Rearrangement pseudorotation mechanism

Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudorotation in permutational isomerization, 9, 25 Photochemistry of aryl halides and related compounds, 20, 191 Photochemistry of carbonium ions, 9, 129 Photosubstitution, nucleophilic aromatic, 11, 225... 

The pseudorotation phenomenon is vital to structural chemistry of phosphorus, silicon, transitional metals, and some other elements. Apart from pseudorotation, there are other rearrangement mechanisms, for example, trigonal twist (Fig. 48a) examined in dynamic stereochemistry of icosahedral carboranes, tourniquet mechanism (Fig. 486), etc. 

Because the rearrangement of a trigonal bipyramid is so widely important, it is useful to have a graphical representation of how the various permutations of vertex atoms are interrelated by the relevant pathway for rearrangement, which is assumed to be the Berry mechanism or pseudorotation just discussed. Necessary but not sufficient criteria for defining such a graph are ... 

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... 

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... 

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. ... 

General.—Possible routes for intramolecular rearrangements of four-, five-, and six-co-ordinated structures have been reviewed. The isomerization pathways are represented by Schlegel diagrams these representations are correlated, where relevant, with specific mechanisms such as those discussed by Muetterties, and the Berry pseudorotation process for five-co-ordinate species. 

---