Hiickel calculations nucleophilic attack

The molecular orbital structure of 1,2,3-benzotriazine has been calculated using the Hiickel method, and the energy levels, charge densities, and wave functions were obtained.Theoretically the charge densities thus obtained should be of some value in predicting the positions of electrophilic and nucleophilic attack, but in the absence of information on reagents and reaction conditions, such predictions cannot be made with any degree of accuracy. 

Calculated reactivity indices are at one with qualitative electronic theory in accounting for ready nucleophilic attack at C-6 in the phenanthridine molecule. However, the limited data available on positional reactivities in electrophilic substitutions is not accounted for satisfactorily by any of the available treatments (see later) and it has been pointed out that the simple Hiickel treatment used by some authors,223, 220 is generally inapplicable in heteroaromatic systems.228... 

However, if we look at the LUMO, we find that it has the form 4.65, namely that of ift4 of benzene, but polarised by the nitrogen atom. This polarisation has reduced the coefficient at C-3, and the coefficient at C-4 is larger than that at C-2, as can be seen from the simple Hiickel calculation for pyridine itself in Fig. 4.11, which gives LUMO coefficients of 0.454 and —0.383, respectively, and an energy of 0.56/3 (compare benzene with 1/3 for this orbital). Thus, soft nucleophiles should attack at C-4, where the frontier orbital term is largest. Again this is the case cyanide ion, bisulfite, enolate ions, and hydride delivered from the carbon atom of the Hantsch ester 4.67 react faster at this site. 

Dienamines differ from simple enamines in that (i) there is an additional nucleophilic site at the -position and (ii) an equilibrium mixture of three isomeric dienamines is frequently formed, consisting of the linear s-trans isomer 1, the linear s-cis isomer 2 and the cross-conjugated isomer 31 (Scheme 1). A variety of factors influence the outcome of electrophilic attack on such an equilibrium system. Hiickel molecular orbital calculations indicate a significantly higher electron density at the -positions compared to the -positions of the dienamine system2 (Figure 1). 

The striking difference in reactivity and stability of the 4//-l,3,4-thiadiazin-5(67/)-one (15 X = O) and the 5-ol (16) have been discussed in terms of resonance stabilization based on spectral data and extended Hiickel MO calculations <87NKZ1312>. The lactam exhibits typical amide resonance (15<- 15a) and is resistant to attack by electrophiles, nucleophiles, and free radicals, whereas the reactivity of the 5-ol (16) towards displacement of the hydroxy group by nucleophiles is attributed to resonance stabilization of the thiadiazinium cation (16a) formed by heterolysis of the C—OH bond. Extended Hiickel MO calculations on the thione analogue of the thiadiazinone, coupled with spectroscopic data, indicate that this system is best represented as the thione (15 X = S) <89CE797>. 

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