Nucleophilic addition molecular

Information on nucleophilic addition chemistry of quinones and various mechanistic rationali2ations have been discussed, and molecular orbital calculations have been proposed as more definitive approaches for explanation and prediction (63). 

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

Display and describe the lowest-unoccupied molecular orbital (LUMO) for cyclohexenone. This is the orbital into which the nucleophile s pair of electrons will go. Does it anticipate both carbonyl and Michael products of nucleophilic addition Explain. A clearer picture is provided by a LUMO map for cyclohexenone. This gives the value of the LUMO on the accessible surface of the molecule, i.e., on the molecule s electron density surface. Does it anticipate both of the observed products If so, which should be the dominant Explain your choice. 

Next, examine the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of dichlorocarbene. Were the reaction a nucleophilic addition , how would you expect CCI2 to approach propene Were the reaction an electrophilic addition , how would you expect CCI2 to approach propene Which inteqDretation is more consistent with the geometry of the transition state ... 

The following molecular model represents a tetrahedral intermediate resulting from addition of a nucleoph ile to an aldehyde or ketone. Identify the reactants, and write the structure of the final product when the nucleophilic addition reaction is complete. 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

Self-consistent field molecular orbital calculations by Fenske and coworkers have confirmed that nucleophilic additions to Fischer and related complexes [e.g., (CO)sCr=CXY, (T)5-C5H5)(CO)2Mn=CXY], are frontier orbital-controlled rather than charge-controlled reactions (7-9). Interaction of the HOMO of the nucleophile with the carbene complex LUMO (localized on Ca) destroys the metal-carbon w-interaction and converts the bond to a single one. 

The high activating power of the furoxan ring in nucleophilic addition has also been observed by Bailey et al.2 9 in 7-nitro-l,2,5-oxadiazolo[3,4-c]-pyridine 3-oxide, a nitropyrido[3,4-c]furoxan that easily undergoes covalent addition of water by nucleophilic attack at the position para to the nitro group. The structure of the covalent hydrate is supported by elemental analysis, osmometric molecular weight determination, and H-NMR spectra in DMSO-[Pg.429]

The molecular mechanisms for the nucleophilic addition of lithium enolates and silyl ketene acetals to nitrones in the absence and in the presence of a Lewis acid catalyst to give isoxazolidin-5-ones or hydroxylamines have been investigated by DFT methods at the B3LYP/6-31G level.13 An analysis of the global electrophilicity of the reagents accounts for the strong electrophile activation of the Lewis acid-coordinated nitrone, (g) and the analysis of the local indices leads to an explanation for the experimentally observed regioselectivity. 

Molecular dynamics simulations have been used to predict solvent and temperature effects in the nucleophilic addition of a-chiral carbonyl compounds.253 Prediction of diastereoselectivity break temperatures (i.e. inversion points) has been achieved with fair accuracy by comparison with experimental data on n-BuLi addition. Dramatic differences are seen for additions to 2-phenylpropanol in pentane solvent, compared with octane. 

The interception of the radical anion by add or the formation of 28 should have the same effect. In fact, the back electron-transfer reaction from (TCA ) to (DPA ) is suppressed, or at least reduced, maximizing the cage escape efliriency of the radical cation and its further reaction with molecular oxygen. In these reactions benzil 26 is probably formed via the dioxete intermediate, also suggested by de Mayo and co-workers [100], whereas the mechanism leading to benzoic add is still uncertain. Other examples of photoinduced oxygenations, preceded by nucleophilic addition, have been also reported. 

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