the fact that in the transition state the five atoms, 0=C—C—C=0 of the anion and the four atoms, C=C-—C=0 (or C=C—C=N) of the a,/ -unsaturated carbonyl (or nitrile) system are all restricted to one plane to allow maximum 

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