Reactions Hiickel calculations

The orbital coefficients obtained from Hiickel calculations predict the terminal position to be the most reactive one, while the AMI model predicts the Cl and C3 positions to be competitive. In polyenes, this is true for the addition of nucleophilic as well as electrophilic radicals, as HOMO and LUMO coefficients are basically identical. Both theoretical methods agree, however, in predicting the Cl position to be considerably more reactive as compared to the C2 position. It must be remembered in this context that FMO-based reactivity predictions are only relevant in kinetically controlled reactions. Under thermodynamic control, the most stable adduct will be formed which, for the case of polyenyl radicals, will most likely be the radical obtained by addition to the C1 position. 

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

To summarize, the HSAB principle is a very good first approximation but is usually inadequate for detailed analysis of reaction mechanisms. This is not really surprising. After all, this principle is nothing else than a two parameters relationship each reactant is characterized by its acidic or basic strength and by its hardness (softness). And obviously, we cannot expect to describe the complexity of chemistry with only two parameters. On the other hand, one should not underestimate its utility. Simple Hiickel calculations are also a two parameters treatment where the initial choice of the coulombic and resonance integrals a and )3 is critical. There is no doubt however that, handled with care, these calculations may give valuable insights. The same may be said for the HSAB principle. 

Hiickel calculations have been employed extensively in other approaches such as the angular overlap model and the method of moments developed by Burdett and coworkers. Stabilities of crystal structures, pressure- and temperature-induced transitions, dynamical pathways in reactions and other phenomena have been analysed using angular overlap models. Thus, the electronic control of rutile structures and the stability of the defect structure of NbO have been examined (Burdett, 1985 Burdett Mitchell, 1993). In the case of NbO, the structure is stable at involving the formation... 

The complexes with the coordination mode (5) have a characteristic IR frequency in the region 1000-1200cm-1 (v(C—S, exocyclic)). They are generally prepared by ligand substitution. A few X-ray structures confirm the coordination mode of these complexes,9 e.g. compound (6) (C—S 147.8, C—S 172, Pt—S 232.8, Pt—C 206.3 pm, S—C—S 136.2° C—Pt—S 45.5°.10 Extended Hiickel calculations show that a reaction path starting from an end-on approach of the CS2 molecule is energetically most accessible.10b... 

The unsubstituted metallacyclobutane formed from Ti(=CH2)(Cl)2 +CH2=CH2 is calculated to have a planar but easily puckered ring. Even a substituent in the 2-position (opposite to Ti which is numbered 4) is known to cause very little puckering124. However, in 1,3-disubstituted tungstacyclobutanes, extended Hiickel calculations show that the ring has a puckered ee configuration, as required by the interpretation of the cis/trans stereoselectivity in the metathesis reactions of alk-2-enes148 (see Section IV). 

FO Study of Ionic Reactions Using Hiickel Calculations... 

For example, reactions which have been treated by MNDO and PM3 methods include Cheney B. V., J. Org. Chem., 1994, 59, 773 Matsuoka T., Harano K., Hisano T., Heterocycles, 1994, 37, 257.1 have been unable to reproduce the experimental results using Hiickel calculations. 

For benzo[b]furan and indole no such precise data are available, but it is possible to adduce some information from the various reactions described below. The positional reactivity orders for these molecules and also for benzo[b]thiophene, which have been calculated by various methods, are given in Table 8.1. In principle the ab initio calculations should be the more reliable, but neither the tt nor the (a + it) order is correct for benzo[6]thiophene, suggesting that these are incorrect for the other molecules also. The calculations using the STO-3G basis set certainly wrongly predict the site of most rapid protonation. Notably, only the Hiickel calculations give the correct order for benzo[b]thiophene and indeed they are usually the most reliable indicators for electrophilic aromatic substitution. 

This suggests that the C-Fe bond remains intact as the C-C bond forms, consistent with the results of an extended Hiickel calculation of the reaction coordinate for migration of Me in MeMn(CO)5. ... 

In the photoaddition of acetone and other ketones to 1-, 2- and 1,2-di-methylimidazoles the products sire a-hydroxyalkylimidazoles (153) which are derived from the selective attack of excited carbonyl oxygen at C-5. In the case of 2-methylimidazole the products are the 4-mono- (8%) and 4,5-di- (14.5%) substituted compounds, but imidazole itself does not react. The suggestion that it is not a sufficiently electron-rich substrate is not particularly convincing. The reaction mechanism (Scheme 72) may reflect the greater radicd reactivity at C-5, and the comparative stabilities of the radical intermediates derived from carbonyl attack at this position. Hiickel calculations of radical reactivity indices show that, indeed, C-5 is more reactive, and the radical intermediate at C-5 is more stable than that at C-4, but a concerted cycloaddition could also give rise to the oxetane (152). Such an oxetane can be isolated in the photochemical addition of benzophenone to 1-acetylimidazole. 

The catalytic activity of the mixed-bridged dinuclear complex [Rh2( i-pz)(/ -S-f-Bu)(CO)2 P(OMe)3 2], 111, in the selective hydroformylation of 1-hexene to afford the corresponding aldehydes was explored and the results were compared with those observed for the symmetrical pyrazolate-free precursor [Rh2(CO)2(ft-S-f-Bu)2 P(OMe)3 2] (117, 212). Very mild conditions (5 bar, 80°C) were chosen in both cases. Complex 111 exhibits lower activity however, the selectivity is about the same as for the symmetrical precursor ( 80% heptanal and 20% 2-methylhexanal). Complex 111 does not give rise to a detectable dismutation and was found to be practically unchanged after reaction. A catalytic cycle in which all the intermediate species remain dinuclear was proposed for the hydroformylation of 1-hexene catalyzed by the symmetrical complexes [Rh2(CO)2(M-S-f-Bu)2L2] [L = P(OMe)3, PPh3, P(OPh)3] (213). This proposal is based mainly on the results obtained by varying the phosphorus ligands and on extended Hiickel calculations carried out for several proposed intermediates. It is possible that Complex 111 could follow such a cycle (117). 

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