Resonance Anions

It, therefore, appears that the preferred nucleophilic center in resonant anions of the type shown in Scheme 4 is nitrogen rather than oxygen. 

Above 15eV the mechanism of chemical bonds dissociation in DNA irradiated with LEEs is probably dominated by direct excitation of dissociative electronically excited states [15], On the other hand, at lower energies the cleavage process is due to the formation of transient resonance anions [1, 2, 15-18], Thus, the SSB and DSB maxima on the yield function observed around 8 and 10 eV (Figure 21-3),... 

Inductive stabilization of charge in the resonating anionic penta-dienoid system is produced when an electron-attracting sp -nitrogen is next to one or more of the 1-, 3-, or 5-positions where the negative charge needs to be distributed in the transition state. Induction is considered by Taft to be a u-bond effect and by Dewar and... 

In pyrimidines, a 4-alkyl- is deprotonated more readily than a 2-aUcyl-gronp here again one sees the greater stability associated with a y-qninonoid resonating anion. Side-chain radical halogenation selects a pyrimidine-5-methyl over a pyrimidine-4-methyl the reverse selectivity can be achieved by halogenation in acid solntion - presnmably an iV-protonated, side-chain-deprotonated species, i.e. the enamine tantomer, is involved. ... 

The resonant anion can act as a typical carbanion and add to C=0 or C=N bonds, just as a Gtignard reagent does. In a true aldol condensation, the anion after formation proceeds to interact with another molecule of the starting aldehyde or ketone, giving a product that is a beta-hydroxy aldehyde or ketone known as an aldol (Scheme 4.29). Frequently, water is eliminated from this compound to produce an a, P-unsaturated carbonyl compound this process is made easy by the resonance stabilization of such conjugated systems. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

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