Frontier control reactivity

Aminothia2ole-3-oxides in neutral medium react differently on alky) isocyanates (355) and arylisocyanates (356) (Scheme 204) (684). The interpretation of this difference rests on the Concepts of charge control versus frontier control reactivity (1594 see also p. 63). Esters of... 

Klopman, G. 1968. Chemical reactivity and the concept of charge and frontier-controlled reactions. J. Am. Chem. Soc. 90 223-234... 

In order to understand the importance of frontier orbitals in chemical reactivity, Berkowitz [213] studied the frontier-controlled reactions within the purview of density functional theory. It is evident that the directional characteristics of frontier orbitals determine the extent of charge transfer, and soft-soft interactions are frontier-con-trolled. A somewhat similar analysis showed that charge transfer would be facilitated at a place where the difference in local softness of two partners is large [87], It may be noted that Fukui function is obtainable from local softness but the reverse is not true. On the other hand, local hardness suffers from the drawback of ambiguity [87], which allows one to even consider it to be equal to global hardness without disturbing their... 

When E% — E 0, interaction of the frontier orbitals becomes important, and a strong electron transfer takes place (frontier-controlled reaction). The reactivity... 

Jacobs A, Hoy T, Humphrys J, Perera P (1978) Iron overload in Chang cell cultures biochemical and morphological studies. Br J Exp Pathol 59 489-498 Kattamis C (ed) (1989) Iron overload and chelation in thalassemia. Huber Toronto Klopman G (1968) Chemical reactivity and the concept of charge- and frontier-controlled reactions. J Am Chem Soc 90 223-234 Kontoghiorghes GJ, Bartlett AN, Hoffbrand AV, Goddard JG, Sheppard L, Barr J, Nortey P (1990) Long-term trial with the oral iron chelator l,2-dimethyl-3-hydroxypyrid-4-one (LI). I. Iron chelation and metabolic studies. Br J Haematol 76 295-300... 

Klopman, G. Chemical Reactivity and the Concept of Charge and Frontier-Controlled Reactions. J. Amer. Chem. Soc. 90, 223 (1968). 

Other reactions are controlled kinetically, and the most stable product is not the major one observed. In these cases, you must look at the reactant side of the reaction coordinate to discover factors determining the outcome. Klopman and Salem developed an analysis of reactivity in terms of two factors an electrostatic interaction approximated by atomic charges and a Frontier orbital interaction. Fleming s book provides an excellent introduction to these ideas. 

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

Brown s reactivity index, Z-value 73), is also the one in which the frontier term solely controls the intramolecular orientation. 

Apeloig and Kami (13) have also studied the effects of substituents on the reactivity of silenes by the frontier molecular orbital (FMO) approach. They have concluded that, concerning electronic factors, the polarity of the carbon-silicon double bond, and thus the coefficients of the frontier orbitals, play a more important role than the energies of these orbitals in controlling the reactivity of silenes. 

The importance of both frontier orbital-controlled and electronic charge-controlled factors in determining chemical reactivity has been recognized (16). These concepts are the key to interpreting two types of reactivity expected for carbene complexes, i.e., reactions with nucleophilic... 

Frontier orbital approaches are not yet implemented in EROS. Nor does EROS take account of the features of reactivity which are controlled by orbital symmetries. This will follow the current work on stereochemistry and conformation. 

The reactions of electrogenerated cation radicals of diarylsulfldes are mainly orbital-controlled and at this level the electronic structure of their frontier orbitals (HOMO-SOMO) has very interesting synthetic consequences. The 3p orbitals of sulfur are conjugated with only one aromatic ring even if there are two aryls bound to sulfur. Therefore, only one ring can be activated electrochemically. The degree of the charge delocalization in the ArS moiety of a cation radical on the one hand, and the availability of p- and o-positions for the substitution on the other, determine quite different reactivity of such species. 

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

Dnring an electron transfer, the acceptor places its LUMO at the electron disposal and the donor releases an electron that is located on its HOMO. These orbitals are frontier orbitals. In the corresponding ion-radicals, the distribution of an unpaired electron proceeds, naturally, under frontier-orbital control. This definitely reflects in the ion-radical reactivity and not always by a self-obvions manner. Let ns concisely trace peculiarities of ion-radical fragmentation reactions that are very important in organic synthesis. 

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