Lewis acids theoretical studies

The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

In a combined experimental and theoretical investigation it was found that the / -alkyl group in the dienophile gave a steric interaction in the transition-state structure which supported the asynchronous transition-state structure for the Lewis acid-catalyzed carbo- and hetero-Diels-Alder reactions. The calculated transition-state energies were of similar magnitude as obtained in other studies of these BF3-catalyzed carbo-Diels-Alder reactions. 

Very few theoretical studies of hetero-Diels-Alder reactions have been performed [25] and, furthermore, theoretical studies of the Lewis acid-catalyzed hetero-Diels-Alder reactions are even more limited. 

The nature of the donor-acceptor interactions in main group complexes has also been studied theoretically [88-91], The calculated BDEs of main-group complexes predicted at the MP2/II level of theory are in very good agreement with experimental results [88], This is an important difference from the performance of the MP2/II level of theory for TM complexes, where the computed BDEs are always too high. Table 7.18 lists the calculated BDEs for complexes of group-13 Lewis acids EX3 with various Lewis bases. 

Ciloslowski, J., Boche, G. (1997) Geometry-tunahle Lewis acidity of amidinium cations and its relevance to redox reactions of the Thauer metal-free hydrogenase - a theoretical study. Angeiv. Chem. Int. Ed. Engl. 36, 107-9. 

Theoretical studies on the Beckmann rearrangement mechanism over zeolite catalyst supported by experimental data have increased. The catalytic activity of the zeohte is determined by Brpnsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and flexibility of the zeolite framework ". ... 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

As mentioned above, the cyclic silylenes 75c and also 76c (or 79a and 79b) are dramatically less reactive than other known silylenes which are all transients. Thus, 75c has such a low Lewis acidity that, unlike other silylenes, it does not react as an electrophile. However, 75c reacts as a Lewis base, i.e. as a nucleophilic reagent980. The known reactions of Arduengo-type silylenes are summarized in References 2e, 98c, 98d and 99b and the references cited therein. The theoretical study of the possible reactions of Arduengo-type silylenes is quite limited and awaits the attention of computational chemists. 

A combined theoretical and experimental study has been reported for the formation of silylated (3-lactams, via Staudinger [2+2] cycloaddition reaction from silylketenes and imines, in the presence or in the absence of a Lewis acid... 

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Diacyl dications have been examined in both experimental and theoretical studies. Attempts to directly observe the oxalyl dication (174) were not successful as ionization of oxalyl chloride (170) with SbFs leads to the chlorocarbonyl cation (172, Scheme 4).65 The initially formed chloroxa-lyl cation 171 immediately decarbonylates under the reaction conditions. Interestingly, complexation of the chloroxalyl cation 171 with excess Lewis acid is expected to generate superelectrophile 173. Theoretical calculations (MP2/6-31G level) indicate that the oxalyl dication (174) is at a minimum on the potential energy surface.66 Calculated bond lengths for... 

Anomerization through the agency of a Lewis acid (electron acceptor) has found numerous applications for the preparation of the more stable form of a sugar acetate—usually a member of the a-D-series. Several of these applications are listed in Table III. Acetic anhydride, and mixtures of acetic anhydride and acetic acid, are the most commonly used solvents. Painter96 has shown that the rate of reaction is increased by addition of an inert solvent, carbon tetrachloride. Zinc chloride and sulfuric acid have been most frequently used as catalysts. Although these transformations have found frequent application, few theoretical studies have been made. 

Polymerization of vinyl ethers (VE) has been the subject of a considerable amount of theoretical studies. These monomers can be polymerized through radical initiation but the reaction is very slow and leads only to oligomers. Cationic polymerization initiated by a wide variety of Lewis acids is much more efficient and definitely preferred for homopolymer synthesis. Detailed theoretical aspects, and particularly recent developments concerning the controlled/living cationic polymerization of these monomers, have been discussed as well in previous exhaustive review [1,13,98,99] as in the present book (Chapters 4 and 5), and they will no longer be considered here. 

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