Electrophilic substitution stereochemistry

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... 

Clearly, a study of the stereochemistry can distinguish between Se2 (back) on the one hand and Se2 (front) or Ssi on the other. Many such investigations have been made. In the overwhelming majority of second-order electrophilic substitutions, the result has been retention of configuration or some other indication of frontside attack, indicating an Se2 (front) or SeI mechanism. For example, when cis-l was treated with labeled mercuric chloride, the 2 produced was 100% cis. The bond between the mercury and the ring must have been broken (as well as the other Hg—C... 

Lithiated allyl carbamates stereochemistry of electrophilic substitution... 

This electrophilic assistance could account for the observed solvent and counterion effects. The stereochemistry of substitution of alkoxy, mercapto and fluorosilanes with alkoxide ion depends dramatically on the composition of the solvent4. As the percentage... 

The possibility of electrophilic substitution at saturated carbon as an independent mechanism was considered by Hughes and Ingold2 in 1935, but this mechanism was not kinetically demonstrated with metal alkyls as substrates until 1955, when Winstein and Traylor3 published their results on the acetolysis of dialkylmercurys. At about the same time, stereochemical studies on electrophilic substitutions at saturated carbon were commenced by Winstein and by Reutov, again using alkylmercury compounds as substrates. Notable studies on the kinetics and stereochemistry of substitution at saturated carbon have been carried out by Ingold and his co-workers and by Reutov and his co-workers. Ingold4... 

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... 

Organometallic reagents in /3-lactam chemistry 88T5615. Stereochemistry of electrophilic substitution in /3-lactams 90KGS1155. /3-Lactam antibiotics (see Section II,D,3,c). 

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. 

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... 

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. 

The stereospecificity of deprotonation of ketone NA -dialkylhydrazones is less defined because of the problem of making unambiguous stereochemical assignments. However, based on the observed stereoselectivity of electrophilic substitutions on these anions, on chemical shift data and on analogies to enolate chemistry, it appears that ( )c—c stereochemistry again predominates, at least with LDA de-protonations. ... 

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