Electrophilic substitution reactions stereochemistry

The chiral allylsilanes (3b) were used to determine the stereochemistry of the electrophilic substitution reactions (SE1). Typical results are shown in equations (II) it rid (III). The (Z)-allylsilanes give products of (R)-configuration and the (E)-allylsilanes give the (S)-isomers. In each case, the electrophile attacks the double bond anti to the leaving group.6... 

Hayashi, T., Konishi, M., Ito, H., Kumada, M. Optically active aiiyisiianes. 1. Preparation by palladium-catalyzed asymmetric Grignard crosscoupling and anti stereochemistry in electrophilic substitution reactions. J. Am. Chem. Soc. 1982,104,4962-4963. 

These X-ray structures and detailed multinuclear studies of hydrazone anions are the most recent structural details available about these anions. While they are likely to prove invaluable in understanding the mechanistic basis of hydrazone selectivity in electrophilic substitution reactions, other questions of stereochemistry and regiochemistry in anion generation are of more immediate synthetic interest. Both questions have received considerable study within the last 10 years. 

Neutral Complexes. Interaction of acetylacetone and hydrous Rh2G3 gives the trisacetylacetonate, which has been resolved into enantiomeric forms. It undergoes a variety of electrophilic substitution reactions of the coordinated ligand, such as chlorination. The stereochemistry and racem-ization of the cis- and trans-isomers of the unsymmetrical trifluoroacetyl-acetonate have been studied by nmr spectroscopy the compound is extremely stable to isomerization. 

As the o-complexes in these azo coupling reactions are steady-state intermediates (Wheland intermediates, named after Wheland s suggestion in 1942), their stereochemistry cannot be determined directly. Bent structures like that in Figure 12-6 can, however, be isolated in electrophilic substitutions of 1,3,5-triaminobenzene... 

Although they really belong in Chapter 17 with other nucleophilic substitution reactions, we included the last few examples of epoxide-opening reactions here because they have many things in common with the reactions of bromonium ions. Now we are going to make the analogy work the other way when we look at the stereochemistry of the reactions of bromonium ions, and hence at the stereoselectivity of electrophilic additions to alkenes. We shall first remind you of an epoxide reaction from Chapter 17, where you saw this. 

Substitution reactions at 1° and 2° (but not 3°) C(sp3) usually proceed by the SN2 mechanism under basic or neutral conditions. In the Sn2 mechanism, the nucleophile Nu approaches the electrophilic center opposite X and in line with the C-X bond. The lone pair on Nu is used to form the C-Nu bond, and the pair of electrons in the C-X bond simultaneously leaves with X as the bond breaks. The other three groups on C move away from Nu and toward X as the reaction proceeds, so that when the reaction is complete, the stereochemistry of C is inverted. The charges do not have to be as shown the nucleophile may be anionic or neutral, and the electrophile may be neutral or cationic. 

The halogen atom of a halonium ion has its octet and a formal positive charge, so the C atoms of the halonium ion are electrophilic and are susceptible to Sn2 nucleophilic attack. Because the Sn2 substitution reaction results in inversion at one of the C atoms, the overall reaction—addition of the electrophilic halogen and nucleophile across the 77 bond—usually proceeds with anti stereochemistry. 

The stereochemistry of the direct attack can be expected to resemble the corresponding reactions at a saturated carbon (Sections 5.1.1.1 and 5.1.1.2)—inversion for nucleophilic substitution, and retention, or perhaps occasionally inversion, for electrophilic substitution. In practice, SN2 reactions at trigonal carbon are rare,503 and their stereochemistry, where inversion is known,504 barely established. Electrophilic attack, like the reactions of vinyllithium reagents with protons, aldehydes and carbon dioxide, takes place invariably... 

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