Electron-deficient species

The poorer electron sinks would be any of the above species with poorer leaving groups, less electronegative atoms, or poorer electron-withdrawing groups. 

Although an electron flow path is shown for each sink in this chapter, it is included only to point out the atom that would be attacked by the electron source. A detailed discussion of the electron flow paths is provided in Chapter 7. 

The best electron sinks, occurring almost exclusively in acidic media, are reactive cations such as carbocations (Section 4.2.4). Most are such good electron sinks that they can react with even very poor electron sources like aromatic rings. The most stable carbocations are the least reactive. Highly stabilized carbocations like +C(NH2)3 are so stable that they can exist in basic media and make very poor electrophiles. The following are some of the more common reactive carbocations. 

Benzyl cation /-Butyl cation Ally lie cation 

Although not really electron deficient, metal ions may bear a high positive charge and can act as electrophilic catalysts because they can complex with many electron sinks 

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Reactions with Radicais and Electron-deficient Species... 

Reactions with Free Radicals and Other Electron Deficient Species... 

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. 

Reactions with Radicals and Electron-deficient Species Reactions at Surfaces 4.02.1.8.1 Carbenes and nitrenes... 

Reaction with radicals and electron-deficient species reaction at surfaces... 

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

Perfluorinated carbonyl compounds, especially hexafluoroacetone, are highly electron-deficient species and react vigorously with a wide variety of HX nucleophiles The reaction of these ketones and of most polyfluonnated imines toward nucleophiles can be generahzed by the scheme shown m equation 1... 

Bis(trifluoromethyl)-substituted heterodienes are electron-deficient species They therefore react preferentially with electron-rich multiple bond systems to give [4+2] cycloadducts (Diels-Alder reaction with inverse electron demand) [238]... 

The reactivity of neutral dienophiles is greatly increased by converting them to the corresponding cation radicals because these highly electron-deficient species can then react readily with dienes. 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

Rearrangements may also proceed via intermediates that are essentially cations, anions, or radicals, though those involving carbocations, or other electron-deficient species, are by far the most common. They may involve a major rearrangement of the carbon skeleton of a compound, as during the conversion of 2,3-dimethylbutan-2,3-diol (pinacol, 42) into 2,2-dimethylbutan-3-one (pinacolone, 43, cf. p. 113) ... 

This concentration of charge might be expected to shield the ring carbon atoms from the attack of nucleophilic reagents and, by contrast, to promote attack by cations, X , or electron-deficient species, i.e. by electrophilic reagents this is indeed found to be the case. 

The pair of electrons in the n orbital are more diffuse and less firmly held by the carbon nuclei, and so more readily polarisable, than those of the a bond, leading to the characteristic reactivity of such unsaturated compounds. As the it electrons are the most readily accessible feature of the carbon-carbon double bond, we should expect them to shield the molecule from attack by nucleophilic reagents and this is indeed found to be the case (cf. p. 198, however). The most characteristic reactions of the system are, hardly surprisingly, found to be initiated by electron-deficient species such as X and X (radicals can be considered electron-deficient species as they are seeking a further electron with which to form a bond), cations inducing heterolytic, and... 

Reactions of 1,3,4-oxadiazoles at the ring atoms with radicals, carbenes, and nitrenes or with other electron-deficient species are rather uncommon. CHEC(1984) and CHEC-II(1996) have reported very few examples of such reactions concerning oxadiazolinones and oxadiazolinethiones. This situation has not changed. 

If trivalent phosphoms compounds are to be treated as electron-deficient species, then reactions of oxadiazoles with some Lewis acids should be reported here. 2-Phenyl-l,3,4-oxadiazole reacting with phosphoms trichloride in pyridine solution in the presence of triethylamine at low temperature furnished the respective dichlorophosphine and chlorophosphine, which were trapped by dimethylamine to give the corresponding amides. 2-Phenyl-l,3,4-oxadiazole also interacts over 24 h with the less reactive chlorodiphenylphosphine and dichlorophenylphosphine at room temperature to give phosphines (Scheme 14) <1999CHE1117>. These reactions of oxadiazoles resemble the behavior of 1-alkylimidazoles toward trivalent phosphorus derivatives. 

Reactions Involving Radicals, Electron-Deficient Species, Reducing Agents, and at... 

The only significant reactions that have appeared regarding this section involve reducing agents. For additional information on reactions with radicals and electron-deficient species, refer to CHEC(1984) <1984CHEC(6)513> and CHEC-II(1996) <1996CHEC-II(4)355>. 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

This reaction includes loss of CO prior to reaction of the rhodium fragment and the manganese fragment to form more reactive, electron deficient species, which we have omitted from the reaction equation. 

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