Electrophilicity frontier orbitals

Nevertheless, the puzzling fact to be explained is that the harder ring nitrogen prefers the softer electrophilic center and that this preference is more pronounced than the one observed for the amino nitrogen. Much remains to be done to explain ambident heterocyclic reactivity it was shown recently by comparison between Photoelectrons Spectroscopy and kinetic data that not only the frontier densities but also the relative symmetries of nucleophilic occupied orbitals and electrophilic unoccupied orbitals must be taken into consideration (308). 

You can interpret the stereochemistry and rates of many reactions involving soft electrophiles and nucleophiles—in particular pericyclic reactions—in terms of the properties of Frontier orbitals. This applies in particular to pericyclic reactions. Overlap between the HOMO and the LUMO is a governing factor in many reactions. HyperChem can show the forms of orbitals such as HOMO and LUMO in two ways a plot at a slice through the molecule and as values in a log file of the orbital coefficients for each atom. 

Frontier orbital theory predicts that electrophilic substitution of pyrroles with soft electrophiles will be frontier controlled and occur at the 2-position, whereas electrophilic substitution with hard electrophiles will be charge controlled and occur at the 3-position. These predictions may be illustrated by the substitution behaviour of 1-benzenesulfonylpyr-role. Nitration and Friedel-Crafts acylation of this substrate occurs at the 3-position, whereas the softer electrophiles generated in the Mannich reaction (R2N=CH2), in formylation under Vilsmeier conditions (R2N=CHC1) or in formylation with dichloromethyl methyl ether and aluminum chloride (MeO=CHCl) effect substitution mainly in the 2-position (81TL4899, 81TL4901). Formylation of 2-methoxycarbonyl-l-methylpyrrole with... 

An alternative approach is in terms of frontier electron densities. In electrophilic substitution, the frontier electron density is taken as the electron density in the highest filled MO. In nucleophilic substitution the frontier orbital is taken as the lowest vacant MO the frontier electron density at a carbon atom is then the electron density that would be present in this MO if it were occupied by two electrons. Both electrophilic and nucleophilic substitution thus occur at the carbon atom with the greatest appropriate frontier electron density. 

Interaction of f. irmeilileliyde frontier orbitals with E and Nu Fig. 1.25. PMO description of interaction of ethylene and formaldehyde with an electrophile (E ) and a nucleophile (Nu ). 

Soft electrophiles will prefer carbon, and it is found experimentally that most alkyl halides react to give C-alkylation. Because of the n character of the HOMO of the anion, there is a stereoelectronic preference for attack of the electrophile approximately perpendicular to the plane of the enolate. The frontier orbital is ip2, with electron density mainly at O and C-2. The tpi orbital is transformed into the C=0 bond. The transition state for an 8 2 alkylation of an enolate can be represented as below. 

Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

Delocalized frontier orbitals provide a different kind of problem. The ester enolate shown below might react with electrophiles at two different sites. 

Naphthalene undergoes electrophihc substitutions at the a rather than p position. The Hueckel molecular orbital calculations show that all the carbons have the same jt electron density 1.0. This is not in agreement with the theory of organic reactions based on the Coulombic interaction that electrophilic attack occurs on the most negatively charged atom. Fukui [7] proposed the frontier orbital theory for the discrepancy between the theory and the experimental observation. The importance of... 

The amplitude of the frontier orbitals determines the selectivity. The most reactive atom in a molecule has the largest amplitude of the frontier orbitals. The frontier orbitals overlap each other to the greatest extent at the sites with the largest amphtudes. Reactions occur on the atoms in the electron donors and acceptors, where the HOMO and LUMO amplitudes are largest, respectively. Electrophiles prefer the a position of naphthalene, an electron donor, with the larger HOMO amplitude (Scheme 21). Nucleophiles attack the carbons of the carbonyl groups, an electron acceptor, with the larger LUMO amplitude (Scheme 7). 

The n orbital amplitudes of ethene are identical on both carbons. Unsymmetrical substitutions polarize the n orbital. Electron acceptors or electrophiles attack the carbon with the larger r amplitude. The polarization of frontier orbitals is important for regioselectivities of reactions. Here, mechanism of the n orbital polarization of ethene by methyl substitution [4] is described (Scheme 5). 

The frontier orbital theory was developed for electrophilic aromatic substitution (Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). Application is successful to the ortho-para orientation (Scheme 23a) for the benzenes substituted with electron donating groups. The ortho and para positions have larger HOMO amplitudes. The meta orientation (Scheme 23b) for the electron accepting groups is under control of both HOMO and the next HOMO [25]. 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

Based on the orbital approximations, it is clear that/(r) is the DFT analog of the frontier orbital regioselectivity for nucleophilic (f (r)) and electrophilic (/ (r)) attack. It is then reasonable to define a reactivity indicator for radical attack by analogy to the corresponding orbital indicator,... 

The preferred orientation for electrophiles is out of the plane defined by the Y-S-Z bonds, and about 20° from the normal (n) to the plane [50], whereas nucleophiles tend to lie in the plane, and cluster close to the line defined by the extension of the Y-S (or Z-S) bond. Similar interactions have been observed for selenium (Ramasubbu and Parthasarathy, 1984). The conclusion, as before, is that these interactions are frontier-orbital controlled, with the HOMO being a sulphur lone pair, and the LUMO an antibonding o- C-y(Z) orbital. 

A case for the irontier orbital hypothesis is more difficult to establish on similar physical grounds. For both electrophilic and nucleophilic attack the frontier orbitals are amongst those which are repelled om the position under attack and become increasingly associated with the residual molecule. It appears therefore that no physical significance can be attached to the fact that the numerical values of frontier orbitals in the molecule under attack predict the active positions successfully. On the other hand, when no polarization occurs, as in free radical reactions, the frontier orbitals are precisely those that combine to form a localized orbital at the position of attack. It seems logical, therefore, to bestow some significance on the frontier orbitals in the case of free radical reactions only. 

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. 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

The stable anion-radical in Scheme 3.63 contains two perchlorotriphenylmethyl radical units linked by an all-trani-p-divinylbenzene bridge. At 200 K, the unpaired electron of the anion-radical is localized (within the ESR timescale) on one stilbenelike moiety only. At 300 K, thermal activation forces the nnpaired electron at one strong electrophilic center to move to another one. Such an electron transfer takes place between two eqnivalent redox sites (Bonvoisin et al. 1994). In contrast to this situation, no electron transfer was observed for the anion-radical that contains two perchlorotriphenylmethyl radical units linked by an all-trani -m-divinylbenzene bridge (Rovira et al. 2001). Such results can be ascribed to the localization of frontier orbitals in the meta-isomeric anion-radical because of the meta connectivity of this non-Kekule structure. 

For pyrroles with electron acceptor substituents in the 1-position electrophilic substitution with soft electrophiles can be frontier orbital controlled and occur at the 2-position, whereas electrophilic substitution with hard electrophiles can be charge controlled and occur at the 3-position. 

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