Covalent anionic adducts, formation anion

In going to polyaza derivatives, the tendency to form adducts decidedly increases. Very reactive compounds, such as 1,2,4-triazines and pteridines, undergo covalent addition in liquid ammonia alone, i.e., without any added amide salts, presumably by a two-stage process via anionic ff-adduct formation. 

Analysis of the spectrum indicated that we are dealing here with the spectrum of 4-aminodihydro-l,5-naphthyridinide (3). The results obtained by 13C-NMR spectroscopy support the formation of 3 (see Table III). From these data it is evident that the covalent amination of 1 by the amide anion is subjected to a kinetic and thermodynamic control. At low temperature ( — 40 C) the kinetically favored cr-adduct (2) is formed, and at higher temperature (10 C) the thermodynamically favored one (3) is formed. 

As with other aromatic substitutions, the substitution step itself can be considered to involve an approximately sps hybridization at the carbon atom under attack (10). In the idealized substitution process shown in Eq. (16), 10 may constitute either an intermediate or a transition state. If proton loss ensues directly, the process is properly called a substitution. In other situations the intermediate 10 may become allied with a radical or an anion, leading thereby to a covalent adduct 11. The final substituted product 12 may then be formed either by the elimination of H—Z (first H, then Z) or by the reversal to 10, followed by proton loss. The first case is a classical example of an addition-elimination halogenation, where the adduct is an essential species in the process. In the second case, structure 10 is a common intermediate for both the substitution and the addition reactions. Being merely a diversion of 10, the addition product is not essential to the substitution. In consequence of this, the isolation of adduct 11 may not mean that addition-elimination is the principal pathway of substitution reversal to 10 may be faster than the elimination of H—Z ( 2, k3>ki). On the other hand, the mere failure to detect adduct 11 does not rule out an addition-elimination process, for dehydrohalogenation of adduct 11 may be much faster than its formation (ki>klt k2). 

When an appropriate alkene is added, color and conductance disappear due to the formation of covalent adducts (Scheme 34). For the success of the method it is essential that the chloride transfer from the complex anion to the new carbocation is fast and complete, because only then the selective formation of 1 1 products and the controlled decay of absorbance and conductance is warranted. Ideally, both quantities (absorbance and conductance) yield the same dependence of carbocation concentration on time, and the second-order rate law (18) is generally obeyed. 

Termination also occurs if ligand exchange produces a weaker Lewis acid which is unable to reactivate the covalent adduct. In this case, addition of another aliquot of the stronger acid may reactivate the system. For example, hydroxy or alkoxy groups are exchanged in polymerization systems based on boron trifluoride and adventitious water or alcohol, respectively. Termination occurs infrequently by irreversible cleavage of B—F bond from the BF3OH anion, with irreversible C—F bond formation [Eq. (127)]. 

Figure 13.10. Structure of daunorabicin (daunomycin) and its intercalation into DNA (a), and catalytic formation of peroxide anions by iron complexed to it (b). Reactive oxygen species released in the immediate vicinity of the DNA will form strand breaks and also promote formation of covalent adducts of daimorabicin and DNA that involve the amino group of the former.

One-electron transfer from the substrate amino group to flavin (FI) results in the formation of the aminium radical and the flavin radical anion (FC) (Scheme 15). Deprotonation of the aminium radical to yield an a-aminoalkyl radical followed by a second electron transfer to the flavin radical anion will result in the formation of the reduced flavin and iminium ion. Alternatively the iminium ion can be formed by path d in Scheme 15 this involves formation of a covalent adduct which can... 

By considering the above-mentioned solution studies and the refined three-dimensional structure of the S. cerevisiae flavocytochrome 62 active site, Lederer and Mathews proposed a scheme for the reverse reaction (the reduction of pyruvate) (39). They did not discuss how the transfer of electrons took place except to say that the structure did not rule out the possibility of a covalent intermediate (39). Ghisla and Massey (116) considered the anionic flavin N5 to be too close to the pyruvate carbonyl (3.7 A) without the formation of a covalent adduct taking place. Covalent intermediates between substrate and flavin have been observed for lactate oxidase (117, 118) and o-amino acid... 

The mechanism of decarboxylation of pyruvic acid is shown in Scheme Xll. The first step is formation of the anion, which then adds to C-2 of pymvate, forming a covalent adduct. This compound has been prepared chemically and its... 

Thiols may be oxidised to disulphides by the intermediate formation of covalent 4a flavin-sulphur adducts subsequent nucleophilic attack by another thiol anion displaces dihydroflavin as a good leaving group (XXXXIV). 

Cooperativity in binding potassium cyanide is shown in the system reported by Miyaji et al.P in which the receptor, a ferrocenyl derivative decorated with a crown ether and a trifluoroacetylcarboxanilide group, 4, recognizes anions by the formation of a reversible covalent adduct. 

Figure 23 The relationship of ion formation and ionization efficiency with the electrical propensity of an analyte. The formation of an adduct ion of a covalent-linked polar compound depends on the availability of the small cation (X ) or anion (T") in the solution and the affinity of X (or Y ) with the analyte.

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