Plausible transition states

Sketch plausible transition states for (a) the dissociation of a molecule in the gas phase (b) the reaction of cyclopropane to give propene (c) the isomerization of CH3CN to CH3NC (d) the desorption of an atom from a surface (e) the dissociation of an adsorbed molecule such as CO on a metal surface. 

From a historical standpoint, the Zimmerman-Traxler study on the stereochemical aspects of the Ivanov reaction is of considerable significance (22). Their investigation revealed that the magnesium enediolate 19a, upon condensation with benzaldehyde (Et2 O, reflux 5 hr), afforded the erythro and threo acids 20E and 20T, respectively, in a 24 76 ratio (eq. [13]). In the analysis of plausible transition states, both chair A and boat B geometries were considered. Zimmerman and Traxler concluded that the major diastereomer 20T could... 

Draw a plausible transition state for the bimolecular reaction of nitric oxide with ozone. Use dashed lines to indicate the atoms that are weakly linked together in the transition state. 

Figure 4.11 Plausible transition state for the asymmetric epoxidation.

Figure 9.1 Plausible transition-state model for nitroaldol reactions...

Fig. 15.10. Frontier orbital interactions in plausible transition states of the one-step [2+2]-cycloadditions of ethene and ethene (left) and of ethene and acetylene (center and right).

The stereochemical results can be explained by the plausible transition state A (Fig. 12.57). Because additional water is added and the reaction is conducted in wet solvents, the transition state is stabilized by hydrogen bonding between the nitrogen atom of the imine and the nitrogen atom of the imidazolium moiety of the catalyst. A switch of the facial selectivity is disfavored because of steric repulsion between the Ar group of the imine and the imidazolium moiety of the catalyst. 

Figure 14. Plausible transition states on the nucleophilic attack of tTans-2-butene (tbn) upon [PtCl3(S,S-tbn)] (13 ...

It is possible to propose plausible transition states that are similar on the (100) and (111) surfaces yet do not resemble an adatom dihydride. Suppose that the diffusing species on the (111) surface is not H but SiH. Adatoms... 

Carbonyl insertion forms an important step in the reaction sequence for substitution into Mo(7r-indenyl)(CO)3, and in the reaction of MoR(77-C5H5)(CO)3, where R = benzyl or allyl, with phosphorus bases. In this latter reaction the rate is independent of the nature and concentration of the incoming ligand a plausible transition state is that shown (40). A Group VII example is provided by the reaction of MnR(CO)s... 

Scheme 9.7 Plausible transition state proposed by Uang.

Scheme 17.5 Plausible transition-state models for cross-aldol reaction.

Methylenecyclopropanes. An extensive ab initio study of all, even remotely plausible, transition states for the degenerate rearrangement of methylenecyclopropane shows the orthogonal methylene-allylic trimethylenemethane biradical to be the most stable of all possible midpoints for the reaction. There is a secondary potential minimum at this point, making this an interesting target for experimental detection. ... 

A plausible transition state (TS) to explain the origin of the activation and the asymmetric induction through a dualactivation mode by the catalyst has been envisioned by the authors (Fig. 9.3). The synergic effect provided by the involvement of both an achiral Brpnsted add and the chiral secondary amine 38 as the catalytic systan in this multicomponent process was crucial for the success of this reaction since neither by themselves could catalyze the formation of DHPM efficiently. Thus, the acid additive could be involved in the formation of... 

Later, Sun s group also used their pipecolinic acid derivative 87 for the first efficient reduction of aliphatic and aromatic ketones with silane 2 in moderate to high enantioselectivity [184]. A plausible transition state was proposed in order to explain the results observed, where the catalyst 87 would act as a tridentate activator and would promote the hydrosUylation of ketones through the heptacoordinate silicon structure depicted in Fig. 12. 

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