Inversion conformer interconversion

SCHEME 1. Conformational map of the RcRn and RcSn diastereomers of /V-ethyl-iV-methyl-2-aminobutane (EMAB). Interconversions among conformers within dashed boxes are fast on the NMR time scale at 104 K. Those between dashed boxes occur via rotations about the methine carbon-nitrogen bond with barriers which are DNMR-visible. The interconversion between the solid boxes occurs via nitrogen inversion (disstereomeric interconversion). The values in parentheses are MM2-80 results. Reprinted with permission from Reference 71. Copyright (1988) American Chemical Society... 

Equation (4) describes a partition between two forms of the complex one, with fraction f, that exhibits quenching and one that does not. Equation (5) corresponds to a distribution of forms with a range of quenching rates whose breadth varies inversely with n the occurrence of distributions is well-established for proteins at low temperatures [29]. In either case, the non-exponential kinbtics necessitate that conformational interconversion in the transition range is slow compared to the lifetime of the triplet state and the gating limit is applicable. 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

In addition to rotation and inversion, there are other mechanisms by which conformational interconversion may occur. One of these, pseudorotation , is most easily seen in a molecule like phosphorous pentafluoride which adopts a trigonal bipyramidal equilibrium geometry with distinct equatorial and axial fluorines. 

The stereodynamics of N-substituted 2,2,6,6-tetramethylpiperidines 99 (cf. Scheme 37a) were studied by Abraham and Lunazzi et al. (93JCS(P2)1299). The temperature dependence of the NMR spectra showed that a conformational interconversion takes place this could be ring inversion, N-inversion, rotation about the exocyclic bond or a complex combination of these processes. Barriers to this process are shown in Table XII. The rate-determining process changes as the nitrogen... 

Compound Substituents Solvent Conformational interconversion (AG /kcal mol-1) ring interconversion N-inversion (eae to aee)... 

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. 

One of the difficulties encountered when investigating or discussing rotation barriers about C-0 and especially C-N bonds is that this process may be overshadowed by a competitive process, namely inversion. Inversion barriers for simple amines are in the region of 5 kcal/mol [86], thus somewhat above the rotation barrier reported in Table 7 for methylamine. However, bulky substituents may markedly increase the rotation barrier and cause inversion to become the preferred process of conformational interconversion. This is precisely what happens with ferf-butylamines, where inversion (LXXVa-LXXVb interconversion) but not C-N bond rotation (LXXVa-LXXVc interconversion) is observed [87],... 

Figure 33. The conformation interconversion of 3-phospholene, associated with the ring pucking coordinate x and the PH inversion coordinate y. [From M. S. Zhao and S. A. Rice, J. Chem. Phys. 98, 2837 (1993).]...

FIGURE 7 Graph of conformational interconversion paths for NHDOH. Edges represent 90° rotations (r) about the N — O bond or inversions (/) at nitrogen. Planar conformations are represented by solid circles. 

NMR spectroscopy has been widely used in particular to study the conformations and conformational interconversions of reduced pyridazines. The conformations of tetra- and hexahydropyridazines have been extensively studied by several groups in order to examine the influence of steric and electronic effects upon the geometry of the hydrazine part of the molecule. The temperature dependence of their NMR spectra was used to examine ring inversion and nitrogen inversion processes. 

The kinetic processes associated with the conformational interconversions of two conformers of equal energy of [Mo204EDTA] " have been studied and the enthalpy of activation calculated to be 54 kJ moP. The rather high AH is accounted for by the semi-rigid nature of the glycinate chelate rings during the inversion process. 

In the calix[5]arenes it is not yet known whether the p-H compound can engage in upper rim through the annulus conformational inversion, but it seems quite certain that the p-tert-buty compound cannot do so at temperatures ordinarily encountered in laboratory procedures. As anticipated, larger OR groups are necessary to curtail conformational interconversion of calix[5]arenes than for the calix[4]arenes it has been determined that the threshold size requires a group slightly more bulky than n-butyl (AG of penta-n-butyl ether of 51-Bu jYioP for ethers and n-butanoyl (AG for penta-n-butanoate of... 

In general, free radicals are rapidly equilibrating intermediates, which makes stereoselective radical reactions extremely challenging. In ring systems that have little or no conformational bias, reactive radical intermediates can racemize either by a conformational interconversion (i.e., ring flip) or by a simple radical inversion. For simple 2-tetrahydropyranyl radicals, the barrier to radical inversion has been estimated to be <1 kcal/mol, while the barrier to ring inversion is 10 kcal/mol. Therefore, if conformational interconversion is slow relative to reaction of the radical intermediate, then non-equilibrium radical reactions are possible. Recently it has been shown that reduction of 2-tetrahydropyranyl radicals can be competitive with conformational interconversions, which allows for a new strategy for the control of stereochemistry in radical reactions [28]. 

The ring inversion of the six-membered ring in cis-decalin " and cis-perhydroindane " " has barriers of about 12.4 and 7.7kcalmol respectively. The barrier is particularly high in the decalin case, since in the middle of the conformational interconversion both rings must be in unstable twist conformations at the same time. 

Ring inversion (Section 3.9) Process by which a chair conformation of cyclohexane is converted to a mirror-image chair. All of the equatorial substituents become axial, and vice versa. Also called ring flipping, or chair-chair interconversion. 

Ring inversion, leading to interconversion of different ring conformers, is typically as facile a process as single-bond rotation. Particularly important are six-membered rings, where interconversion leads to interchange of axial and equatorial positions. 

Fig. 18. Top transition coordinates (with symmetry species) of conformational transition states of cyclohexane (top and side views). Hydrogen displacements are omitted. The displacement amplitudes given are towards the C2v-symmetric boat form, and towards >2-symmetric twist forms (from left), respectively. Inversion of these displacements leads to the chair and an equivalent T>2-form, respectively. Displacements of obscured atoms are given as open arrows, obscured displacements as an additional top. See Fig. 17 for perspective conformational drawings. Bottom pseudorotational normal coordinates (with symmetry species) of the Cs- and C2-symmetric transition states. The phases of the displacement amplitudes are chosen such that a mutual interconversion of both forms results. The two conformations are viewed down the CC-bonds around which the ring torsion angles - 7.3 and - 13.1° are calculated (Fig. 17). The displacement components perpendicular to the drawing plane are comparatively small. - See text for further details.

Inversion at the N center is coupled to conformational changes in a chelate ring. The kinetics of inversion at asymmetric N centers in complexes of tetraaza linear or macrocyclic ligands have received scant attention. There are five configurational isomers of the planar complex Ni([14]aneN4) +, Sec. 3.1.1. The interconversions between such structures are base catalyzed with second-order rate constants covering a small range from 1.2 x 10 to 2.4 x 10 M- s- Refs. 108-110. 

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