Oxyphosphoranes

By methods analogous to those used for the tetrahedral intermediates related to carboxylic acid derivatives, Guthrie proceeded from the heat of formation of pentaeth-oxyphosphorane to free energies of the P(OEt) (OH)5 species. °° This allowed the calculation of the equilibrium constants for addition of water or hydroxide to simple alkyl esters of phosphoric acid see Table 1.7. 

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. 

The pioneering work of Denney et ai19 on the synthetic utility of oxyphosphoranes has been thoroughly exploited by Evans et al. in demonstrating that diethoxytriphenylphosphorane promotes mild and efficient cyclodehydration of diols (e.g. 11) to cyclic ethers (e.g. 13) via the cyclic phosphorane (12)20>21. Simple 1,2-, 1,4-, and 1,5- diols afford good yields of the cyclic ethers but 1,3-propanediol and 1,6-hexandiol give mainly 3-ethoxy-l-pro-panol and 6-ethoxy-l-hexanol respectively whereas tri- and tetra-substituted 1,2-diols afford the relatively stable 1,3,2- diox-phospholanes. In some instances (e.g. 14), ketones (e.g. 16) are formed by a synchronous 1,2-hydride shift within (15). The synthetic utility has been extended to diethoxyphosphoranes supported on a polystyrene backbone22. 

There have been a number of hydrolysis studies. The mechanism of alkaline hydrolysis of phenyl dimethylthiophosphinate has been compared to that of phenyl acetate.27 Evidence for the formation of five coordinate oxyphosphorane intermediates in the alkaline hydrolysis of aryl diphenylthiophosphinates is based on the lack of development of negative charge on the leaving group in the transition state.279... 

Doubts have recently been expressed regarding the validity of the metaphosphate pathway for hydrolysis of the monoanion of 2,4-dinitrophenyl phosphate (111) 70,71,72) since the basicity of the 2,4-dinitrophenolate group is insufficient to produce a zwitterion corresponding to 106 or even a proton transfer via intermediates of type 103 or 105 (pKa values in water 4.07 for 2,4-dinitrophenol, 1.0 and 4.6 for 2,4-dinitrophenyl phosphate). Instead, hydrolysis and phosphorylation reactions of the anion 111 are formulated via oxyphosphorane intermediates according to 114. 

Evidence for the participation of oxyphosphorane intermediates in the reactions of monoanion 111 comes from ... 

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

Less is known about unequivocal reactions of the POf ion with amines. This is partly because the phosphate esters examined undergo direct SN2(P) reaction with amines via oxyphosphoranes or can at least react in the boundary area between SN1 (metaphosphate mechanism) and SN2(P)37) or because the reaction actually occurs at another part of the molecule (simple primary and secondary amines react with 2,4-dinitrophenylphosphate to give 2,4-dinitroanilines " ). 

The study of [4.4.0] ring systems has resulted primarily from the study of various other aspects of phosphorus chemistry. An investigation into the effect of nitrogen donor action on the increase in coordination at phosphorus in a series of oxyphosphoranes led Holmes and co-workers <1998IC4945> to compounds 24 and 25. The compounds were fully characterized by NMR spectroscopy and X-ray diffraction. Compound 24 was heated for 30 min at 140°C in an NMR tube. The reaction was followed by 31P NMR spectroscopy which indicated that conversion to a phosphorane 26 and a small amount of phosphate had taken place (Equation 4). The pentaoxyphosphorane 25 was successfully produced via an oxidative addition reaction between the diol 27 and triphenyl phosphate in the presence... 

Preparation of 2,2,2-trimethoxy-3-phenyl-4-acetyl-5-methyl-A4-oxaphospholene — Preparation of an oxyphosphorane by reaction of a trialkyl phosphite with an a,(l-unsaturated carbonyl compound... 

Preparation of 2,2,2-triethoxy-2,2-dihydro-5-methyl-l,2A,5-oxaphospholene —Preparation of an oxyphosphorane... 

These developments are derived from the early work of Ramirez and coworkers16 17, which was concerned with the formation of a type of cyclic oxyphosphorane that is formed by the addition of a trialkyl phosphite to either a diketone (Figure 5.8) or an a, 3-imsaturated carbonyl compound (Figure 5.9). Initially, developments from this effort... 

Figure 5.9 Formation of an oxyphosphorane by the addition of a phosphite to an a, fi-unsa tu ra ted carbonyl compound.

A particular application of this type of synthesis involving aldehyde addition to the intermediate oxyphosphorane is in the preparation of... 

Figure 5.10 Continuing reaction of an unsaturated oxyphosphorane with an aldehyde.

Reaction of the oxyphosphorane intermediate with arylisocyan-ates has also provided an approach to uracil phosphonate derivatives via a double addition of the arylisocyanate (Figure 5.15).49 Monoaddition products are also isolated in this system. 

Figure 5.11 Approach to neocnidilides proceeding through an oxyphosphorane.

Figure 5.12 Preparation of fS-phosphono-a,fS-unsaturated carbonyl compounds from oxyphosphoranes.

Figure 5.14 Reaction of azodicarboxylates with oxyphosphorane intermediates.

Figure 5.15 Reaction of arylisocyanates with oxyphosphorane intermediates.

Alkaline hydrolysis of the salt (133) occurs with exocyclic loss of phenyl, presumably via an oxyphosphorane in which the ring system spans diequatorial positions.128... 

Y = Cl dp +131 p.p.m.). Formation of six-co-ordinate compounds from spiro-oxyphosphoranes has been reported.41 The chemical shifts are to high field of the corresponding oxyphosphoranes. 

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