Phosphorus Berry pseudorotation

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

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.

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. 

As indicated in Table 10-4, the four pentafluorides are all well known. Phosphorus pentafluoride, which may be prepared by fiuorinating PC15 with AsF3 or CaF2 is molecular, with a tbp structure in the solid state13 and under all other conditions. It is a colorless gas (bp -102°C), which reacts readily with Lewis bases such as amines and ethers, as well as F , to form six-coordinate complexes. The fluxional character of the molecule, whereby the axial and equatorial F atoms change places rapidly on the nmr time scale is generally explained as a Berry pseudorotation. 

Phospholanium salts have been of considerable interest in the last two decades with respect to the stereochemistry of the interconversion of these salts to phos-pholane oxides and phospholanes. These investigations have centered mainly on understandirtg the steric requirements of phosphorus in a five-membered heterocyclic system with respect to the process of Berry pseudorotation and the existence of pentacoordinated phosphorus intermediates. ... 

Physical and Theoretical Aspects.— Phosphorus pentafluoride continues to attract attention from those interested in theoretical aspects of structure, and from those more inclined to test the predictions of others. The most detailed ab initio calculations of the year lead to confirmation of Berry pseudorotation... 

Phosphorus v) Compounds. According to CNDO/2 calculations, the only way in which the axial and equatorial fluorines in PF, can interchange is by the Berry pseudorotation mechanism,and an estimate of the barrier to such an interchange, (1371 cm or 3.92 kcal moP ) has been obtained from a detailed Raman study of the Vj, equatorial bend, of PF.-,. A new normal-co-ordinate analysis for the PF5 molecule has been published. 

Enzymatic phosphoryl transfer reactions are ubiquitous in nature and play significant roles in ATP hydrolysis and protein phosphorylation processess. As previously described, pentacoordinate phosphorus species have been assumed as transient intermediates or transition states in these pathways and their structural and electronic properties are undoubtedly related to the outcome of the process. Therefore, to aid understanding of the phosphorus-catalyzed biological routes, many model pentacoordinated phosphoranes have been synthesized. While most studies have focused on aspects of apicophilicity, anti-apicophilicity or Berry pseudorotation, there have been limited investigations on the stereochemistry of pentacoordinated spirophosphoranes with a chiral phosphorus atom. In the past year, much attention has been paid to the synthesis and determination of absolute configuration of several chiral pentacoordinate spirophosphoranes derived from D- and L-aminoacids. Some significant achievements in this area will be discussed in this section. 

A remarkable feature of this transformation is its total stereoselectivity, with clean retention of configuration at the phosphorus atom, as confirmed by H NMR and X-ray diffraction studies. This fact can be explained by considering a mechanism via pentacoordinated trigonal bipyramidal (TBP) species and a sequential apical addition-pseudorotation apical elimination (Scheme 3.8). Apical attack of the anionic ortho carbon in 14 yields intermediate 15, which rearranges to 15 via the well-known Berry pseudorotation or, alternatively, the... 

Figure 10.4. PF5 (phosphorus pentafluoride) a symmetrical tetragonal pyramid. A representation (using phosphorus pentafluoride) of the process by which axial and equatorial substituents are interchanged by intramolecular tunneling (called a Berry pseudorotation).

The subject of pentacoordinated phosphorus has been considered in depth in two volumes entitled Structure and Spectroscopy and Reaction Mechanisms. The latter volume covers two major topics, namely, a comparison of the Berry pseudorotation process and the turnstile mechanism for explaining the interconversion of rotamers, and the consideration of reactions (from simple to enzymatic) which involve five-coordinate phosphorus intermediates. 

It has been suggested that pseudorotation, i.e. intramolecular ligand rearrangement by the Berry mechanism, should be faster at arsenic(v) incorporated into a five-membered spiroarsorane ring than at phosphorus(v) in an analogous spirophosphorane. However, variable-temperature n.m.r. studies of compounds (22), with R = Me, CHaPh, Ph, p-QHa OMe, or p-CeHi NOa, and (23), with R = Me or Ph, show that barriers are comparable, at around 20—23 kcal mol, for such arsenic and phosphorus compounds. Pseudorotation in AsFs has been discussed theoretically. ... 

The and F n.m.r. spectra of the phosphorane (47) indicate rapid positional exchange of the ligands attached to phosphorus. This has been quoted as evidence for a (2 + 3)-turnstile process of ligand reorganization, the normal Berry (1 + 4)-pseudorotation being held to be impossible in (47) because of increased strain. Details of this work are awaited with interest. 

It is interesting to note that many of the techniques developed in phosphorus chemistry are npw being routinely applied to hypervalent molecules of other elements. For instance, Martin et al. have studied the pseudorotational (Berry) mechanism for the inversion of 10-Si-5-siliconates (1) by 19F n.m.r. and demonstrated a linear correlation between AG for inversion at silicon and the a values of the variable ligand, Y The energy barriers for... 

The very fast Berry and tumstile pseudorotation processes can lead to the presence of a number of stereoisomers. The Berry process occurs through a transition state possessing the C4V square-pyramidal geometry with an energy barrier of ca. 2-3 kcal/mol in the case of phosphorus derivatives.48,49 (Scheme 2.5)... 

Bicyclic phosphoranes (87a-d) prepared by the reaction of (85a-d) with (86) were isolated and then characterised by m.s., elemental analysis, i.r. and multinuclear n.m.r. spectroscopy no pseudorotation was observed at ambient temperature. In a related study, tricyclic phosphoranes (89a-d) were obtained from the reaction of (88a-d) with (86). An analogous reaction between (90a-i) and (86) furnished another series of tricyclic phosphoranes (91a-i). The single crystal X-ray structure of (91a) revealed a tbp configuration about phosphorus with a 26% distortion along the Berry coordinate using P-01 as the pivot. 

This is the geometrical coordinate for the Berry-type pseudorotation of ligands about phosphorus, leading to interconversion between TBP structures. 

The study of pseudorotation in five-co-ordinate phosphorus(v) compounds is undertaken not only out of intrinsic interest but also for its relevance to rearrangements in transition states for nucleophilic substitution at tetrahedral phosphorus(v) compounds (c/. pp. 118—120). Pseudorotation in the stable compound MeaN PF4 takes place via a square-pyramidal transition state. The variation of line widths with temperature for various lines in the n.m.r. spectrum permits a distinction to be drawn between possible processes involving simultaneous interchange of two pairs of fluorine atoms, as required for the Berry mechanism for pseudorotation, or of only one pair. Inversion or pseudorotation in... 

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