Cycloadditions frontier molecular orbital theory

Frontier molecular orbital theory correctly rationalizes the regioselectivity of most 1,3-dipolar cycloadditions (73JA7287). When nitrile ylides are used as 1,3-dipoles, the dipole... 

An interpretation based on frontier molecular orbital theory of the regiochemistry of Diels Alder and 1,3-dipolar cycloaddition reactions of the triazepine 3 is available.343 2,4,6-Trimethyl-benzonitrile oxide, for example, yields initially the adduct 6.344... 

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. 

The Diels-Alder reaction (47t 2ir cycloaddition) is by far the best studied reaction of dienes from both theoretical and experimental viewpoints. Frontier molecular orbital theory predicts three types of Diels-Alder reaction. Structural effects on rate constants show the existence of two types of reaction ... 

Hydroxy-l-methylquinolinium iodide is deprotonated by tertiary base giving A-methylquinoIinium-3-olate (283 R = Me) which can be trapped by dienes giving adducts 284. These cycloadditions have been discussed in terms of frontier molecular orbital theory (see Section N,C,2)3° ... 

Cycloadditions that involve two unsymmetric reactants can lead to regioisomers. The regioselectivity of these adducts can be predicted with a high degree of success through the use of frontier molecular orbital theory.22 25 The ortho product (this nomenclature follows the analogy of disubstituted aromatic systems) is usually the preferred isomer from 1-substituted dienes, whereas 2-substituted dienes provide the para isomer as the major adduct. However, when a Lewis acid is used as a catalyst in the reaction, the ratio of these isomers can alter dramatically and, occasionally, can be reversed.22... 

Two studies on the mechanism of this type of [4 + 2] cycloaddition which have led to very di erent interpretations have appeared. Mock and Nugent suggested that the Diels-Alder reactions of N-sulfi-nyl-p-toluenesulfonamide are stepwise, ionic processes. On the other hand, Hanson and Stockbum prefer a concerted, pericyclic mechanism in accord with frontier molecular orbital theory. Both proposals satisfactorily rationalize the observed regioselectivity of these reactions. 

The variations of periselectivity exhibited in the cycloadditions of fulvenes to dienes have been rationalized by rqjplication of frontier molecular orbital theory. The fulvene orbitals of interest in these reactions are depicted in Figure 2. - The controlling orbitals in the reaction of fulvene with an... 

Houk, K. N. Frontier molecular orbital theory of cycloaddition reactions. Acc. Chem. Res. 1975, 8, 361-369. 

A novel photochemical cycloaddition between 2-cyanofuran and 2-alkoxy-3-cyanopyridines gave the 4+4] product as the major isomer. The regioselectivity and stereoselectivity of this singlet photoaddition process was explained by frontier molecular orbital theory <04TL4437>. 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

Frontier molecular orbital theory predicts that 4 + 2 cycloadditions in which the reactants interact in a suprafacial maimer is allowed but that analogous 2 + 2 cycloadditions are forbidden. 

Frontier molecular orbital theory. A. Analysis of a [2+2] cycloaddition, showing unfavorable mixing between the HOMO and LUMO of the two ethylene fragments. B. Analysis of a [4+2] cycloaddition, showing the favorable HOMO-LUMO mixings. 

The second-order rate constant of 1,3-dipolar cycloadditions of benzonitrile oxide (85) with various dipolarophiles A-E were determined in water and organic solvents. The greatest accelerating effect was observed in water for the cycloaddition of 85 with electron-rich dipolarophiles A and B and in -hexane for the cycloadditions with electron-poor dipolarophiles C-E (Table 5.6). The results were explained in terms of frontier molecular orbital theory and enforced hydrophobic interactions. 

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