Frontier Molecular Orbital Considerations

The adsorption of CO on the 100 face of nickel, for instance, has been calculated to take place on top of the atom as shown in Fig. 4.8.5 This is the type of adsorption discussed previously for 100 face atoms using only the d orbitals. However, since nickel is a djo element the d orbitals on this site are filled, so this simplified adsorption scheme does not apply. The 100 site energy levels shown in Fig. 4.8b indicate, though, that the pz is the LUMO and is ideally situated for accepting the 2ct electrons from the CO. The HOMO of this site is the dxy that is directed toward the neighboring atoms in the 100 plane so it is not available for backbonding. Neither is the next lower orbital, the dx2.y2 which is also oriented in the 100 plane. Backbonding can only take place with me dxz or dyz orbitals. 

however, is still a simplified approach in that it does not take into consideration the formation of the primary metalalkyl nor the possible intermediacy of an adsorbed n-allyl species as occurs over palladium and, to some extent, nickel catalysts.  

Goodenough, Magnetism and the Chemical Bond, Interscience, New York, 1963. 

Physical Methods in Inorganic Chemistry, Saunders, NY, 1977, pp. 396-406. 

---

To explain tlie stereodieniistiy of tlie allylic substitution reaction, a simple stereoelectronic model based on frontier molecular orbital considerations bas been proposed fl55. Fig. G.2). Organocopper reagents, unlike C-nudeopbiles, possess filled d-orbitals fd - configuration), wbidi can interact botli witli tlie 7t -fC=C) orbital at tlie y-carbon and to a minor extent witli tlie cr -fC X) orbital, as depicted... 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

As with the chloropalladation reaction (vide supra)m the rearrangement of >/2-methyl-enecyclopropane to >/4-TMM was shown experimentally to proceed stereoselectively by disrotatory ring cleavage of the distal phase interaction between the metal orbital in the distal ring-opening of -methylenecyclopropane complexes can be minimized by bending the bond up away from the metal (equation 348)410 ... 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

Fulvenes, like their troponoid counterparts, are capable of engaging dienes in a number of different pericyclic reactions. A reasonably well-defined reactivity profile of these systems has emerged as the result of extensive scrutiny of the cycloaddition behavior of the fulvene nucleus. To a large extent, fulvenes undergo concerted cycloadditions to dienes as either the 6ir or 2ir participant and the factors governing which of these reactivities is expressed in a particular circumstance has been elucidated employing frontier molecular orbital considerations. ... 

In one study, various substituted allyl radicals were generated by sulfenate photolysis, and it was shown that coupling was controlled both by steric and by frontier molecular orbital considerations [153]. p-Scission is favored by a-substitution and the stability of the putative alkyl radical. A particularly clever device used in another study was the thermal equilibrium between allylic sulfoxides and sulfenates used to generate allyloxy and other C3H5O radicals [154]. 

Scheme 14). The regiochemical outcome of the 1,3-dipolar cycloaddition reactions of the cyclic five-membered ring carbonyl yUde 48 with a variety of acycUc and cycHc alkenes having activated or inactivated r-bonds can be ra-tionaUzed [78,79] on the basis of frontier molecular orbital considerations, with the HOMO and LUMO of the carbonyl ylides dominating the reactions with electron-deficient and electron-rich dipolarophiles, respectively (Scheme 14). 

The regioselectivity of the reaction is consistent with die Frontier Molecular Orbital considerations for a concerted cycloaddition process. The stereochemistry of the reaction products 2 indicates a clear preference for the endo-appoach of the dipolarophile. 

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

Sun Shuiyu, Wang Dianzuo, Long Xiangyun, 1994b. Frontier molecular orbital theory consideration for electron transfer process across sulphide mineral-solution interface. J. BGRIMM, 3(1) 34 - 39 (in Chinese)... 

The relative frontier molecular orbital (FMO) energies of the reagents are very important for the catalytic control of 1,3-dipolar cycloadditions. In order to control the stereochemical outcome of a reaction with a substoichiometric amount of a ligand-metal catalyst, it is desirable that a large rate acceleration is obtained in order to assure that the reaction only takes place in the sphere of the metal and the chiral ligand. The FMO considerations will be outlined in the following using nitrones as an example. 

The structural requirements of the mesomeric betaines described in Section III endow these molecules with reactive -electron systems whose orbital symmetries are suitable for participation in a variety of pericyclic reactions. In particular, many betaines undergo 1,3-dipolar cycloaddition reactions giving stable adducts. Since these reactions are moderately exothermic, the transition state can be expected to occur early in the reaction and the magnitude of the frontier orbital interactions, as 1,3-dipole and 1,3-dipolarophile approach, can be expected to influence the energy of the transition state—and therefore the reaction rate and the structure of the product. This is the essence of frontier molecular orbital (EMO) theory, several accounts of which have been published. 16.317 application of the FMO method to the pericyclic reactions of mesomeric betaines has met with considerable success. The following section describes how the reactivity, electroselectivity, and regioselectivity of these molecules have been rationalized. 

Initial work indicates that dispersed metals may be used to promote a variety of organometallic reactions. The Heck Arylation proceeds smoothly over supported Pd catalysts while diene cyclizations can be catalyzed by dispersed Rh metal. The use of these heterogeneous species facilitates product isolation and permits the application of flow systems rather than batch reactors for these reactions. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions take place on the coordinately unsaturated comer atoms on the metal surface. 

Although IMDA reactions are entropically less disfavored than the intermolecular versions, they are nonetheless not as simple as might at first appear. The well-known Alder endo rule and its frontier molecular orbital theoretical interpretation involving secondary orbital interactions, together with steric considerations, serve to explain the kinetic preference for the endo-product and the thermodynamic preference for the < o-product in IMDAs. For the IMDA reaction, an additional parameter, the effect of the tether that connects the diene to the dienophile to control the conformation available to a transition state has to be considered. 

---