Axial-equatorial interchange

The nature of pseudorotation restrictions of stable five-coordinate phosphorus compounds has been studied , 23,24) pSeudorotations which move groups into restricted positions do not occur. Pseudorotations involving axial-equatorial rings occur by interchange of the axial for the equatorial group. However, only those pseudorotations about the leaving group as pivot alter the stereochemical outcome of substitution. 

The axial-equatorial conversion is generally quite rapid for M(L)j systems, but a series of M(P(OR)3)j complexes have rates that are accessible on the NMR time scale. Meakin and Jesson" found values for Aft of 8 to 12 kcal mol" and showed that the process required the simultaneous interchange of two axial and two equatorial substituents. The pseudorotation mechanism suggested by Berry," and shown in Scheme 4.4, is consistent with the observations. The equatorial e and e" ligands become axial, whereas the axial a and a become equatorial. 

The stereochemistry of spiroacetals (both synthetic and naturally occurring) can largely be divided into two categories—the so-called anomeric or axial/axial conformers, and non-anomeric or axial/equatorial conformers. In 6,6-spiroacetals, four possible conformations exist (Fig. la-2d) with varying relative stabilities, which are interchangeable via ring flipping [2]. 

Figure 12.13 Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

P-F 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol". ( ... 

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. 

FIGURE 14.8 The Berry pseudorotation that leads to interchange of axial and equatorial groups. The letters a, e, A, etc., indicate only positions because all five groups are identical. 

Valuable information on conformational equilibrium can be obtained particularly by N.M.R. technique. When a molecule can exist in several conformations which rapidly interchange, then any proton which assumes all possible positions in a very short time, the n.m.r. spectrum would show only one peak. This happens in most open chain compounds and even in cyclohexanes where the interconversion is very rapid. But if the interconversion is slowed or prevented, either by cooling or due to the inherent structure in the molecule, the hydrogens of each conformer appear separately and so more than one peak would appear. For example by cooling cyclohexane to -110°C, two peaks appear, one due to equatorial and the other to the axial hydrogens. 

However, equatorial and axial fluorines rapidly interchange. The two out-of-plane equatorial fluorines bend outward at the same time as the top ar/a/fluorine bends downward and the bottom axial fluorine bends upward. 

The designation, equatorial or basal, has been applied to the three shorted bonds, equatorial appears to be the more widely acid connotation. Similarly the terms apical and axial, are used interchangeably. 

Perhaps more subtle is a molecule such as PF3 (Fig. 6.le), which has symmetry. The three equatorial fiuonne atoms can be interchanged by reflection or by rotation about the C3 axis. Similarly, the two axial atoms can be reflected or rotated into each other. However, no operation allows interchange of an axial and an equatorial fluorine atom. Thus we have two sets of symmetry (and chemically) equivalent fluorine atoms. As a consequence, we would not expect P—F,v bond lengths to be the same as P—Teq bond lengths (and they are not), nor would we expect the flve fiuonne... 

The studies of cyclohexane and its derivatives by Hassel and co-workers in the late thirties and early forties using mainly the electron diffraction method laid the foundation of conformational analysis. In 1943 Hassel295 summarized that cyclohexane exists mainly in the chair conformation as distinct from any other possible conformation. The chair conformation will have distinguishable axial, a, and equatorial, e, substituents. (See Fig. 13). The equatorial position is the energetically favored one. Furthermore, Hassel stated that there is a rapid inversion of the ring with an associated low barrier. This motion interchanges the a and e positions with the result that a and e conformers cannot be isolated. 

The averaging sets can be correlated with rearrangement mechanisms (Section III.A) which give the same site interchange as follows p-C3 or r-C3 twists, A6 i-C3 twists, A3 TBP-axial, A7 TBP-equatorial, A3 SP-axial, A3 + A6 + A8. 

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