Nucleophilic site

On curing, amino resins not only react with the nucleophilic sites (hydroxyl, carboxyl, amide) on the other film formers in the formulation, but also self-condense to some extent. Highly alkylated amino resins have less tendency to self-condense (33,34) and are therefore effective cross-linking agents, but may require the addition of a strong acid catalyst to obtain acceptable cure even at bake temperatures of 120—177°C. 

Protonation or Lewis acid complexation of a heteroatom invites nucleophilic attack, including nucleophilic attack by a parent molecule. Oligomerization and polymerization are thus often the results of bringing heterocycles into an acid environment without making sure that all of the potentially nucleophilic sites are protonated. 

Nucleic acids in the DNA contain a high number of nucleophilic sites that can be attacked by electrophilic intermediates (metabolites) of chemical compounds. DNA adducts formed may cause alterations in the expression of a critical gene in the cell and thus lead to cell death. For example, modification of p53 tumor suppressor gene may inactivate the functions of the p53 protein and render cells sensitive to malignant transformation. Also, formation of RNA adducts may inhibit key cellular events because RNA is essential for protein synthesis. 

According to the resonance picture, where is the excess negative charge in azide anion Will the center nitrogen or a terminal nitrogen act as the nucleophilic site Examine atomic charges and the electrostatic potential map. Do they substantiate your conclusion Explain. 

Compare atomic charges for the enolate anion and the lithium salt. Are there major differences, in particular, for the oxygen and the a carbon Also compare the highest-occupied molecular orbital (HOMO) in the two molecules. This identifies the most nucleophilic sites, that is, the most likely sites for attack by electrophiles. Are the two orbitals similar or do they differ substantially Elaborate. 

HOMO of cyclohexanone enolate reveals most nucleophilic sites. 

HOMO of the potassium enolate of ethyl acetoacetate is the electron-acceptor orbital and reveals most nucleophilic sites. 

HOMO of cyclohexanone lithium enolate reveals the most nucleophilic sites. 

Electrostatic potential map for amine-amide shows negatively-charged regions (in red) as likely nucleophilic sites. 

Similar to phosphur ylides, sulfur ylides 1 and 2 possess the nucleophilic site at the carbon atom and the pendant leaving group at the heteroatom (sulfur). Different from the Wittig reaction, the Corey-Chaykovsky reaction does not lead to olefins. 

An intramolecular ring expansion of aziridine esters can be accomplished by installing an appropriate nucleophilic entity in these substrates. Conversion of the ester moiety into carboxamides derived from aminomalonate ester gives compounds 44 containing the requisite nucleophilic site in the malonate moiety (Scheme 35). 

Nitrosamines require metabolic activation in order to produce a chemical species that will alkylate nucleophilic sites on a biomolecule such as DNA (1, . The crucial initial step in the... 

Since the results of our experiments with isolated rat liver fractions supported a reaction sequence Initiated by microsomal oxidation of the nitrosamine leading to formation of a carbonium ion, the results of the animal experiment suggested that in the intact hepatocyte, one of the earlier electrophilic intermediates (II, III or V, Figure 1) is intercepted by nucleophilic sites in DNA (exemplified here by the N7 position of guanine) before a carbocation is formed. 

Nucleophilic Sites in DNA and the Factors that Determine the Sites... 

Alkylation at several sites on the DNA bases affords an attachment that is chemically stable. Nucleophilic sites that typically yield stable alkyl attachments include the exocyclic nitrogen atoms ISPG, N A, N C, the amide-type nitrogens NIG, NIT, 0 G, and Alkyl groups on phosphate residues in DNA are typically stable... 

DNA alkylation has the potential to yield a time-dependent spectrum of adducts, in which initially formed kinetically favored lesions give way to the thermodynamically favored adducts over time. Reversible alkylation has been observed at several of the nucleophilic sites in DNA, including N3A (CC-1065,7, Scheme 8.10, duocarmycin, 8)," " NIA (qui-none methide, 9)," N7G (leinamycin, Schane 8.11, aflatoxin Bj epoxide, 10 and quinone methide, 9),57.ii4.ii8 (quinone methide, 9)," and bPG (ecteinascidin 743,11)." The bidentate Nl/ISPG adduct of malondialdehyde also forms reversibly. ... 

The (3-carbon atom of an enamine is a nucleophilic site because of conjugation with the nitrogen atom. Protonation of enamines takes place at the (3-carbon, giving an iminium ion. 

Part B of Scheme 4.5 gives some examples of cyclizations induced by selenium electrophiles. Entries 9 to 13 are various selenyletherifications. All exhibit anti stereochemistry. Entries 14 and 15 are selenyllactonizations. Entries 17 and 18 involve amido groups as the internal nucleophile. Entry 17 is an 5-exo cyclization in which the amido oxygen is the more reactive nucleophilic site, leading to an iminolactone. Geometric factors favor N-cyclization in the latter case. 

The 2-azonia analog of the Cope rearrangement is estimated to be accelerated by 106, relative to the unsubstituted system.270 The product of the rearrangement is an isomeric iminium ion, which is a mild electrophile. In synthetic applications, the reaction is often designed to generate this electrophilic site in a position that can lead to a cyclization by reaction with a nucleophilic site. For example, the presence of a 4-hydroxy substituent generates an enol that can react with the iminiun ion intermediate to form a five-membered ring.271... 

The effect of the aqueous medium on the reactivity and on the stability of the resulting adducts has been investigated to assess which adduct arises from the kinetically favorable path or from an equilibrating process. The calculations indicate that the most nucleophilic site of the methyl-substituted nucleobases in the gas phase is the guanine oxygen atom, followed by the adenine N1, while other centers exhibit a substantially lower nucleophilicity (see activation Gibbs energies in Table 2.2). 

The N7 of dG is one of the most nucleophilic sites within duplex DNA and is relatively accessible within the major groove.59 However, QM-dependent alkylation... 

In keeping with the anisotropic electrostatic potential model for the halogen atoms, halogens show an amphiphilic character and can work both as the electrophilic sites and the nucleophilic sites when involved in short contacts (Fig. 3). 

The importance of hydrophobic binding interactions in facilitating catalysis in enzyme reactions is well known. The impact of this phenomenon in the action of synthetic polymer catalysts for reactions such as described above is significant. A full investigation of a variety of monomeric and polymeric catalysts with nucleophilic sites is currently underway. They are being used to study the effect of polymer structure and morphology on catalytic activity in transacylation and other reactions. 

Pad-dry-cure for 3 min at 100 °C. Scheme 10.62 represents the reactive and nucleophilic sites that may exist on the surface of the treated fibre... 

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