C transition state

Figure A3.7.1. Two-dimensional contour plot for direct collinear reaction A + BC —> AB + C. Transition state is indicated by J.

Cyclic and open transition state models have been used to explain syn/anti stereoselectivity in these reactions1. The possible transition states (including boat B and chair C transition states) can be deduced from the E/Z geometry of the crotyl reagent and the imine. The postulated cyclic transition states for the preferred E geometry of the imine arc shown below. 

An analogous set of four transition states for the (Z)-imine geometry could also be constructed. The transition state descriptors, such as B(Z,E) and C(Z,E), may be employed to denote boat (B) or chair (C) transition states respectively, possessing (Z)-enolate and ( )-imine geometries. The interrelationships between the stereochemical features of reactants, transition states, and products are summarized in Table 28. Unfortunately, the kinetic diastereoselection in such condensations as a function of either enolate or imine geometry has not been systematically studied. In all the early work in this field... 

FIGURE 7.18 Shape-selective catalysis (a) reactant, (b) product, and (c) transition state. 

Figure 7.5 (a) Transition state for f 2H-syn elimination, (b) Transition state for fi H-anti elimination, (c) Transition state for E2C elimination. 

The Hammett p values in Table 28, found by changing the p-substituent on the nucleophile in DMF and methanol, support this conclusion. The larger p value in DMF indicates that the change in charge on the nucleophilic sulfur atom is greater on going from the reactant to the transition state in DMF. Therefore, the S—C transition state bond is shorter in DMF than in methanol. 

O and OH products are predominantly scattered in the specular direction (Fig. 14). However, the specular direction in the laboratory frame corresponds to sideways scattering, with respect to the direction of the incident O atom, in the c.m. frame. Sideways scattering suggests a strong interaction between the incident atom and a hydrocarbon group on the surface. In the case of an abstraction reaction, these dynamics indicate a mechanism where the incident O atom reacts through a relaxed collinear 0-H-C transition state. 

Figure 13. Formation and collapse of the tetrahedral oxy-radical intermediate. Optimized structures for A) transition state of the thiyl radical addition to pyruvate (TSl), B) tetrahedral oxy-radical intermediate, and C) transition state of the dissociation of formyl radical (TS2). The energies of these species are 12.3 kcal/mol, 9.9 kcal/mol, and 12.7 kcal/mol, respectively, relative the energy of (methylthiyl + pyruvic acid).

Figure 12 Schematic representation of the proposed cat2dytic cycle [138]. Labels refer to A resting state B protonated intermediate C transition state of the H-abstraction step, D product of the abstraction step, semi semi reduced form, ox oxidized form. (Reproduced with permission fix>m ref [159], Copyright 2000 Springer.)...

Aplot of the oc-carbon KIEs versus the Wiberg Ca-C transition state bond order12 (Fig. 7) shows that all of the calculated KIEs are within 1 % of the largest KIE even though the Ca-C transition state bond orders vary from 0.32 to 0.73, the Ca-Nu transition state bond orders vary from 0.21 to 0.70 and the transition states range... 

Figure 5 Optimized transition states for the hydroiysis of (1S,2S)-/3-methyistyrene oxide (MSO, attack at 01) in the sEH active modei. (a) Schematic representation of the modei (b) Transition state of the alkylation step (c) Transition state for water attack and (d) Transition state for dissociation of the enzyme-product bond.

Figure 28. View of the (a) meso-like and (b) rac-like rotamers of the bis(2-phenylindenyl)ZrCl2 complex and of the (c) transition state connecting them. ...

Fig. 5 Selected geometric parameters (A) of the optimized ri -s/n,ri (C ) transition-state structure TSefc[2] for allylic enantioface conversion and of the optimized rotational q -syn,r (C ) transition-state structure TSiso[3] for allylic isomerization occurring in the [Ni (octadienediyl)(ethylene)] complex. Activation free energies (kcal mob ) are given relative to the [Ni"(q -syn,q (C ),A-cis,-octadienediyl)(ethylene)] isomer of 2...

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