Stereochemistry of Electrophilic Additions

For a summary and detailed discussion of the stereochemistry of electrophilic additions to alkenes and alkynes see R. C. Fahey, in Topics in Stereochemistry, Vol. 3,... 

Fahey, R. C., The Stereochemistry of Electrophilic Additions to Olefins and Acetylenes, 3, 237. Farina, M., The Stereochemistry of Linear Macromolecules, 17, 1. 

For a review of the stereochemistry of electrophilic additions to double and triple bonds, see Fahey Top. Ster-eochem. 1948, 3. 237-342. For a review of the synthetic uses of stereoselective additions, sec Bartlett Tetrahedron 1980, 36. 2-72. pp. 3-15. 

The effects of the cr—JT interaction on the ground-state properties of allyltrimethylmetal compounds are paralleled by the effect on reactivity towards electrophilic reagents. Mayr demonstrated that allyltrialkylsilanes, allyltrialkyl-germanes, and trialkylstannanes react with diphenylcarbenium ions at rates 105,5.6 x 105, and 109, respectively, relative to propene.158 The reaction rates were also found to be sensitive to the inductive effects of the other substituents attached to the metal. A theoretical evaluation of the factors determining the regiochemistry and stereochemistry of electrophilic addition to allylsilanes and other allyl systems is reported by Hehre et al.159 They predict a preference for electrophilic attack anti with respect to the silane substituent, a prediction that is supported by many experimental studies.82,160... 

Now we can move on to look at the stereochemistry of electrophilic addition to alkenes. 

To understand the stereochemistry of electrophilic addition, recall two stereochemical principles learned in Chapters 7 and 9. 

Figure 14 Stereochemistry of electrophilic additions to chiral allylic silanes...

The regio- and stereochemistry of electrophilic additions to allylsilanes were explained in terms of attack of the electrophile at the terminal olefin position and anti to the silyl group, allowing better stabilization by the / -silyl substituent. The model was based on 6-31G //3-21G calculations for two conformations of CH2=CHCH(CH3)SiH3334. 

Before we examine the stereochemistry of electrophilic addition reactions, we need to become familiar with some terms used in describing the stereochemistry of a reaction. 

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes... 

Now that you are familiar with electrophilic addition reactions and with stereoisomers, we can combine the two topics and look at the stereochemistry of electrophilic addition reactions. In other words, we will look at the stereoisomers that are formed in the electrophilic addition reactions that were discussed in Chapter 4. 

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