Cyclopentane pseudorotation

Pitzer strain in any planar cyclic alkane makes that conformation higher in energy and relatively unstable, and the molecule will not exist in that form if pseudorotation leads to a lower energy conformation. In the case of cyclopentane, pseudorotation about the carbon-carbon bonds alleviates the Pitzer strain and leads to a twisted structure such as 47B. This structure is discussed in more detail later, but for the moment note that the eclipsing interactions are largely removed, and the conformation is much lower in energy when compared to 47A. The space-fllling molecular model of this pseudorotated structure is 47D, where the relief of Pitzer strain is apparent when compared to 47C. 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Electron diffraction data on gaseous cyclopentasilane (61) fit both the C2 and C, models. Rustad s simple MM method calculates virtually equal energies for these conformers, and 61 is likely to undergo pseudorotation, as does cyclopentane (200). 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Die Konformation des Fiinfringes (Cyclopentan- oder Tetrahydrofuran-ring) ist normalerweise nicht festgelegt. So findet z. B. im unsubstituierten Cyclopentan eine Pseudorotation seiner Methylengruppen um eine S5tu-metrieebene statt. Zwei bevorzugte Konformationen, die hierbei durch-... 

Sometimes the barriers to internal rotations are very small, allowing for almost unhindered movement. This is the case in cyclopentane, which executes pseudorotation [49], and in toluene C6H5CH3 with a barrier of 0.014 kcal/mol [50]... 

In contrast to the findings for the cyclobutanes where the large amplitude motions mainly consist of conversion between rather rigid forms, the cyclopentanes exhibit more complex conformational and dynamic properties. Pseudorotation is a prominent large amplitude motion prevailing not only in cyclopentane but also in other five-membered rings. If the barrier to pseudorotation is high, distinct conformations may exist. In this case, the envelope conformation which has maximum Cs symmetry... 

Cyclopentane was studied as early as in 1931 by Wierl258 who found that the molecule was planar. In 1946 Hassel and Viervoll259 found that the carbon ring deviates slightly from planarity. This has later been confirmed by repeated investigations241, 26°. Spectroscopic studies239 have shown that this molecule undergoes nearly free pseudorotation. 

J The Berry mechanism in the case of PF5 gives the impression that the whole molecule has been rotated, but since in Berry s definition there is no rotational motion, the process was named pseudorotation. The term pseudorotation was originally applied to the rapid up-and-down motion of the carbon atoms in cyclopentane, and later extended to the puckering of rings in general see Ref. 54. Objections can be raised to this duplication in terminology, and in this review we denote the Berry mechanism simply as BPR. 

The two conformers LXXVlIIa and LXXVIIIb are extremes of symmetry in what is known as the pseudorotational circuit of cyclopentane. If in structure LXXVlIIa, the out-of-plane carbon (arbitrarily designated C-l here) is pushed down together with C-2, a half-chair is obtained (C-l above, C-2 below the plane). If the motion is continued, another envelope is reached (C-2 below the plane). The process then repeats with C-2 and C-3, and so on 10 envelope forms and 10 half-chair forms interconvert by this process, which is not unlike a wave on a water surface ([100], and refs, therein). 

The pseudorotation circuit of cyclopentane is essentially of constant strain, and therefore without maxima and minima in contrast, the fully planar conformer is less stable by ca. 5 kcal/mol [101]. An extensive treatment of the conformational aspects of cyclopentane and of many five-membered-ring compounds has been published by Legon [92]. 

The above descriptions well simulate the pseudorotation of the cyclopentane, (CH2)s, molecule (Figure 3-46), although on a different time scale. This molecule has a special degree of freedom when the out-of-plane carbon atom exchanges roles with one of its two neigh-... 

Figure 3-46. Two models of the cyclopentane molecule from its pseudorotation.

The term pseudorotation was first appUed to cyclopentane like inversion, it has an atomic analogue in 5-coordinate compounds (e.g. PF5). ) The name means false rotation , and it is therefore appropriate for any conformational process which results in a conformation superposable on the original, and which differs from the original in being apparenUy rotated about one or more axes. Pseudorotation, in analogy with real molecular and internal rotations, can be free, as in cyclopentane, or more or less hindered, eis in cycloheptane and higher cycloalkanes. In moderately to severely hindered pseudorotation, it is appropriate to consider distinct stable conformations which are pseudorotation partners, and these cases are often amenable to study by dynamic nmr methods. When the barrier to pseudorotation is very low, or in the limit when pseudorotation is free, it is not really justified to talk about separate stable conformations (e.g. the C2 and Cg forms of cyclopentane), because strictly there is only one conformation, and the pseudorotation is simply a molecular vibration. 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

Pseudorotation The progression of one conformer of a five-membered ring to another conformer. In the case of cyclopentane there is no planar intermediate all conformers have at least one carbon atom out of the plane of the other carbon atoms. The maximum pucker can, in this case, rotate with almost no potential energy barrier between conformers. Each of the multitude of possible conformers can be described in terms of the maximum pucker and the pseudorotation phase angle, that is, where the conformer lies on a pseudorotation cycle (with an arbitrarily chosen origin). 

The symmetric puckered conformations of cyclopentane are the Cs symmetric envelope (E) (10) with four carbon atoms in a plane and the C2 s)mrmetric twist (T) (11) with three carbon atoms in a plane [88]. Unlike cyclohexane, these conformations are of almost equal energy and are separated by barriers of about RT or less [89]. There are ten envelope conformations, each with one of the five carbon atoms out of the plane in one of the two directions, and ten corresponding twist conformations. The individual conformations freely exchange which atom or atoms are out of the plane, a process termed pseudorotation, and the whole sequence of conformations is called the pseudorotational itinerary (O Fig. 9). 

The cyclobutane ring exists in a non-planar, open-book conformation, which may flex further open or shut. The cyclopentane ring exists in two puckered conformations, the half chair and the envelope, both of which are non-planar. The position of the puckering may migrate around the ring, and this is called pseudorotation. 

Cyclopentane adopts an envelope, or puckered, conformation the puckering rotates rapidly around the ring in a process called pseudorotation. 

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