Transition states theoretical studies

The redox mechanism operates with late transition metals, as exemplified by Mn, Fe, and Ru, which have easily accessible multiple oxidation states. Theoretical studies and trapping experiments suggest that in porphyrins and salen compounds, the mechanism involves the formation of a radical intermediate. 

In addition to the experimental applications of chiral copper Lewis acids in organic synthesis discussed previously, a number of theoretical investigations on the nature of chiral copper Lewis acids have been reported. Much of this work has been focused on the investigation of chiral bisoxazoline copper(ll) complexes and/or reactions catalyzed by these chiral Lewis acids. Deeth and Fey reported a molecidar mechanics study of Cu(II)/bisoxazoline-catalyzed Diels-Alder reactions [131). The work details conformational preferences of various catalyst/substrate complexes and likely transition states. The study also found good success in predicting... 

Oxazolidinone-, oxazolidinethione-, oxazolidineselone-, and thiazolidine-thione-based enolates react svith aldehydes via the svell-established six-membered Zimmerman-Traxler [3] chair-like transition state. Exhaustive studies and analysis by Crimmins have established the theoretical basis of these reactions [33]. These transition states can proceed svithout chelation betsveen carbonyl or thiocarbonyl (84) or svith an additional chelation to titanium (85), as shosvn in Scheme 2.9. To proceed via the chelated transition structure 85, one of the ligands on titanium (typically chloride) must be displaced by the carbonyl or thiocarbonyl group. Although these groups are not sufficiently nucleophilic to completely displace this ligand on their o vn. 

For equilibrium states, theoretical studies have related domain dimensions and lattice parameters, and the composition of transitional boundaries separating spherical, cylindrical and lamellar morphologies, to block types and molecular weight and to the nature and concentration of any diluent (see Section 6.3). 

The above discussion represents a necessarily brief simnnary of the aspects of chemical reaction dynamics. The theoretical focus of tliis field is concerned with the development of accurate potential energy surfaces and the calculation of scattering dynamics on these surfaces. Experimentally, much effort has been devoted to developing complementary asymptotic techniques for product characterization and frequency- and time-resolved teclmiques to study transition-state spectroscopy and dynamics. It is instructive to see what can be accomplished with all of these capabilities. Of all the benclunark reactions mentioned in section A3.7.2. the reaction F + H2 —> HE + H represents the best example of how theory and experiment can converge to yield a fairly complete picture of the dynamics of a chemical reaction. Thus, the remainder of this chapter focuses on this reaction as a case study in reaction dynamics. 

Alkylation of enamines can take place on carbon or on nitrogen (see Section I). The theoretical considerations and reaction conditions which determine whether C or N alkylation takes place have already been studied extensively 26-32). These studies have shown that the facility with which alkylation takes place depends on the basicity of theenamine, on the ease of formation of a trigonal atom in the transition state, and on the nature of the enamine, the alkylating agent, and the solvent. 

Another application of catalyst 8 is to the reaction of acetylenic aldehydes [10c] (Scheme 1.18, Table 1.6). Two acetylenic dienophiles have been reacted with cyclo-pentadiene or cyclohexadiene to give bicyclo[2.2.1]heptadiene or bicyclo[2.2.2]octa-diene derivatives in high optical purity. A theoretical study suggests that this reaction proceeds via an exo transition state. 

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. 

Polar factors can play an extremely important role in determining the overall reactivity and specificity of hotnolytic substitution.97 Theoretical studies on atom abstraction reactions support this view by showing that the transition state has a degree of charge separation.101 10 ... 

Metcalfe107,108 has recently modeled electrochemical promotion using 02 conductors and derived108 equation (6.66) using transition state theory and the concept of a partially charged transition state.108 Despite this interesting theoretical study,108 which is consistent with the basic experimental electrochemical promotion observations Eqs. (4.49) and (4.50) little is still known, experimentally or theoretically about the parameter AR and its possible relationship to A and Aa. Consequently, and in order not to introduce adjustable parameters, we will set XR equal to zero in the subsequent analysis and will show" that it is possible to derive all local and global promotional rules in terms of only four parameters... 

The work of Melander and Carter (1964) on 2,2 -dibromo-4,4 -di-carboxybiphenyl-6,6 -d2 (1) has been referred to above in the introductory and theoretical sections, where it was pointed out that the availability of two detailed theoretical computations of the inversion barrier (Westheimer and Mayer, 1946, Westheimer, 1947 Hewlett, 1960) made this system especially attractive for the study of steric isotope efifects. Furthermore, in the preferred initial-state conformation the two bromines are probably in van der Waals contact (cf. Hampsoii and Weissberger, 1936 Bastiansen, 1950), and thus initial-state steric effects are unaffected by deuterium substitution in the 6 and 6 positions. The barrier calculations provided two different theoretical values for the non-bonded H Br distance in the transition state which, together with the corresponding H Br potential function, could be inserted in equation (10) to yield values for A AH. For... 

The second group of studies tries to explain the solvent effects on enantioselectivity by means of the contribution of substrate solvation to the energetics of the reaction [38], For instance, a theoretical model based on the thermodynamics of substrate solvation was developed [39]. However, this model, based on the determination of the desolvated portion of the substrate transition state by molecular modeling and on the calculation of the activity coefficient by UNIFAC, gave contradictory results. In fact, it was successful in predicting solvent effects on the enantio- and prochiral selectivity of y-chymotrypsin with racemic 3-hydroxy-2-phenylpropionate and 2-substituted 1,3-propanediols [39], whereas it failed in the case of subtilisin and racemic sec-phenetyl alcohol and traws-sobrerol [40]. That substrate solvation by the solvent can contribute to enzyme enantioselectivity was also claimed in the case of subtilisin-catalyzed resolution of secondary alcohols [41]. 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

Before 1980, force field and semiempircal methods (such as CNDO, MNDO, AMI, etc.) [1] were used exclusively to study sulfur-containing compounds due to the lack of computer resources and due to inefficient quantum-chemical programs. Unfortunately, these computational methods are rather hmit-ed in their reliability. The majority of the theoretical studies under this review utilized ab initio MO methods [2]. Not only ab initio MO theory is more reliable, but also it has the desirable feature of not relying on experimental parameters. As a consequence, ab initio MO methods are apphcable to any systems of interest, particularly for novel species and transition states. 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

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