Oxyphosphoranes structure

Reaction with aldehydes.1 The reagent (1) reacts with propionaldehyde to form one diastereoisomeric form of a cyclic saturated oxyphosphorane structure with the 1,3-dioxaphospholane ring system (2). The product is hydrolyzed by water in benzene solution to give an erythro-a,/8-dihydroxy ketone (3). Compare 1,1233. 

Simple aliphatic aldehydes are also said to yield products which contain the equivalent of 2-3 mol of RCHO per phosphorus atom, some of which have been shown to have an oxyphosphorane structure. (For an introductory account of this fascinating area of phosphorus chemistry, the reader should consult early reviews by Ramirez . ) The phosphite ester obtained from 2-hydroxybenzaldehyde and diethyl chlorophosphite cyclizes with rearrangement to the l,2-benzoxaphosph(V)ole derivative 205 (R = H, R = EtO), and a similar process was observed in the case of phosphites and phosphonites derived from 2-hydroxyacetophenone to give 205 (R = Me) . 

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. 

B. Molecular Structure of Oxyphosphoranes from, X-Ray Analysis 1. Five-membered oxyphosphoranes... 

Scheme 3 contains several of the fundamental structures with penta-valent phosphorus, among them phosphorane itself, PH6 (21), and pentahydroxyphosphorane, P(OH)6 (26). The latter is the hydrate of phosphoric acid H3P04+H20 - P(OH)5. A hydrate of methylphos-phonic acid (CH3)(HO)4P (29), is also included as a model for its esters. Mono- and di-ionized forms, (27), (28), (30), are also given, since their stabilities in various isomeric forms provide important data concerning the role of intermediate oxyphosphoranes in the chemistry of phosphoric acid and its derivatives. A model compound of the 1,2-oxaphospholene ring (31), is provided, since this system, in the form of several derivatives, will be discussed extensively in Section VIII. 

A third type of five-membered cyclic oxyphosphorane is represented by derivatives of the 1,3,2-dioxaphospholene -10,12,18 and the 1,3,2-dioxaphospholane ring systems. These compounds are made from the reactions of trivalent phosphorus compounds and certain mono- and dicarbonyl functions, (73).J5 Some a-diearbonyl structures furnish... 

It has, however, been postulated by Ramirez et al37,89), in the development of their oxyphosphorane concept, that four-/five-membered rings are placed exclusively ae in the trigonal bipyramid even for transitory intermediates. According to their turnstile rotation mechanism an ee intermediate would not, therefore, be involved in the permutational isomerisation of such derivatives. However, a recent structural study of a tricyclic derivative [(I) in Fig. 2.7.1.] has demonstrated that the equatorial angle at phosphorus may be narrowed sufficiently to allow a four-membered ring to be placed ee. This enables one to accept the findings of Bone et al more readily. 

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. 

Five- and six-coordinate compounds. Evidence for an increase in coordination geometry to pseudo-trigonal bipyramid and TBP in respective cyclic phosphites and phosphates containing sulfur via sulfur donor action) has been illustrated using P NMR spectroscopy and X-ray diffraction studies, e.g, 19, whereas in a cyclic phosphite (6) with a methylene group in place of the sulfur atom this was not observed." The sulfonyl-substituted oxyphosphoranes 20-24 have been examined structurally by NMR spectroscopy and X-ray diffraction. P and H NMR spectral data indicated the presence of two isomeric forms for each of the phosphoranes 20-22. 

Since then we have carried out X-ray and NMR studies (8-13) of a variety of cyclic oxyphosphoranes containing ring sizes from five- to eightmembered in an attempt to learn structural and conformational preferences as ring size varies and to understand what the important factors are that may induce structural and conformational changes. The results should prove useful in modeling proposed intermediates and activated states in a host of reactions that have been studied and for ones that may be developed in future work. 

An interesting observation is found in the first X-ray structures (12, 32, 33) of oxyphosphoranes containing trans-fused five-membered rings. Trans annelation of the ribose component is a property of cAMP. 

The above discussion emphasized phosphorinane ring orientation in isolated and structurally characterized cyclic oxyphosphoranes and their relation to proposed P activated states in enzyme reactions of cAMP. Here, we concentrate on ring conformation and its projected role in cAMP interactions based largely on our recent structural work and preliminary investigations of new systems. 

Recently, we synthesized the related phosphorane W and showed the formation of the (e-e) ring orientation by X-ray analysis (Prakasha, T. K. Day, R. O. Holmes, R. R., unpublished work). This represents the first solid state structural evidence of a diequatorial ring orientation in any cyclic oxyphosphorane system. However, when the phenyl ring substituents are replaced by protons. X-ray analysis of X (Prakasha, T. K. ... 

Only one other study exists suggesting (e-e) ring orientation, a recent NMR study by Broeders et al. (36) on the interesting bicyclic oxyphosphorane Y which has a ribose component trans-annelated to a phosphorinane ring, as in cAMP. Indirect evidence from an orientational effect indicated to be present in the axial acyclic attachment suggests that isomers of Y with an (e-e) and (a-e) ring orientations coexist in solution. It would prove worthwhile if this or related derivatives can be isolated and structurally identified to verify this structural indication. 

In conclusion, we may expect further advances in understanding factors controlling structural and conformational preferences of cyclic oxyphosphoranes, both from an experimental as well as a theoretical point of view. As the area continues to develop, the use of oxyphosphoranes as models in mechanistic interpretation should enhance our understanding of pathways followed in nucleophilic displacement reactions of tetracoordinated phosphorus compounds. Acknowledgment... 

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