Carbon electron-deficient

Several types of functional groups that contain C-Z o bonds are listed in Table 3.2. The electronegative heteroatom Z creates a polar bond, making carbon electron deficient. The lone pairs on Z are available for reaction with protons and other electrophiles, especially when Z = N or O. 

The electronegative oxygen polarizes the carbon-oxygen bond, rendering the carbon electron deficient and hence subject to nucleophilic substitution. 

In general, the acid-catalysed esterification proceeds by the protonation of the alcohol (ROH) with subsequent proton transfer to the carboxylic acid (R COOH), rendering the carboxyl carbon electron deficient. Nucleophilic attack of the alcohol is thus facilitated, and water liberated. In an ionic liquid [X][Y], interactions will occur with the solutes, as exemplarily depicted in Fig. 15. 

The carbonyl group, C=0, provides a site for nucleophilic addition. The funtional group contains mobile tt electrons that pull strongly toward oxygen, this makes the carbonyl carbon electron-deficient and the carbonyl oxygen electron-rich. The molecule is flat due to the sp hybridization of the carbon. Consequently, it is open to relatively unhindered attack from above or below. Since the carbonyl carbon is electron-deficient, it is susceptible to attack by electron-rich, nucleophilic reagents such as bases. This typical reaction of aldehydes and ketones may be written as ... 

A selective cw-carboxylation was obtained with a dialkyl substituted acetylene (entry 8). With a phenyl and an alkyl substituent (entry 10) preferential carboxylation occurred at the alkylated carbon. Electron-deficient alkynes led to the saturated dicarboxylic acids (entry... 

Scheme 1.2 shows a hyperconjugative effect of the -CF3 group in an aromatic ring. Because of its hyperconjugative effect, the -CF3 group makes the o-and p-carbons electron deficient. Therefore, the electrophile attacks the relatively less electron-deficient m-carbon to give an m-substitution product in electrophilic aromatic substitution. 

Accordingly, the exterior surface is much more reactive than planar analogues, and is comparable to those of electron deficient polyolefins. This, in turn, rationalizes the high reactivity of the fullerene core towards photolytically and radiolytically generated carbon- and heteroatomic-centred radicals and also other neutral or ionic species [8]. The interior, in contrast, is shown to be practically inert [9]. Despite these surface related effects, the... 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

It is remarkable that chemists long resisted making the connection between boron and electron-deficient carbon, which, after all, are analogs. I was thus given the opportunity to be able to establish the general concept of five and six coordination of electron-deficient carbon and to open up the field of what I called hypercarbon chemistry. 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Electron-Deficient Boron and Carbon Clusters (ed. with Wade and Williams), 1991. 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Boron is electron deficient relative to carbon. Therefore, small amounts of boron, replacing carbon in a diamond lattice, causes electron holes. As electrons move to fill these lattice vacancies, infrared light is absorbed causing the blue color of the Hope diamond and other blue diamonds. 

Ring syntheses in which a bond is formed between the heteroatom and a carbon atom are conveniently considered according to whether the heteroatom functions as a nucleophile, an electrophile, a radical or other electron deficient species. 

By definition, members of this group have a vicinal arrangement of their electron-deficient centers. They may be conveniently considered according to their atom composition and the hybridization state of any carbon atoms involved. 

The above examples illustrate reactions at an electron-deficient carbon atom. Other 1,1-bielectrophiles allow the direct introduction of a heteroatom into the resultant heterocycle. The most widely applicable and versatile methods for the synthesis of 1,2,5-thiadiazoles and 1,2,5-selenadiazole rely on this approach. 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

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