Pseudorotating isomer

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

It has been shown that alkaline hydrolysis of both isomers of (91) gives the same ring-expanded oxide (92) of unknown geometry, presumably because rapid pseudorotation leads to equilibration of the intermediates. Alkaline hydrolysis of the dialkoxyphosphetanium salts (93) proceeds with almost complete lack of stereospecificity. In this case the authors... 

O-as- S p i ro p h os p h or a n e s 69, which display anti-apicophilicty, exhibit enhanced reactivity toward nucleophiles compared to 0-/ra r-spirophosphoranes 70 for example 69 produces hexacoordinate 157 on treatment with MeLi at 0°C while 70 is unreactive under the same conditions (Scheme 16). Compound 69 (R = Bn) is more reactive toward bases than 70 (R = Bn) with KHMDS deprotonating 69 (R = Bn) in THF at 0 °C but not 70 (R= Bn). -BuLi was sufficiently basic to deprotonate both 69 and 70 <2002JA13154>. Further differences are observed in the stability and stereochemistry of the brominated products formed between the reaction of the anion with BrCF2CF2Br. The effect of the bromide substituent in the O-m-derivative is such that it accelerates pseudorotation to the O-trans-isomer (Scheme 17). 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

Bestmann et al.298 showed that there was a fairly small energy difference between the pseudorotational isomers of the oxaphosphetane from H3P=CH2and H2C=0. 

Fig. 7. Itinerary for permutational isomerizations by single turnstile rotation, or by Berry pseudorotation, when the five ligands in trigonal bipyramidal phosphorus are different. Isomers are denoted by Permutational Notation.

Pentacoordinate centers are mainly exemplified by phosphorus, whereas some pentacoordinate sulfur derivatives exist, and a few other elements can be envisaged [36], In the case of phosphorus derivatives (phosphoranes), the trigonal-bipyramidal arrangement is the low-energy geometry, while interconversion of isomers occurs by pseudorotation through the tetragonal-pyramidal transition state [23,36]. 

Tetravalent phosphorus radicals have the geometry of an incomplete trigonal bipyramid in which the p-orbital on phosphorus containing the unpaired electron can occupy an equatorial or axial position, giving rise to conformational isomers. These can interconvert by bending motions of the substituents, as in Berry pseudorotation (Scheme 21), or by pseudoinversion [223]. Although tetravalent 9-P-4 phosphorus radicals have been studied quite in detail by ab initio theory [224], there are only scant experimental data describing their properties. 

With X Y and R H, formation of isomers has been observed equilibration occurs on heating. Pseudorotation involves a trigonal-bipyramidal intermediate with the four-membered ring in the diequatorial position (221). 

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