Transition state structures theoretical calculations

Both experimental [7] and theoretical [8] investigations have shown that the anti complexes of acrolein and boranes are the most stable and the transition states were located only for these four anti complexes. The most stable transition-state structure was calculated (RHF/3-21G) to be NC, while XT is the least stable of the four located. The activation energy has been calculated to be 21.6 kcal mol for the catalyzed reaction, which is substantially above the experimental value of 10.4 1.9 kcal mol for the AlCl3-catalyzed addition of methyl acrylate to butadiene [4a]. The transition-state structure NC is shown in Fig. 8.5. 

Efforts to deduce transition state structures theoretically have until recently been retarded by the failure of even the more sophisticated molecular orbital treatments to predict accurate activation energies, and the need to avoid geometric and mechanistic assumptions has made the calculation of reaction pathways prohibitively expensive. The introduction of efficient gradient methods for minimizing energy with respect to all geometric parameters, coupled with the advent of faster computers, has now virtually overcome the latter problem, and careful parameterization of semiempirical molecular orbital methods has led to more... 

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. 

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. 

The four different transition states in Fig. 8.10 were considered with BF3 as a model for the BLA catalyst in the theoretical calculations. It was found that the lowest transition-state energy for the BF3-catalyzed reactions was calculated to be 21.3 kcal mol for anti-exo transition state, while only 1.5 kcal mol higher in energy the syn-exo transition state, was found. The uncatalyzed reaction was calculation to proceed via an exo transition state having an energy of 37.0 kcal mol . The calculations indicated that the reaction proceeds predominantly by an exo transition-state structure and that it is enhanced by the coordination of the Lewis acid. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

It has been suggested that, regardless of the transition state structure during solvolysis, the norbomyl ion might have yet another structure under the stabilizing influence of strong acid media. Before considering those systems it is appropriate to discuss the current theoretical status as provided by molecular orbital calculations. 

The use of zeolite clusters in quantum chemical calculations has now progressed to quite a sophisticated level. Elementary steps of reaction mechanisms can now be characterized and the results used to distinguish which steps are the most plausible. Computational power is such that clusters and methods can avoid obvious pitfalls (too small a cluster, basis set, etc.). Several key concepts that have arisen from theoretical studies are illustrated in the preceding discussion. These include the following carbo-cations exist as parts of transition state structures, rather than as stable intermediates, and their stabilization is controlled by the zeolite lattice. The transition states are very different from the ground states to either side of them, and each different reaction has been shown to proceed via a different transition state. 

The cy-carbon k12/kn KIEs calculated for the. S N2 reactions between methyl chloride and 22 different nucleophiles at the BILYP/aug-cc-pVDZ level of theory64 showed that all the KIEs were near the theoretical maximum and did not change significantly with transition state structure. A review showed that the a-carbon KIEs found experimentally for. S n2 reactions were also all near the theoretical maximum KIE. This suggests that the Melander-Westheimer curve,65,66 relating the magnitude of these... 

The kinetics of the gas-phase elimination of ethyl and /-butyl carbazates have been studied in a static reactor system over the temperature range 220.3-341.7 °C and pressure range 21.1-70.0 torr.13 Theoretical calculations on the thermal decomposition of ethyl carbazate (4) suggest that the reaction proceeds by a concerted non-synchronous mechanism, through a quasi-three-membered ring transition state (Scheme 4). In contrast, the transition state structure for the thermal decomposition of /-butyl carbazate is an almost planar six-membered ring. 

A number of theoretical studies have been conducted to understand the mechanism of the Cope rearrangement.16 According to calculations by Houk and co-workers, the chairlike transition state is more stable than the boatlike transition state by 7.8 kcal/mol (Scheme l.XII). When Schleyer and colleagues performed calculations to compute the magnetic properties of the transition-state structures, transition states A and B had a magnetic susceptibility of—55.0 and—56.6, respectively. These values are comparable to that of benezene (—62.9), confirming the existence of an aromatic transition state in the Cope rearrangement. 

Theoretical investigations have become more and more important for the development of new catalysts. The interaction of experimental and quantum chemists is fruitful because of the better accuracy and the possibility to calculate the molecules instead of model systems. Quantum-chemical calculations now allow for the determination of transition state structures and an analysis of the factors which have an impact on the reaction. 

A combined experimental and theoretical study using kinetic isotope effects (KIEs) to compare experiment and theory provided additional evidence that the reaction proceeds via a [3 + 2] pathway. The KIEs were measured by a new NMR technique38 and were compared to values, which can be obtained from the calculated transition state structures. Two sets of data were measured for the experimentally used alkene (H3C)3C-CH=CH2 (Scheme 4), and the structures of the corresponding transition states for substituted alkenes were also calculated. 

This solvation rule for 5n2 reactions can be useful in predicting the influence of a change in solvent on the structure of activated complexes. It is in agreement with studies involving leaving group heavy atom and secondary a-deuterium kinetic isotope effects, as well as theoretical calculations of solvent effects on transition-state structures. Possible limitations of this solvation rule have been discussed see [498] and relevant references cited therein. 

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