HOMO-LUMO interactions substitution

An HSAB analysis of singlet carbene reactivity based on B3LYP/6-31G computations has calculated the extent of charge transfer for substituted alkenes,122 and the results are summarized in Figure 10.3 The trends are as anticipated for changes in structure of both the carbene and alkene. The charge transfer interactions are consistent with HOMO-LUMO interactions between the carbene and alkene. Similarly, a correlation was found for the global electrophilicity parameter, co, and the ANmax parameters (see Topic 1.5, Part A for definition of these DFT-based parameters).123... 

Many aspects of intramolecular nitrile oxide cycloadditions are similar to those of the intermolecular ones. Due to the proximity of the reacting groups, however, there are also several items that differ significantly. While HOMO-LUMO interactions and steric effects direct the intermolecular nitrile oxide cycloaddition to 1-alkenes to produce 5-substituted isoxazolines, the intramolecular cases often show a different behavior. With most of them, regioselectivity is determined by geometric constraints and cycloadditions occur in the exo mode to furnish the annulated bicycle (Scheme 6.42). 

Thus far, in the alkaloid series discussed, the nitrogen atom has always been part of the core of the alkaloid structure, rather than acting in a dipolarophilic manner in the cycloaddition of the carbonyl ylide. Recently, Padwa et al. (117) addressed this deficiency by conducting model studies to synthesize the core of ribasine, an alkaloid containing the indanobenzazepine skeleton with a bridging ether moiety (Scheme 4.57). Padwa found that indeed it was possible to use a C = N it-bond as the dipolarophile. In the first generation, a substituted benzylidene imine (219) was added after formation of the putative carbonyl ylide from diazoketone 218. The result was formation of both the endo and exo adduct with the endo adduct favored in an 8 1 ratio. This indicates that the endo transition state was slightly favored as dictated by symmetry controlled HOMO—LUMO interactions. 

Figure 10.16 The electrophilic bimolecular substitution can occur from the front, because symmetry permits HOMO-LUMO interaction and stabilization.

Pyrolysis of organoboranes at > 200°C can lead to substitution of hydrogen with boron accompanied by evolution of Hj [see Eq. (e), 5.3.2.6]. Little is known about the mechanism, but it does not involve radicals. A single-bonded version of hydroboration may be taking place on the C—H bond, in which the HOMO-LUMO interactions shown in I and II are important. 

Electron-donor and electron-acceptor substituents selectively interact with different ring orbitals. Compare the HOMO and LUMO of azobenzene with the corresponding orbitals of the two substituted molecules. Which orbitals show signficant substituent contributions What are the nature of these contributions, bonding or antibonding Try to relate this to the effect which the substituents have on orbital energies and on the HOMO-LUMO gap in azobenzene. 

The alkene substituted with the electron accepting group has the LUMO (it ) lowered by the interaction with the vacant orbital of the substituent. The high-lying SOMO interacts with the LUMO of the alkene more effectively than with the HOMO. The interaction is the symmetry-allowed it - n interaction (Scheme 30a). The configuration of the alkene is retained. 

The Z-substituted benzene (benzaldehyde, Figure 11.2) is not activated toward electrophilic attack since the HOMO of benzene is scarcely affected. No preferred site for attack of the electrophile can be deduced from inspection of the HOMOs. The interaction diagram for a Z-substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with a formyl substituent are shown in Figure 11.4. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 9.204 / , 6a — 9.2031/ , and 6a -9.1291/ , respectively. Thus, by SHMO, the ortho and meta channels are favored over the para channel, with no distinction between the ortho and meta pathways. Experimentally, meta substitution products are usually the major ones, contrary to the SHMO predictions. Either the SHMO method fails in this case or the predominance of meta products may be attributed to steric effects. 

A series of silyl and disilyl substituted thioketene dimers (2,4-diylidene-1,3-dithietane), including polymers, have been synthesized (33, 34) [117]. HOMO-LUMO calculation by PM3 semi-empirical molecular orbital method predicted an intramolecular CT from the thioketene dimer to the Si-Si bond in disilyl thioketene dimer (33a) and no electronic interaction between the... 

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