Anion abstraction

Other gas phase reactions can occur under Cl conditions. They are anion abstraction, charge exchange and electrophilic addition. Furthermore, Cl can be used to produce negative ions, such as [M-H] or radical anions M. These reactions are beyond the scope of this chapter and more specialized literature is recommended. [13]... 

Shida and Hamill23 found that the positive and negative molecular ions of 1,3-butadiene and its homologs have similar absorption spectra. Band maxima of the anions are not sensitive to substituent alkyl groups, whereas those of the cations are red-shifted as the number of substituent methyl groups increases. In alcoholic matrices the butadiene anions abstract the alcoholic proton to form an allylic radical (equation 23), as was proven by ESR spectroscopy. 

This is easiiy formuiated as production of an enoiate anion foiiowed by nucieophiiic attack of this anion on to the carbonyi group of a second moiecuie of acetaidehyde. Aidoi is then produced when the addition anion abstracts a proton from soivent. The reaction is reversibie, and it is usuaiiy necessary to disturb the equiiibrium by some means. Removai of product is possibie, but, as seen beiow, the dehydration part of the sequence may be responsibie for pushing the reaction to compietion. 

The reaction is usually performed with homogeneous basic catalysts such as alkali hydroxides, alkoxides, and tetraalkyl ammonium hydroxide (161,162). The mechanism accepted for this transformation starts with the abstraction by the base catalyst of a proton from the hydroxyl group of the alcohol to generate the alkoxide anion, which reacts with acrylonitrile to form the 3-alkoxypropanenitrile anion. The 3-alkoxypropanenitrile anion abstracts a proton from the catalyst to yield 3-alkoxypropane nitrile. 

MgCl+ ions undergo anion abstraction reactions with organic halides (equation and nitric acid (equation 31). 

Step 3 Completion of addition phase. The aryl anion abstracts a proton from the ammonia used as the solvent in the reaction. 

Initiation takes place by rapid reaction of an ammonium salt with the anhydride (Eq. (46)) whereby ammonium carboxylate is formed. In the propagation step, the carboxylate anion opens an epoxy ring and forms an ammonium alcoholate (Eq. (47)). The latter reacts with the anhydride to yield another ester bond, and ammonium carboxylate is recovered (Eq. (48)). Termination occurs through decomposition of the ammonium counter ion, the alkoxide anion abstracting a proton from the quaternary nitrogen with the formation of a deactivated tertiary amine. 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

The mechanism of the Birch reduction (shown next) is similar to the sodium/liquid ammonia reduction of alkynes to fnmy-alkencs (Section 9-9C). A solution of sodium in liquid ammonia contains solvated electrons that can add to benzene, forming a radical anion. The strongly basic radical anion abstracts a proton from the alcohol in the solvent, giving a cyclohexadienyl radical. The radical quickly adds another solvated electron to form a cyclohexadienyl anion. Protonation of this anion gives the reduced product. 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

In addition, the endo double bonds activate neighboring hydrogen atoms or alkyl groups to hydride and alkyl anion abstraction, respectively, to produce tertiary carbenium ions stabilized by both the double bond and the a-substituent [Eq. (91)]. 

Platinum-silylenes are important to numerous transformations involving organosilicon compounds. Base-stabihzed Fisher-type cationic silylene complex of platinnm(II) may be prepared by anion abstraction from a silyl group, as shown in Scheme 51. The counter anion is cracial... 

Chemical ionization (Cl) is one of the most versatile ionization techniqnes as it relies on chemical reactions in the gas phase [6]. Cl is an important ionization techniqne in LC-MS. It is based on ion-molecule reactions between reagent-gas ions and the analyte molecules. Gas-phase ion-molecule reactions comprise proton transfer, charge exchange, electrophilic addition, and anion abstraction in positive-ion Cl and proton transfer (abstraction) in negative-ion Cl. Cl can be performed nndervarions pressnre conditions ... 

More recently DDT-dehydrochlorinase has been isolated and purified ( 660-fold) to apparent homogeneity from houseflies (49). In contrast to that described in earlier studies, this enzyme was found to be a dimer with subunits of molecular weights of 23,000 and 25,000. It was also found to possess substantial GSH-S-transferase activity towards 2,4-dinitrochlorobenzene and 3,4-dichloro-nitrobenzene. Based on its structure, catalytic activity and chromatographic behavior it was concluded that the purified heterodimeric DDT-dehydrochlorinase was indeed a GSH-S-transferase isozyme (49). It was proposed that instead of the nucleophilic substitution usually observed in GSH-S-transferase activity, DDT-dehydrochlorination by this enzyme involves an E2 elimination reaction in which the GS thiolate anion abstracts the hydrogen on the C-2 of DDT and this initiates the departure of the chlorine atom from C-1 ( 9) (Figure 9). 

Scheme 6.273 illustrates the photocatalytic addition of a tertiary amine (571) to the furanone 572,1474 which is initiated by electron transfer from the amine to a photoexcited benzophenone to form a radical ion pair.1475 The ketyl radical anion abstracts a proton from an amine radical cation and subsequently donates a hydrogen atom to the previously coupled radical intermediate 573 to regenerate the benzophenone molecule and to give the final addition product 574 (<94% chemical yield). 

The first step in the mechanism of this reaction is transfer of the s orbital electron from sodium (or lithium) to an sp carbon to form a radical anion—a species with a negative charge and an unpaired electron. (Recall that sodium and liAium have a strong tendency to lose the single electron in their outer-shell 5 orbital Section 1.3.) The radical anion is such a strong base that it can remove a proton from ammonia. This results in the formation of a vinylic radical—the unpaired electron is on a vinylic carbon. Another single-electron transfer from sodium (or lithium) to the vinylic radical forms a vinylic anion. The vinylic anion abstracts a proton from another molecule of ammonia. The product is the trans alkene. 

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