Substitution at Saturated Atoms

Fukui and Pujimoto (1966b) consider stereoselection in cases of a-n interactions at carbon. With respect to substitutions, the qualitative frontier-electron viewpoint is adequate for a general view of the field. It is of little help, however, in sorting out the numerous and subtle variations in the kinds of substitution at a single center. 

Idealized concerted bond making and/or bond breaking in conjugated polyenes in substitution, addition and elimination. 

The predictions based on this approach are given in Table 6, where syn and anti refer to a symmetry plane through the tt system—the corresponding terms used for displacements are usually retention and inversion. There is, of course, an overwhelming amount of data for two or three of the possible cases covered in this table and little or nothing 

Bimolecular electrophilic reactions (SE2) involving hydrogen exchange at a saturated center are rare. Although retention at tetracoordinated 

By contrast, prototropy at sp2 carbon, e.g. in polyenes such as in trans-i/t-ionone, 13, or a-ionone, 14 (Roest et al., 1967), is well known. 

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Substitutions at saturated carbon atoms that are subject to control by remote functionalities may be best illustrated by the ring opening of aziridines [35] and epoxides [36, 37]. 

If R is an alkyl group, reaction (1) leads to the familiar mechanism of nucleophilic substitution at saturated carbon whilst reaction (2) leads to an electrophilic substitution of saturated carbon. Of course for these mechanisms to be followed it is not necessary for a completely developed carbonium ion or carbanion to be formed, and both nucleophilic and electrophilic substitution at saturated carbon may proceed by mechanisms in which the carbon atom undergoing substitution has a carbonium ion character or a carbanion character respectively. 

When an electrophilic substitution at saturated carbon occurs, either a car-banion is liberated as such or, if no carbanion is actually formed, the carbon atom undergoing substitution has a certain amount of carbanion character . Thus a knowledge of the factors governing the formation and the stability of carbanions might be of help in the understanding of the mechanism of electrophilic substitution at saturated carbon. 

A. R. Katritzky, B. E. Brycki, Nucleophilic Substitution at Saturated Carbon Atoms. Mechanisms and Mechanistic Borderlines Evidence from Studies with Neutral Leaving Groups, J. Rhys. Org. Chem. 1988, 1, 1-20. 

Simple nucleophilic substitutions at saturated carbon atoms are fundamental reactions found wherever organic chemistry is practised. They are used in industry on an enormous scale to make heavy chemicals and in pharmaceutical laboratories to make important drugs. They are worth studying for their importance and relevance,... 

Ring formations by nucleophilic substitution at saturated carbon atoms with primary amines as nucleophiles have rarely been carried out because the resulting secondary amines as a rule are more nucleophilic than the primary ones, and therefore competition reactions are favored. The synthesis of secondary amines often starts from toluene sulfonamides which can easily be deprotonated and alkylated. A large number of methods for detosylation exists especially the acidic cleavage with H2SO4 or with HBr/phenol have proved to be reliable. 

Some carbonium-ion- and carbanion-pro-ducing heterolyses of carbon-metal bonds Electrophilic substitutions at saturated carbon atoms... 

Homolytic substitution at mrrltivalent atoms also occtus, but neither normally occurs at saturated carbon centers. Sh2 reactions can be synchronous, occurring via a transition state, or stepwise via an intermediate that may exist for only a finite time. 

We will come back to this concept again in Chapter 15, where you will see that this principle does not apply to substitution at saturated carbon atoms. 

This contrasts markedly with most electrophihc additions to carbon—carbon double bonds and with nucleophilic substitutions at saturated carbon atoms. The latter reactions are essentially irreversible, and overall results are a function of relative reaction rates. 

Polymer-supported podands appear to be more commonly used in solid-liquid systems. A variety of chemical transformations including nucleophilic substitutions at saturated carbon atoms [214,215], esterifications [214], sodium bo-rohydride reduction [214], fluoride-catalyzed Michael addition [214], phenacyl ester synthesis, Darzen s reaction, Wittig alkenylation, and dichlorocarbene reactions can be carried out under these conditions [214],... 

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