Electron-deficient centers

The unshared pairs of electrons on hydroxyl oxygens seek electron deficient centers. Alkylphenols tend to be less nucleophiUc than aUphatic alcohols as a direct result of the attraction of the electron density by the aromatic nucleus. The reactivity of the hydroxyl group can be enhanced in spite of the attraction of the ring current by use of a basic catalyst which removes the acidic proton from the hydroxyl group leaving the more nucleophiUc alkylphenoxide. 

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. 

Intramolecular alkylnitrene addition to an alkenic moiety situated S,e to the electron deficient center has been utilized for the preparation of bi- and tri-cyclic aziridines (Scheme 11) (68JA1650). Oxidation of the primary alkylamine can be effected cleanly with NCS, LTA or mercury(II) oxide. 

Qualitative models of reactivity and quantum mechanical calculations of reaction paths both indicate an angular approach of the attacking nucleophile to the first-row sp -hybridized electrophilic centers M at intermediate and reactive distances, 29. The geometry of 29 is also characteristic for the case of nucleophilic addition to electron-deficient centers of main-group 12 and 13 elements. By contrast, a linear arrangement 30 of making and breaking bonds is required for sp -hybridized first-row centers (C, N, O)... 

Many reductive cyclizations, including many of those that are not initiated electrochemically, correspond to variations on the electrohydrocyclization theme. The so-called electroreductive-cyclization reaction, for example, involves cyclization between the /I-carbon of an electron-deficient alkene and an aldehyde or ketone tethered to it, to form a new a-bond between these formally electron deficient centers (Scheme 2). 

As indicated, these transformations lead to the formation of a new sigma bond between two formally electron-deficient centers [4,22,23], in this instance between the )9-carbon of an electron-deficient alkene and a carbonyl carbon. 

Following another line of thought, ligands with electron deficient centers may bind anions. Macrobicyclic ammonium salts displaying such properties have already been described (164) other ligands containing, for instance, trivalent boron sites might also be suitable. 

This rearrangement is unusual because hydrogen usually migrates toward electron-deficient centers as calculated for vinylcarbene. 

The relative rates for electrocyclic ring openings of cyclopropyl ions [200] are shown in Eqs (173,174). For the cations the faster rates are exhibited by compounds in which an acceptor is directly bonded to the electron deficient center (a-a arrangement), whereas precursors with a donor substituent at the center open most slowly. 

In many ways, cyclopropane behaves in the same fashion as an alkene. This is particularly evident in its interactions with electron-deficient centers. Thus, it undergoes a relatively facile reaction with a proton, and it interacts strongly with an attached cationic center.1... 

Many of the unique properties of cyclopropanes, and to a lesser extent, cyclobutanes, are derived from the formation of bent bonds. They may act in a fashion similar to 7r-bonds in interacting with electron-deficient centers, and are more easily cleaved thermally via electrophilic attack than are ordinary C-C bonds. The strain energy associated with bond angle deformation is also an important quantity, especially when considering thermal reactions. 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

Schemes I—III do not differ significantly from those reported in the literature (8,12). First, the electron-deficient centers in the zeolites must arise at the expense of proton-donating sites. Secondly, the nonproton centers formed in decationized zeolites are essentially different from each other. Both facts are confirmed by the results of our investigations on the electronic spectra of decationized zeolites.

The etcroospocific character of this reaction was indicated by the nature of produets secured from cw- and (rana-stilbeue oxides. Whereas the former afforded only deeoxybenzoin, (he latter yielded diphenyl-acetaldehyde exclusively (Eq. 492). T njike acid-catalyzed migrations to electron-deficient centers, therefore, the base-catalyzed epoxide jiuimerizations under scrutiny appear to involve cis-migratioa and fronts attack upon an anionic center. 39... 

Lewis acidity, arising from electron deficient centers, does not exist in silica. In alumina this acidity is associated with surface aluminum ions which are coordinated in a tetrahedral manner. Such sites may be generated either by removal of water coordinated to an aluminum center or by dehydration of two adjacent hydroxyl groups above 500°C. 

Positively charged or neutral electron-deficient groups may serve as interaction sites for anion binding. Ammonium and guanidinium units, which form +N-H" X bonds, have mainly been used, but neutral polar hydrogen bonds (e.g., with -NHCO- or -COOH functions), electron-deficient centers (boron, tin, mercury, [3.6, 3.7] as well as perfluoro crown ethers and cryptands [3.8], etc.), or metal-ion centres in complexes also interact with anions. 

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