Molecular-orbital calculations cycloaddition

Fig. 22 The Diels-Alder cycloaddition between the dienophile [54] and diene [53 yields two diastereoisomers [55] and [56]. Attenuated substrate analogues [57] and [58 were used in molecular orbital calculations of this reaction.

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

On the other hand, molecular orbital calculations predict that the cycloaddition of ketenes to olefins docs not take place by a [,2, +, 2.) mechanism Wang Houk J. Am. Chem. Soc. 1990, 112. 1754 Bemardi Bottoni Robb Venturini J. Am. Chem. Soc. 1990,112, 2106 Valenti Peric s Moyano J. Org. Chem. 1990, 55, 3582. 

A four-center molecular syn addition ([2 + 2]-cycloaddition) is also consistent with experimental observations.202 Molecular-orbital calculations and experimental observations by NMR spectroscopy in superacidic systems rule out the existence of the bridged fluoronium ion.204-206... 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

Ab initio molecular orbital calculations, coupled with activation energies and entropies from experimental data, have been employed to determine the nature of the intermediates in the reaction of singlet oxygen with alkenes, enol ethers, and enamines.214 Allylic alkenes probably react via a perepoxide-like conformation, whereas the more likely pathway for enamines involves a zwitterionic cycloaddition mechanism. The reactions of enol ethers are more complex, since the relative stabilities of the possible intermediates (biradical, perepoxide, and zwitterionic) here depend sensitively on the substituents and solvent polarity. 

Because the carbonyl groups are part of the tt system of the dienophile, there is an opportunity for secondary interactions between orbitals that are not involved in the bonding changes taking part in the cycloaddition. Molecular orbital calculations have indicated that stabilizing interactions can take place when the reactants are oriented in such a way as to produce the endo isomer. 

Their calculated value for the 7r-ionization potential was 14.111 eV no experimental value is available. Galasso94 has also used Pariser-Parr Pople calculations on the three benzoquinolizinium ions to produce charge densities and transition energies. Molecular orbital calculations of LUMO and HOMO levels in the benzo[/>]quinolizinium ion have been used in a discussion of the mechanism of cationic polar cycloaddition (Section IV,E).95 The binding energy of the positive nitrogen in the benzoquinolizinium salts has been measured relative to that of nitrate, and has been found to be 7.8-8.1 eV.96... 

On the basis of Monte Carlo simulations [40] and molecular orbital calculations [26a], hydrogen bonding was proposed as the key factor controlling the variation of the acceleration for Diels-Alder reactions in water. Experimental differences of rate acceleration in water-promoted cycloadditions were recently observed [41]. Cycloadditions of cyclopentadiene with acridizinium bromide, acrylonitrile and methyl vinyl ketone were investigated in water and in ethanol for comparison (Scheme 3). Only a modest rate acceleration of 5.3 was found with acridizinium bromide, which was attributed to the absence of hydrogenbonding groups in the reactants. The acceleration factor reaches about 14 with acrylonitrile and 60 with methyl vinyl ketone, which is the best hydrogen-bond acceptor [41]. 

Maryanoff and Turchi pursued a detailed theoretical study of the reaction between 1,2-dicyanocyclobutene 279 and munchnone 280, prepared by cyclodehydration of 278 and acetic anhydride (Fig. 4.100). The results from these AMI molecular orbital calculations led to the conclusions that the transition state leading to the exo cycloadduct 281 is favored electrostatically and that azomethine ylide 282 is a discrete intermediate in the formation of dihydroazepine 283. More recently, Turchi reported cycloaddition reactions between munchnone 285 and 279 to afford dihydroazepine 286 in high yield. Further cyclization of 286 gave tricycle 287. Likewise, diester 288 reacts with munchnone 42 to give dihydroazepine 289. 

The limited number of molecular orbital calculations, which deal with the regioselectivities of isomunchnone 1,3-dipolar cycloaddition reactions, are covered in Section 4.4.3.2. 

Molecular orbital calculations have been widely used both for the investigation of the structure and the aromaticity of these seven-membered heterocycles as well as for the determination of the reaction pathways. A qualitative analysis based on MNDO and ab initio molecular orbital calculations has been reported <87JCS(P1)1579,87JCS(P2)i669> for the cycloaddition of several alkynes to tetrasulfur tetranitride with the formation of trithiadiazepines and 1,2,5-thiadiazoles (see heme 28). 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate) [131, 132], They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffmann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO (HOMO) of the grotmd state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the cmicerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2 [132]. 

A Knoevenagel condensation/Diels-Alder cycloaddition approach has been developed by Delgado et al. [124]. By infrared light irradiation at 50 °C and solvent-free conditions, the desired products 324/325 were isolated in moderate yields (40-65%), with also moderate to good e t/o-selectivity (Scheme 13.71). Frontier molecular orbital calculations rationalized the stereoselectivity of this process. In the cases of diethyl malonate as the... 

A review (37 refs.) on the application of molecular orbital calculations to the stereoselectivity of addition reactions of nitro-enitol derivatives has been published in Japanese. Cycloaddition of 1-acetoxy- or 1-trimethylsilyloxy-butadiene to l,2-dideoxy-l-nitro-D-g/ co-hept-l-enitol tetraacetate gave mainly the cyclohexane derivatives 41. Additions of alcohols, carbon radicals and phosphoiyl... 

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