Super-electrophiles

It is explosive and a super-electrophile, like 4,6-dinitrobenofurazan N-oxide. 

Fluorination of vinorelbine was thus performed in super acid medium (HF-SbFs). A super electrophilic agent, such as a chloromethyl or a Br+ cation is generated in s/fu from a chloromethane (CHCI3, CCI4) or from N-Bromo Succinimide (NBS). It is able to abstract a hydrogen from the protonated alkaloid, leading to a cation that can be trapped by an halide anion present in the medium [105,106], Difluorination remarkably occurs selectively at C-4 of the clavamine fragment (Fig. 37) [105]. 

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) ... 

Some interesting reactions of the super-electrophile 4,6-dinitrobenzofuroxan (151) with aryloxide ions have been reported 130 (151) with phenoxide ion gives the O-bonded ff-adduct (152) which has been fully characterized for the first time. On... 

There have been several studies utilizing the ability of 4,6-dinitrobenzofuroxan (DNBF) to act as a super-electrophile. It has been shown that the initial product of its reaction with phenoxide ions is the O-bonded adduct (29), although this rearranges to give die C-bonded adduct.117 The reaction of DNBF with ethyl vinyl ether yields a dihydrooxazine A -oxidc cycloadduct as a mixture of two diastereoisomers, and in the presence of excess ether a di-adduct (30) is formed.118 Rate constants have been reported for the reaction of DNBF with 3-methoxythiophene in DMSO-water mixtures. The reaction results in a er-adduct which is the product of Se-At substitution in the thiophene, and the results were used to probe the carbon basicity of the thiophene derivative.119... 

Reactions of PhO with the super-electrophilic substrate (163) have been examined by 400 MHz 1 H NMR spectroscopy in MeCN-DME (1 1) as a function of temperature (—40 to 23 °C) and in DMSO at room temperature (Scheme 34). The <9-bonded c-adduct (164), resulting from the attack at C(7) of (163), has been observed and characterized for the first tune no C(5) adduct was detected at —40 °C or in subsequent monitoring the reaction. Apparently, the C(7) attack is favoured by both kinetics and thermodynamics. Upon warming the reaction mixture to ambient temperature, (164)... 

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... 

Alkyl cations like the tert-butyl cation (2) and 2-propyl cation (89) are significantly stabilized by hyperconjugative C-H and C-C a -back donation into the empty carbocationic p-orbitals. Protosolvation involving a -bonds can diminish this hyperconjugative stabilization and thus lead to super-electrophilic carbocationic species. 

One of the proposed intermediates in this transformation is the super-electrophilic species (184), which undergoes deprotonation to give the 2-buten-oyl cation. Further evidence for the superelectrophile 184 is obtained from experiments in which the 2-butenoyl cation (185) is generated in DSOsF-SbFs. Significant deuterium incorporation is found at the a and y positions, suggesting equilibria involving 184-186. In a similar respect, formation of the 4-chloro-3-methylbutanoyl cation (187) in superacid leads to the two acyl dications (188-189, eq 57).69... 

Since benzene and cyclohexane are both fairly weak nucleophiles, this chemistry indicates that such dicationic species (i.e., 93) are indeed super-electrophilic. 

Tetramethyl-2,4-pentanediol forms the oxonium dication 230 in superacid, and dehydration is followed by carbon- carbon bond cleavage. Although such cleavage reactions can occur with monocationic onium ions, in this case the cleavage reaction is likely an indication of the super-electrophilic nature of the dicationic intermediate(s). 

This unusual observation is explained with consideration of the super-electrophilic nature of the dihalonium ions (252a,b). It is thought that the cubyl framework is stabilized by the pair of electron-withdrawing halonium groups, while in the case of the monohalonium cubanes (254a-c),... 

S Ar reactions have been studied mostly with dinitro and trinitro activated benzenes. Recently, the aniline reactions with the highly activated super-electrophile, 4,6-dinitrobenzofuroxan (DNBF), have attracted considerable attention due to its ambident reactivity114. In acidic solvent, where aniline is almost completely protonated, the reaction by nitrogen attack is inhibited. However, a carbon-carbon bonded adduct is slowly formed by reaction at the 4-position of the aniline. Kinetic studies in H2O-DMSO have shown that this adduct formation involves two steps a carbon-carbon bond formation ( a), followed by elimination of a proton, Kiep (vide infra). The reactions of anilines with DNBF in DMSO have shown that the N-bonded adduct (a-N) is kinetically preferred, but the C-bonded adduct (a-C), formed in equilibrium with its deprotonated form, is the thermodynamically favored product (Scheme 14). 

It is explosive and a super-electrophile, like 4.6-dinitrobenofurazan -oxide. See other N-OXIDES, POLYNITROARYL COMPOUNDS, TRIAZOLES... 

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]... 

Salts with carbenium ions and SbFg as counteranion have already been described, such as magic methyl CHj SbFg. However, these super-electrophilic species can be involved in electrophilic additions to double bonds and carbenium cations, and are therefore not suitable for fluorination reactions. 

Super-electrophilic lOrt heteroaromatics. New mechanistic and synthetic appHcations 120PP289. 

A theoretical study of the hydrolysis of arenediazonium ions, combining Marcus theory with density functional theory (DPT) calculations, indicates the possibility of reaction by an 5 2 pathway, with the transition state (1), rather than an 5 1 pathway There has been a survey of the use of arenediazonium ions in synthetic reactions including their use as aryl radical precursors, aryl cation precursors, and super-electrophiles in transition-metal-catalysed reactions ... 

Cyclization of IV-allyl anilines (64) in superacidic medium has been reported to afford anti-Markovnikov products (65). Investigation by the in situ NMR spectroscopy, DFT calculations, and reactions with labelled substrates suggest that new ammonium-carbenium super-electrophiles are involved as intermediates. ... 

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