Abstraction electrophilic

Boranes undergo a variety of reactions, such as proton abstraction, electrophilic substitution, fragmentation and adduct formation. Some of these reactions are highlighted below with selective examples. 

Specifically, hydroxyl radicals can oxidize organics by hydroxylation, hydrogen abstraction, electrophilic addition, and electron transfer, depending upon the nature of organic compounds. 

The higher propensity of excited azoalkanes to abstract electrophilic hydrogens is not only manifested in the strong quenching by protic 0-H bonds of water and alcohols, but also in the efficient interaction with the C-H bonds of acetonitrile and chloroform (cf. Scheme 3.6) [47,54,56,65,71], The latter are unreactive toward excited ketones and are broadly employed as photochemically inert solvents. 

The resulting macrocyclic ligand was then metallated with nickel(II) acetate. Hydride abstraction by the strongly electrophilic trityl cation and proton elimination resulted in the formation of carbon-carbon double bonds (T.J. Truex, 1972). 

With the exception of the nuclear amination of 4-methylthiazole by sodium amide (341, 346) the main reactions of nucleophiles with thiazole and its simple alkyl or aryl derivatives involve the abstraction of a ring or substituent proton by a strongly basic nucleophile followed by the addition of an electrophile to the intermediate. Nucleophilic substitution of halogens is discussed in Chapter V. 

CgH COO from BPO. The first type involves direct radical displacement on the oxygen—oxygen bond and is the preferred mode for nucleophilic radicals, eg, -CH(R)OR7 The second type involves radical addition to, or abstraction from, the hydrocarbyl group adjacent to the peroxide this is the preferred mode for electrophilic radicals, eg, Cl C (eq. 32). In the last type (eq. 33), there is hydrogen donation from certain hydrogen-donating radicals, eg, ketyls (52,187,188,199). 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

The pyrazole molecule resembles both pyridine (the N(2)—C(3) part) and pyrrole (the N(l)—C(5)—C(4) part) and its reactivity reflects also this duality of behaviour. The pyridinic N-2 atom is susceptible to electrophilic attack (Section 4.04.2.1.3) and the pyrrolic N-1 atom is unreactive, but the N-1 proton can be removed by nucleophiles. However, N-2 is less nucleophilic than the pyridine nitrogen atom and N(1)H more acidic than the corresponding pyrrolic NH group. Electrophilic attack on C-4 is generally preferred, contrary to pyrrole which reacts often on C-2 (a attack). When position 3 is unsubstituted, powerful nucleophiles can abstract the proton with a concomitant ring opening of the anion. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

Another important feature of the Nef reaction is the possible use of a CH-NO2 function as an umpoled carbonyl function. A proton at a carbon a to a nitro group is acidic, and can be abstracted by base. The resulting anionic species has a nucleophilic carbon, and can react at that position with electrophiles. In contrast the carbon center of a carbonyl group is electrophilic, and thus reactive towards nucleophiles. 1,4-Diketones 4 can for example be prepared from a-acidic nitro compounds by a Michael additionfNef reaction sequence " ... 

In the first step, the fairly acidic proton on CIO of the red biladiene-ac salt 6 is abstracted and, even in solution in polar solvents, the salts are converted into the corresponding yellow bilatriene-u/ic salts 7. With a base such as piperidine, the salts 7 form the green bilatriene-a/>e free base. For further reaction to the porphyrin it is important that the salts 7 are oxidized to the bilatriene enamines 8 which cyclize via the electrophilic carbon of the terminal pyrrole ring by the loss of the leaving group X to 9. Porphin (10) is finally obtained by the loss of... 

The synthesis of alkoxy amines 2 by addition of organometallic reagents to the C-N double bond of oxime ethers 1 is plagued by the propensity for proton abstraction a. to the C-N double bond, the lability of the N-O bond and the poor electrophilicity of the oxime ethers. Therefore, frequently no products, undesired products or complex mixtures are obtained. The result depends on the substrate, organometallic reagent, solvent, temperature and additives1 6. 

Several studies338,340-342 show that the chlorination does not proceed, as assumed previously343, by proton abstraction followed by reaction of the carbanion thus formed, with electrophilic chlorine. A mechanism involving a chlorooxosulfonium ion formed by attack of a positive chlorine species on sulfur was shown to be more likely344. 

Abstract The dirhodium(II) core is a template onto which both achiral and chiral ligands are placed so that four exist in a paddle wheel fashion around the core. The resulting structures are effective electrophilic catalysts for diazo decomposition in reactions that involve metal carbene intermediates. High selectivities are achieved in transformations ranging from addition to insertion and association. The syntheses of natural products and compounds of biological interest have employed these catalysts and methods with increasing frequency. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

In proceeding to a consideration of the chemical ionization mass spectra of more highly branched paraffins, it will be most convenient to consider separately the several different classes of alkyl ions found in the spectra—i.e., MW — 1+, MW — 15+, MW — 29 +, etc. We can see from Table II that a considerable amount of variation in the relative intensity of the MW — 1 ions (always the highest mass ion for which an intensity is given in the table) occurs. However, we shall show that the observed MW — 1 intensities can be approximately accounted for in terms of the concept of localized electrophilic attack by the reactant ion. First, however, we must consider the energetics of two processes which may be important in generating the spectra of branched paraffins. One of these is the abstraction of a primary hydrogen by the reactant ion. As a typical example we may write... 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

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