Frontier-orbital-controlled process

A detailed interpretation of these results has not been given. However, nucleophilic substitution at silicon can be accepted as a frontier orbital controlled process between the HOMO of the nucleophile and the antibonding orbital as LUMO165. There are important differences in energy levels between octahedral and pentagonal complexes, and molecular orbital calculations might suggest a rationalization of these differences. 

Another aspect of qualitative application of MO theory is the analysis of interactions of the orbitals in reacting molecules. As molecules approach one another and reaction proceeds, there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the new product (or intermediate in a multistep reaction) is formed. PMO theory incorporates the concept of frontier orbital control. This concept proposes that the most important interactions will be between a particular pair of orbitals. These orbitals are the highest filled oihital of one reactant (the HOMO, highest occupied molecular oihital) and the lowest unfilled (LUMO, lowest unoccupied molecular oihital) orbital of the other reactant. The basis for concentrating attention on these two orbitals is that they will be the closest in energy of the interacting orbitals. A basic postulate of PMO... 

Again it has to be noted that the frontier orbitals participating in such a valence isomerization are delocalized over the whole molecule [22]. This has consequences for the orbital symmetry and, thereby, a prior analogy with comparable processes involving 6 t-electrons only is not given. However, compared with smaller Jt-systems the selection rules for orbital symmetry controlled processes in fullerenes seem to be less restrictive, since a large number of tt-orbitals with small energy separation are available. Calculations at the AM 1 and PM3 level show that the photocycKzation... 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

The Diels-Alder reaction of a diene and a dienophile has become one of the most powerful carbon-carbon bond-forming processes [81]. In normal Diels-Alder reactions of an electron-poor dienophile with an electron-rich diene, the main interaction is between the HOMO of the diene and the LUMO of the dienophile. Coordination of a Lewis acid to the dienophile reduces its frontier orbital energies, and this increases the rate of the reaction. Regio- and stereoselectivity are also markedly affected by the Lewis acid. Recent extensive studies on the design of chiral Lewis acids have led to fruitful results in the control of the stereochemistry of a variety of pericyclic reactions. Several chirally modified Lewis acids have been developed for the asymmetric Diels-Alder reaction [82,83] and spectacular advances have recently been achieved in this area. Various kinds of polymer-supported chiral Lewis acid have also been developed. Polymer-supported A1 Lewis acids such as 62 have been used in the Diels-Alder reaction of cyclopentadiene and methacrolein (Eq. 20) [84] as has polymer-supported Ti alkoxide 63 [84]. These Ti catalysts are readily prepared and have high activity in the Diels-Alder reaction. 

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