Pseudorotation five-membered ring

In the area of theory and physical methods there has been a suggestion that five-membered rings may pseudorotate via top structures possessing a diradical five-membered ring. The introduction of ortho and para substituents to all the phenyl rings of triphenylphosphine dramatically increases the basicity, the resulting compounds being more basic than piperidine. 

Intramolecular five-membered ring formation occurs in the alkaline-catalyzed hydrolysis of a series of esters of 2-hydroxypropylphosphate, where a linear free-energy relationship is found for the rate of alkoxide expulsion as a function of the pKa of the alcohol. No evidence for phosphoryl migration was obtained consistent with the fact that pseudorotation of the presumed intermediate leading to ring opening would be inhibited (50), viz. 

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... 

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]. 

Furanose flve-membered rings are never planar. They adopt a puckered form for which a family of pseudorotational conformers [541] are possible. The main conformations are the envelope and twist forms denoted by atom names, Le., C(2 ) and C(3 ), and descriptors endo and exo. They refer to the displacement of these atoms from the mean plane through the other atoms of the five-membered ring, with reference to the exocyclic C(4 )-C(5 ) bond (Fig. 17.4). 

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). 

Laane, J. Pseudorotation of five-membered rings,Chap. 2, in Vibrational spectra and structure, Vol. 1. Durig, J. R. (ed.). New York Marcel Dekker, Inc. 1972, pp. 25-50... 

F. A. A. M. de Leeuw and C. Altona, Computer-assisted pseudorotation analysis of five-membered rings by means of proton spin-spin coupling constants Program PSEUROT, J. Comput. Chem., 4 (1983) 428-437. 

Stereochemical data exist for a large number of displacement reactions on neutral phosphoric acid and thiophosphoric acid esters and amides (64). Many of these occur with retention of configuration, especially when the displacement reactions occur on five-membered ring compounds, and this stereochemical course is explained by pseudorotation of a pentacoordinate intermediate. In contrast, only one stereochemical study has been performed on an anionic phosphate ester with the intention of detecting the participation of a phosphorane intermediate. 

The first-order process in equation 24 for 53 was interpreted as a pseudorotation mechanism. It was proposed that the second-order pathway proceeds through penta-coordinate species as in equation 27. The four-membered ring compound was 800 times faster than the five-membered ring compound which, in turn, was about 700 times as reactive as the corresponding acyclic compound. Since the ring is diequatorial in 55, it is... 

The choice of suitable coordinates to describe the puckering of a five-membered ring has been the subject of innumerable papers. The essential points were formulated in the classic paper introducing the concept of pseudorotation [12] ... 

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