Super-electrophiles, reactivity

A formally antiaromatic 1,4-dihydropyrazinothiadiazole has been prepared and characterized by single crystal X-ray spectroscopy. The antiaromatic character of which has been supported computationally using NICS measurements <20070L1073>. CHIH-DFT computational studies on acenaphtho[l,2-f]-l,2,5-thiadiazole 1,1-dioxide led to simulations of its infrared (IR) and ultraviolate (LJV) spectra, the dipole moment and polarizability <2007JMT373>. 4,6-Dinitrobenzothiadiazole was determined to have an electrophilic reactivity of —8.40 which corresponds to a pK z° of 7.86 for Meisenheimer complexation with water and is close to the demarcation boundary (E = —8.5) between super-and normal-electrophiles and between reactive dienophiles and inert partners in Diels-Alder adduct formation <20070BC1744>. 

Sander applied DFT (B3LYP) theory to carbenic philicity, computing the electron affinities (EA) and ionization potentials (IP) of the carbenes." " The EA tracks the carbene s electrophilicity (its ability to accept electron density), whereas the IP represents the carbene s nucleophilicity (its ability to donate electron density). This approach parallels the differential orbital energy treatment. Both EA and IP can be calculated for any carbene, so Sander was able to analyze the reactivity of super electrophilic carbenes such as difluorovinylidene (9)" which is sufficiently electrophilic to insert into the C—H bond of methane. It even reacts with the H—H bond of dihydrogen at temperamres as low as 40 K, Scheme 7.2) ... 

In a series of papers, Koltunov, Repinskaya, and co-workers have reported the ionic hydrogenation of isomeric naphthols and dihydroxynaphthalenes with alkanes in the presence of aluminum halides. 1-Naphthol and substituted derivatives undergo regioselective reduction under mild conditions in excess alkane and aluminum halides with the involvement of various reactive intermediates to yield a-tetralone derivatives874 [Eq. (5.322)]. Byproducts are 3-, 6-, and 7-alkyl-substituted derivatives. Mechanistic studies875 with cyclohexane-di2 showed that deuterium incorporation takes place exclusively at C(4), indicating the involvement of super-electrophilic dication 263.2-Naphthol is much less reactive and complete conversion cannot be achieved.876... 

Carboxonium ions are indicated to be involved in a number of super-electrophilic reactions. In several cases, the direct observation of the superelectrophiles and reactive dications has been possible using low... 

An advantage of Sander s method is that EA and IP can be calculated for any carbene, and do not depend (as does w xy) 011 specific set of alkenes or carbenes. Thus, Sander is able to examine species he refers to as super-electrophilic carbenes. [82] These include difluorovinylidene, 29 [84] and 4-oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene, 30, a highly electrophilic triplet carbene. [85] With EA= 2.26 (observed) or 2.56 eV (computed), [84] carbene 29 is electrophilic enough to insert into the C-H bond of methane, and even the H-H bond of hydrogen at temperatures as low as 40 K. This type of reactivity is also observed [85] with 30 (computed EA= 3.61 eV). [82]... 

Tsuge and Samura392 used the reactivities of positions 7 and 2 toward electrophiles calculated from frontier electron densities and super-delocalizabilities to assign the structures of disubstituted derivatives... 

Fusion of pyridine to a furan nucleus gives rise to six isomeric furopyridines (6-11 Scheme 1). Members of all these types are known. As is to be expected and as is shown even by simple HMO calculations (B-78MI31700, B-78MI31701, B-78MI31702), derivatives (6-9) of benzo[6]furan (4) are considerably more stable than the o-quinonoid isomers (10,11) derived from benzo[c]furan (5) (Tables 1 and 2). It is also to be noted that if super-delocalizabilities are considered as indices for reactivities, electrophilic substitution will occur in the furan nucleus both when considering the neutral heterocycles and the N-protonated species. 

The concept of superelectrophilic activation was first proposed 30 years ago.20 Since these early publications from the Olah group, superelectrophilic activation has been recognized in many organic, inorganic, and biochemical reactions.22 Due to the unusual reactivities observed of superelectrophiles, they have been exploited in varied synthetic reactions and in mechanistic studies. Superelectrophiles have also been the subject of numerous theoretical investigations and some have been directly observed by physical methods (spectroscopic, gas-phase methods, etc.). The results of kinetic studies also support the role of superelectrophilic activation. Because of the importance of electrophilic chemistry in general and super-acidic catalysis in particular, there continues to be substantial interest in the chemistry of these reactive species. It is thus timely to review their chemistry. 

Zahradnlk et al.160 have combined a set of parameters which may be regarded as aromaticity indices into the stability index. The approach is more comprehensive than that of Balaban, but as the authors point out it is obviously difficult to assess the relative importance of the selected indices. Those included are the specific delocalization energy DE,P34 (Section II, A,3,a) related to the extent of -electron delocalization, Aq the difference between the maximum and minimum ir-electron density in the molecule (related to the reactivity toward nucleophilic and electrophilic reagents see the preceding section), the free valence F (related to radical reactivity) and the radical super-delocalizability ST (related to radical reactivity, oxidizability, and reducibility). 

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