Transition state conformation and

The volume of activation should be formed by an equilibrium (quasi-ther-modynamic) part related to the TST rate, plus an extra part, Vj, originating from the dynamic interaction with the solvent may be estimated with reasonable assumptions about the transition-state conformation, and thus access is gained to that is, the pressure dependence of k/ Tsr- pressure dependence is felt through the friction. 

Use SpartanView to examine bond-rotation sequences about the C2-C3 bond in both 1-butene and 1,3-butadiene. Compare the energies of the lowest-energy and transition-state conformations, and tell in which molecule rotation is more difficult. Identify the two minimum-energy conformations of 1,3-butadiene, and tell which geometry permits Diels-Alder cycloaddition. Is this the preferred geometry ... 

The product ratio is therefore determined not by AG but by the relative energy of the two transition states A and B. Although the rate of the formation of the products is dependent upon the relative concentration of the two conformers, since AGJ is decreased relative to AG to the extent of the difference in the two conformational energies, the conformational preequilibrium is established rapidly, relative to the two competing... 

The transition state for such processes is represented as two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic transition state results, and the process is thermally allowed. Usually, a chairlike transition state is involved, but a boatlike conformation is also possible. 

Regioselectivity in opening of a-functionalized epoxides by treatment with trimethylamine dihydrofluoride has been observed. Thus, cw-isophorol epoxide gives exclusively 3-fluoro-l,2-diol, whereas from the trans isomer, 2-fluoro-l,3-diol is obtained as the main product together with 3-methylenecyclohexane-l,2-diol. This behavior has been discussed m terms of the influence of a-substituents on the transition state conformations [7 J] (equations 11 and 12). 

Additionally, it was found that the energy difference between the two transition states (3 and 4) is determined mainly by the difference in the conformational energy of the a-chloro aldehyde in the two transition states i.e., the energetic preference of transition state 3 over 4 is due to a more favorable conformation of the aldehyde rather than a more favorable interaction with the attacking nucleophile. In fact, interaction between lithium hydride and 2-chloropropanal stabilizes transition state 4, which yields the minor diastereomer. 

We can conclude that anti elimination is generally favored in the E2 mechanism, but that steric (inability to form the anti-periplanar transition state), conformational, ion pairing, and other factors cause syn elimination to intervene (and even predominate) in some cases. 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

Figure 1.6 Schematic representation of the changes in protein conformational microstate distribution that attend ligand (i.e., substrate, transition state, product and inhibitor) binding during enzyme catalysis. For each step of the reaction cycle, the distribution of conformational microstates is represented as a potential energy (PE) diagram.

Intramolecular cyclopropanation is a useful method for construction of [n.l.0]-bicyclic compounds.17-21 225 275 As a matter of course, alkenyl and diazo groups of the substrate are connected by a linker and the transition-state conformation of intramolecular cyclization is influenced by the length and the shape of the linker. Thus, the enantioselectivity of the reaction often depends upon the substrates used. Use of a catalyst suitably designed for each reaction is essential for achieving high enantioselectivity. 

The stereochemical outcomes of the above reactions can be explained by the proposed transition states A and B (Fig. 2.25). Model A, derived from the Houk model for nucleophilic addition to olefins, explains the formation of, v y -adducts. Model B, involving a different nitrone conformation, due to the chelation of diethylaluminum chloride, accounts for the formation of anti -adducts (581). 

However, a quite different situation is observed for ene nitroso acetal (423). First, a stable conformation and, correspondingly, new transition states TSR1 and TSR1 with a low barrier (8.8 kJ/mol) appear on the rotation coordinate about the C,N bond at the place of the transition state of enamine (422). Therefore, the barrier to rotation about the C,N bond decreases so that the process is fast on the NMR time scale and cannot be detected by this method. 

As shown above, C7 has the same type and number of bonds as C4, differs only in conformation, and is 16 kcal/mol less stable than C4(B13). One may expect that the products of reaction C7 and C4 with another H2 molecule also will be different from each other by conformational changes. Therefore, we will not discuss the geometries of transition state C8 and product C9. Here, we would only like to point out that the C7 + H2 —> C8 — C9 reaction takes place with an 18.1 kcal/mol barrier and is exothermic by 10.0 kcal/mol. 

The basic assumption of the chair-preferred transition state (for tetrahedral metal centers) is clearly tenuous, and diastereomeric boat transition state geometries should not be discounted. For example, the diastereomeric chair and boat transition states for (Z)-enolates are illustrated in Scheme 4, For this enolate geometry it is entirely reasonable to consider that the heat of formation of boat transition state B2 might actually be less than chair transition state C4 for certain combinations of substituents Ri, R2, and R3. For example, boat transition state B2 not only disposes substituents R2 and R3 in a staggered conformation as in chair transition states C3 and C4, but also minimizes Rj R3 eclipsing, which must be significant in chan-transition state C3. The change in kinetic aldol diastereoselection of... 

---