Electrophilic reactions formation

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

In the reactions of arylsulfenyl chlorides with enamines one encounters an unusual result for enamine chemistry, in that the formation of 2,6-disubstituted cyclohexanone enamines predominates over the formation of monosubstitution products 474). A rationalization of this result suggests the formation of an intermediate which can act as an intramolecular electrophile in formation of the second carbon-sulfur bond. 

In the cationic domino process which is a synonym for an electrophilic reaction a carbocation is formed first, either formally or in reality, which under bond formation reacts with a nucleophile to form a new carbocation. In most of the known... 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which differs appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Z value . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function for a charge transfer complex is written as a linear combination of a wave function < o describing the unperturbed ground state of the molecule under attack, and a function which differs from (Pq in the replacement... 

The electrophilic reaction of magnesium cyclopropylidene (113) with Ai-lithioaryl-amines was reported (equation 31) . Thus, electrophilic reaction of magnesium cyclopropylidene (113) derived from 112 with iV-lithio iV-methyl p-anisidine resulted in the formation of a-amino-substituted cyclopropylmagnesium (119) in good yield. Methanol-ysis of the reaction mixture with CH3OD gave a-deuteriated Af-cyclopropyl-Af-methyl-p-anisidine (120) in 82% yield with 98% D-content. 

The chemistry of cyclopropanol [7] has long been studied in the context of electrophilic reactions, and these investigations have resulted in the preparation of some 3-mercurio ketones. As such mercury compounds are quite unreactive, they have failed to attract great interest in homoenolate chemistry. Only recent studies to exploit siloxycyclopropanes as precursors to homoenolates have led to the use of 3-mercurio ketones for the transition metal-catalyzed formation of new carbon-carbon bonds [8] (vide infra). 

The electrophilic reaction of NP with sulfite (the Boedeker reaction) has been studied, and follows a similar reaction pattern as with other reactants [Eqs. (5) and (6)]. The red color shows up at 475 nm, and this decays in an unknown way with formation of [Fe(CN)5S03]5 as the... 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

It is well known that trivalent carbenium ions play an important role in electrophilic reactions of 7t- and -donors systems. Similarly, pentacoordinate carbonium ions are the key to electrophilic reactions of o-donor systems (single bonds). The ability of single bonds to act as o-donors lies in their ability to form carbonium ions via delocalized two-electron, three-center (2e-3c) bond formation. Consequently, there seems to be in principle no difference between the electrophilic reactions of n- and O-bonds except that the former react more easily even with weak electrophiles, whereas the latter necessitate more severe conditions. 

The features of the electronic structure of aryl-substituted pyrazolines influence their chemical properties. For example, in the case of 3-substituted 7V-phenyl-pyrazolines 100 reactions of formylation, acylation, nitration, sulfonation, azocoupling and other electrophilic processes involve the para position of the 7V-phenyl ring, with formation of compounds 101 [103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113]. On the other hand, some electrophilic reactions, including nitration, bromination, chlorination, formylation and azocoupling, for 3-unsubstituted pyrazolines 102 occur at position 3, yielding heterocycles 103 and in some cases as a mixture with 104 [108, 114, 115] (Scheme 2.26). This fact provides evidence for orbital control of these reactions. 

The electrophilic reactions of co-ordinated 1,10-phenanthrolines are not always as simple as might be expected. Thus, the nitration of cobalt(m) 1,10-phenanthroline complexes yields 5-nitro-1,10-phenanthroline derivatives at low temperature, but prolonged reaction in hot solution leads to further reaction and oxidation of the ligand to give excellent yields of 1,10-phenanthroline-5,6-quinone complexes (Fig. 8-40). Even after the formation of the quinone, the complexes may exhibit further reaction. For example, reaction of the l,10-phenanthroline-5,6-quinone complexes with base results in the formation of a complex of 2,2 -bipyridine-3,3 -dicarboxylic acid (Fig. 8-41)... 

Despite the fact that H+ (in terms of electrophilic reactions of polyfluorinated compounds only) is a relatively mild electrophile, strong protic acids (H0S02F, H0S02CF3, anhydrous HF) are widely used for generating electrophilic species. Thus, protonation of fluorine in HgF2 by anhydrous HF results in formation of the corresponding metal centered cation 3 [ 17,20] ... 

Carbocations are important intermediates in most electrophilic reactions, since the attack of an electrophile on the C=C bond often results in the formation of these species (see Eq. 2). Factors affecting the stability of carbocations are... 

In contrast to most reported electrophilic reactions of CF2=CFCF3, addition of CF3OCl to this olefin is not regiospecific, and it results in formation of two isomers [9], which maybe an indication that a competetive radical pathway also operates in this reaction ... 

Addition of halogen fluorosulfates XOS02F (X=C1, Br, I) to fluoroolefins is considered an electrophilic reaction [8]. However, the question of whether this process is concerted or the reaction proceeds via an independent carbocationic intermediate (Eqs. 2 and 3) is still open. Formation of carboxylic acid esters as byproducts in the reaction of HFP with C10S02F, which was carried out in trifluoroacetic or heptafluorobutyric acids as solvents, could not be a solid proof of conjugate addition, since formation of esters may be a result of addition of C10C(0)Rf to olefin. These materials are known to be formed in the reaction of ClOS02F with fluorinated carboxylic acids, even at low temperature [99]. 

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