Free energy conformation rotational barriers

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

Asymmetrically substituted thioamides 24e-h exist in equilibrium between rotamers 24-1 and 24-11. The NMR spectrum of AT-benzyl-iV-methylmeth-acrylthioamide 24e showed the distribution of 24e-I 24e-II was 55 45 (Scheme 13 and Table 8) [47]. The free energy of activation of the rotational barrier was measured by the temperature-dependent NMR spectra in DMSO at various temperatures. The free energy of activation of the rotation of the (C=S)-N bond could be estimated as 22.7 kcal mokh Furthermore, Table 8 shows the ratio of I II, and all three thioamides 24f-h favor the conformation I with the situation that a benzyl group was placed closely to the thiocarbonyl group on... 

Conformational isomerism, as already defined (Section 3.b), is a property of stereoisomers separated by a low barrier of energy. The separation of isomers at room temperature requires half-lives of several hours, which correspond approximately to a free energy of activation of AG > 20 kcal/mol [56]. An operational and convenient definition of conformational isomerism is thus to consider as conformers those stereoisomers which are not physically separable under ordinary conditions, in other words, which are separated by an energy barrier lower than 20 kcal/mol. Such a definition is further useful in that it sets no conditions as to the chemical process by which conformer interconversion occurs while bond rotation is the most frequently encountered interconversion process, inversion processes are also important. 

The prismatic cyclopentadienyl and benzene complexes of transition metals are reminiscent of the polycyclic hydrocarbon pris-manes. Figure 3-28 shows ferrocene, (C5H5)2Fe, for which both the barrier to rotation and the free energy difference between the prismatic (eclipsed) and antiprismatic (staggered) conformations are very small [75], Figure 3-28 also presents a prismatic model with Ay, symmetry for dibenzene chromium, (C6F[6)2Cr. 

TABLE 1. Free energy barriers (AG in kcal mol 1 at T K) to rotation about C1—N and C2—(thio)acyl bonds and E conformer population (pE) in simple 2-acyl- and 2-thioacylenamines... 

TABLE 3. Free-energy barriers (kcal mol 1 at T K) to C=C and C—N rotations and E conformer populations (pE) for compounds of the general type A1A2C=C(R)NR1R2... 

On the basis of C satellite observations and iterative computer calculations it has proved possible to make tentative assignments of conformation for 1,3-dimethylimidazolidines. At about -150 °C it was possible to detect both cis (15a) and trans (15b) isomers the free energy barrier to rotation is approximately 6.2 kJ moF In such compounds typical shifts are 41.8 (CH3), 55.0 (CH2—CH2) and 80.2 (NCH2N) p.p.m. (790MR(12)362, 71X2453). 

Rotational barriers and conformational equilibria of 1,3-diphenylallyl lithium and a series of 2-substituted 1,3-diphenylallyl lithium compounds have been determined by Boche and coworkers 18). Table 3 summarizes the amounts of endo,endo, endo,exo-and exo.exo-conformers 27 and the free energies of activation of their mutual transformations. 

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