FMO frontier molecular

FMO Frontier Molecular Orbital TBDPS tert-butyldiphenylsilyl... 

Similarly, reaction of l-ethoxycarbonyl-3-nitroindole 422a with miinchnone 425c also affords nearly exclusively the product 431 (FMO = frontier molecular orbital), whereas miinchnone 425d with the same substrate... 

There have been few applications of theoretical methods in this area, but attempts have been made to rationalize the structure and reactivity toward cycloaddition of the mesoionic compounds (45) using MNDO and FMO (frontier molecular orbital) methods <86CB2308,86CB2317>. 

Contrary to the normal Diels-Alder processes, the FMO (Frontier Molecular Orbitals) involved in inverse electron demand Diels-Alder reactions are the LUMO of the diene (1,2,4,5-tetrazine) and the HOMO of the dienophile (electron-rich al-kene). The reactivity of tetrazines 1 toward ethyl vinyl ether would depend on the LUMO diene-HOMO dienophile gap. Thus, the combination LUMO-HOMO having a smaller energy gap will lead to the faster reaction. In this case the LUMO for la has a lower energy than that of lb and this will be reflected in their relative reactivities. Energy gaps between tetrazines 1 and ethyl vinyl ether are 6.61 eV for la and 6.64 eV for lb. In consequence, tetrazine la will be more reactive toward ethyl vinyl ether than lb. 

In view of this, early quantum mechanical approximations still merit interest, as they can provide quantitative data that can be correlated with observations on chemical reactivity. One of the most successful methods for explaining the course of chemical reactions is frontier molecular orbital (FMO) theory [5]. The course of a chemical reaction is rationali2ed on the basis of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the frontier orbitals. Both the energy and the orbital coefficients of the HOMO and LUMO of the reactants are taken into account. 

The importance of FMO theory hes in the fact that good results may be obtained even if the frontier molecular orbitals are calculated by rather simple, approximate quantum mechanical methods such as perturbation theory. Even simple additivity schemes have been developed for estimating the energies and the orbital coefficients of frontier molecular orbitals [6]. 

The way the substituents affect the rate of the reaction can be rationalised with the aid of the Frontier Molecular Orbital (FMO) theory. This theory was developed during a study of the role of orbital symmetry in pericyclic reactions by Woodward and Hoffinann and, independently, by Fukui Later, Houk contributed significantly to the understanding of the reactivity and selectivity of these processes. ... 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

Aceording to Frontier Molecular Orbital (FMO) theory, thiophene s most reactive site can be identified by examining the shape of its highest-occupied molecular orbital (HOMO). Which atoms contribute to thiophene s HOMO Which atom(s) contributes the most Which nitration product should form preferentially ... 

According to Frontier Molecular Orbital (FMO) theory, Diels-Alder reaction between an electron-rich diene and an electron-poor dienophile involves interaction between the highest-occupied molecular orbital (HOMO) on the diene and the lowest-unoccupied molecular orbital (LUMO) on the dienophile. The better the HOMO/LUMO overlap and the smaller their energy difference, the more favorable the interaction and the faster the reaction. 

A theoretical study based on PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the RHF/6-31-I-G level was performed to examine the reaction of l-amino-2-ethoxycarbonylpyridinium mesitylenesulfonate and acrylonitrile in the presence of Hiinig s base leading to the formation of 3-cyano-4-hydroxy-l,2-dihydropyrido[l,2-ft]pyridazi-nium inner salt (54) (99JOC9001). The calculations indicated that both the... 

One cannot discuss Lewis acid-catalyzed cycloaddition reactions in the present context without trying to understand the reaction course mechanistically, e.g. using a frontier molecular orbital (FMO) point of reasoning, or theoretical calculations of transition state structures. 

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. 

Inagaki, Fujimoto and Fukui demonstrated that ir-facial selectivity in the Diels-Alder reaction of 5-acetoxy- and 5-chloro-l,3-cyclopentadienes, 1 and 2, can be explained in terms of deformation of a frontier molecular orbital FMO [2], The orbital mixing rule was proposed to predict the nonequivalent orbital deformation due to asymmetric perturbation of the substituent orbital (Chapter Orbital Mixing Rules by Inagaki in this volume). 

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. 

Frontier molecular orbital (FMO) theory 62) has provided new insights into chemical reactivity. This, and the simplicity of its application, has led to its widespread use, particularly in the treatment of pericyclic reactions 63). An FMO treatment depends on the energy of the highest occupied (HOMO) and lowest unoccupied molecular... 

When ene-nitrile oxidoisoquinolium betaine 131 was heated as a dilute solution in toluene to 120 °C (Scheme 1.15), near quantitative conversion to the cycloadduct 133, resulting from the undesired regioselectivity, was observed. While the near complete conversion to cycloadduct 133 of oxidoisoquinolinium betaine 131 indeed demonstrated complete avoidance of the conjugate addition pathway in favor of cycloaddition, initial production of undesired isomeric cycloadduct 133 (instead of 136) was disappointing. Notably, cycloadduct 133 is expected to be less kinetically favored based on frontier molecular orbital (FMO) analysis (assuming dipole HOMO-controlled cycloaddition) of the putative transition state. This result stands in contrast to the cycloaddition of nitroalkene oxidoisoquinolinium betaine... 

These relations highlight the fact that the formalism of DFT-based chemical reactivity built by Parr and coworkers, captures the essence of the pre DFT formulation of reactivity under frontier molecular orbital theory (FMO). Berkowitz showed that similar to FMO, DFT could also explain the orientation or stereoselectivity of a reaction [12]. In addition, DFT-based reactivity parameters are augmented by more global terms expressed in the softness. 

Based on the foregoing discussion, one might suppose that the Fukui function is nothing more than a DFT-inspired restatement of frontier molecular orbital (FMO) theory. This is not quite true. Because DFT is, in principle, exact, the Fukui function includes effects—notably electron correlation and orbital relaxation—that are a priori neglected in an FMO approach. This is most clear when the electron density is expressed in terms of the occupied Kohn-Sham spin-orbitals [16],... 

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