Oxyphosphoranes, pseudorotation

It has also been shown by 31P n.m.r., that pseudorotation of the oxyphosphoranes (4) and (5) involves intermediate structures with a diequatorial disposition of the dioxaphospholene ring, thus violating the ring-strain rule16. 

As in the above case for the alkyl fluorophosphoranes. the H1 nmr spectrum of the oxyphosphoranes possesses features which are only readily interpretable in terms of a pseudorotation mechanism68-66. The H1 spectrum of the cyclic tetraoxyalkylphosphorane synthesized from trimethylphosphate and 3-benzylidene-2,4-pentadione shows two kinds of methoxyl groups at low temperature (—65°C), a single methoxyl signal at room temperature, and a... 

This short discussion does not cover the available literature concerning nmr studies of oxyphosphorane derivatives but is meant merely to furnish experimental examples o the operation of the preference rules and pseudorotation. 

The great flexibility of the trigonal bipyramidal skeleton displayed in the structures of many cyclic oxyphosphoranes suggests that the Berry pseudorotation mechanism may be an oversimplified representation for the permutational isomerisation of such species. Theoretical calculations and statistical considerations combined with these studies lead to the conclusion that this isomerisation may take place by a continuum of different routes (including turnstile rotation pathways) over a broad relatively flat potential surface, which have similar energy barriers. The Berry pseudorotation will, however, represent the energetically most favourable pathway for acyclic derivatives. 

The evidence for Berry pseudorotation and turnstile rotation has been reviewed. Further evidence for the turnstile rotation for spiro-penta-oxyphosphoranes has been presented, based on the lowering of the exchange barrier as the distortion of the t.b.p. of the phosphorane increases. A... 

Interest in oxyphosphoranes derived from hexafluoroacetone continues,and their applications in stereochemical studies concerned with phosphorus(v) compounds have been discussed in the review literature. Information concerning the relative apicophilicities of groups (R) has been provided by n.m.r. studies of the pseudorotation (68) (69) in hexafluoroacetone... 

Intramolecular Processes. A -Ray work has shown the existence of a square-pyramidal structure (12). The n.m.r. data on spirocyclic oxyphosphoranes have been discussed, and were concluded to be consistent with the AT-ray data. Mechanisms for intramolecular exchange processes are proposed. The rate of pseudorotation in XPF4 increases in the sequence MejN < SR, H < Cl < Me, F. It was concluded that even if a square pyramid were an intermediate in pseudorotation, it would not be possible to detect it directly with present-day equipment. However, in a study of the H n.m.r. spectrum of 2-isopropylphenylbis-(4,4 -dimethyl-2,2 -biphenylene) phos-phorane, it is suggested that pseudorotation does not occur by a simple Berry mechanism, but involves a square-pyramidal intermediate. An n.m.r. study of (13)... 

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