HOMO—LUMO transition substituent effect

Several oxovanadium(IV) complexes (141) with ON-donor ligands were prepared from [YOCl2(thf)2] (thf = tetrahydrofuran) and /// /////-substituted phenols to study the electronic effects of para substituents in order to develop better [VCl2(OR)2]-type olefin polymerization catalysts.676 The hyperfine coupling constants, the HOMO-LUMO transitions, and the oxidation potentials were all found to be linearly related to the Hammett a constant of the substituent on the monoanionic aryloxy ring.676... 

The first term of Equation 15.3 is responsible for most of the transition state stabilization of a Diels-Alder reaction with normal electron demand. In this case, the first term is larger than the second term because the denominator is smaller. The denominator of the first term is smaller because the HOMO of an electron-rich diene is closer to the LUMO of an electron-poor dienophile than is the LUMO of the same electron-rich diene with respect to the HOMO of the same electron-poor dienophile (Figure 15.24, column 2). Acceptors lower the energy of all 7F-type MOs irrespective of whether these MOs are bonding or antibonding. This is all the more true the stronger the substituent effects and the more substituents are present. 

Fig. 10.6. Schematic diagram illustrating substituent effect on reactivity in terms of FMO theory. HOMO-LUMO gap narrows, transition state is stabihzed, and reactivity is increased in normal electron-demand Diels-Alder reaction as the nucleophilicity of diene and the electrophihcity of dienophile increase.

Cycloadditions of ketenes and alkenes have been shown to have synthetic utility for the preparation of cyclobutanones.101 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.102 To be an allowed process, the [2n + 2n] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the transition state is a [2ns + (2ns + 2ns)] addition.103 Figure 6.6 illustrates these transition states. The ketene, utilizing its low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the... 

X is NR R2. The substituent is converted to a Z substituent via the low-lying a orbital, and the ring is deactivated toward further electrophilic attack. The ortho and para channels lead to products. The interaction diagram for an X -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 an amino substituent are shown in Figure 11.3. Notice that the LUMO of each is suitable to activate the C—H bond at the saturated site toward abstraction by the base. Curiously, the meta cation has the lowest LUMO and should most readily eliminate the proton. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 8.7621/ , 6a — 8.499 / , and 6a — 8.718 / , respectively. Thus the ortho and para channels are favored over the meta channel, and the ortho route is slightly preferred over the para route. Experimentally, para substitution products are often the major ones in spite of there being two ortho pathways. The predominance of para products is usually attributed to steric effects. 

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