Hydroxyphosphoranes

It has been shown that the hydroxyphosphorane (91) has a pK. of 9 - 10 and is in rapid equilibrium with its conjugate anion and the ring opened alcohol.252... 

A full report has now appeared of solvent effects on the rates of hydrolysis of benzyltriphenylphosphonium bromide. The remarkable increase in rate in media of low polarity is largely attributable to a shift of the pre-equilibrium between phos-phonium and hydroxide ions in favour of the intermediate hydroxyphosphorane.129 In a similar vein, a study of medium and deuterium isotope effects on the rate of hydrolysis of tetraphenylphosphonium chloride in acetone-water mixtures has been reported.130... 

Investigation of the influence of the solvent on the kinetics of the reaction corroborate, in the same way as the order of the reaction, the formation of hydroxyphosphorane. Kinetic investigations have been performed with various solvent mixtures EtOH-H2Q601.608,616-620 Me0H-H20610,61 Me0CH2CH20Me-H20583 586,595,621,... 

SCHEME 3. Formation, isomerization and decomposition of the hydroxyphosphoranes from the chiral salt 31... 

Equilibria between hydroxyphosphoranes and phosphoric esters have been claimed to occur as intermediates170-172, or have been shown to exist by trapping the hydroxy compound with diazomethane as above173, or have been displaced by acido-basic variations174, as with 96 and 97 in Scheme 21. 

The first spectroscopic evidence of the simultaneous existence of P(IV) and P(V) in a cyclic phosphate hydroxyphosphorane equilibrium was given by Ramirez et al.115 in a variable-temperature 31PNMR study of 98 and 99 (Scheme 22) at — 39 °C, in acetone-d6 the signals from the two species are narrow, but they broaden at 32 °C and finally coalesce at 55 °C the temperature of coalescence is about 10 °C in acetonitrile-d3. Further, diazomethane yielded the Me ester of 98 and acetyl chloride gave acetylated derivatives of 98 and 99. 

In addition to the possibility of detecting their presence as intermediates during certain reactions, some hydroxyphosphoranes can be isolated as salts or can be detected as unstable intermediates in the course of a reaction178 thus, the phosphorane 102 was prepared179 and the structure of its triethylammonium salt determined by XRD180. 

Similarly, the hydroxyphosphorane 103 with a PH bond was detected during the hydrolysis of the dimethyl-6,6-dioxa-2,8-aza-5-phospha(III)-l-bicyclo[3.3.0]octane181 (Figure 21). 

Pentacoordinate hydroxyphosphoranes are likely intermediates or transition states in substitution reactions at tetracoordinate phosphorus (1). Recently, stable hydroxyphosphoranes (2, 3) and their conjugate bases - metal phosphoranoxides (J, 5) have been isolated. Spectroscopic evidence (J - 7) for equilibria between P(IV) compounds and hydroxyphosphoranes have been reported. Observation (8) and isolation (9) of P(IV) TBP phosphoranide species have also been announced. All these phosphoranes are stabilized by several features dominated by their spirobicyclic nature. 

The phosphites 3 and 4 are oxidized by chlorine and bromine to furnish stable covalent phosphoranes. No Arbuzov reaction was observed. The trichlorophosphorane 5 can be reacted with again to form a spirophosphorane 7 which is hydrolyzed without ring cleavage to give an extremely stable hydroxyphosphorane or a phosphoraneoxide anion . . ... 

Starting with these compounds, we synthetised the corresponding oligomer or dimer hydroxyphosphoranes and adducts with an hexacoordinated phosphorus atom, according to reactions (B) and (C). 

Concerning phosphorus chemistry, the synthesis of condensed compounds, optically active, with pentaco-ordinated phosphorus adopting helicoidal or macrocyclic structures, represents, in our opinion, significant progress. Thus, hydroxyphosphoranes lb, 2b are tautomers of hydroxyphosphoric esters, similar to phosphoric esters of natural polyhydroxylated compounds. 

The crystal structure of the hydroxyphosphorane (88) prepared by N2O4 oxidation of (87) showed an almost perfect tbp structure with the unit cell containing two molecules of the same helicity connected by H-bonds between the P-OH and carbonyl groups.The phosphorus ester (89), fashioned from two n-butyl tartrate moieties exists in solution due to intramolecular hydrogen bonds. On treatment with triethylamine, however, it forms the triethylammo-nium salt (90) of the corresponding hydroxyphosphorane. The pKa value of (89) was determined to be 7.7 in DMF and 4.4 in DMSO, similar to values for dichloroacetic acid in the same two solvents. ... 

Hydroxyphosphoranes, which as such or as their conjugate bases have long been postulated as intermediates in nucleophilic substitutions at phosphoryl centres, are beginning to attract attention. Interconversion of the isomeric phosphinates (5) and (8), via the hydroxyphosphoranes (6) and (7), has been monitored by variable-temperature n.m.r. 17 kcal mol" ) and the intermediates have been trapped as methoxyphosphoranes (9) on treatment with diazomethane. As expected, the... 

H and n.m.r.) when (82 X = O) was treated with alcohols at room temperature the subsequent intermediate (85), which is the result of P-0 and not the expected P-N bond cleavage, was inferred from the results of model reactions. In contrast to (86), the analogue (87) does not show any evidence for a hydroxyphosphorane during hydrolysis, although with alcohols (88) is the stable product. ... 

Further results on the acidities of hydroxyphosphoranes have been published. The p/STa values of the dicarboxy compound (86) were in the range 0—4 depending on the solvent, that of (109 X = CF3) at 2, whilst calculations on tetrahydroxyoxyphosphorane indicated that they should have p Ta values of 10—20. The acidity of the hydroxyphosphorane (110) has been compared with other acidic phosphoranes. ... 

The five coordinate spirophosphoranes have played an important role with respect to understanding the stereochemistry of four coordinate phosphorus reactions which proceed via phos-phorane intermediates. Variable temperature NMR studies have established the apicophilicities of a wide range of groups (i.e. relative preferences of a series of groups to occupy an apical position rather than an equatorial position of a trigonal bipyramidal (t.b.p.) conformation of a phosphorane). The spiro ring system has been used to stabilize transient reaction intermediates of acyclic compounds, for example hydrolysis intermediates such as hydroxyphosphoranes. 

Chlorospiro-phosphoranes, under the action of Lewis acids such as SbCls, may be converted to the quasiphosphonium salt, for example (16) <88Zaac(561)49>. Hydroxyphosphorane (42 R = OH, Y = CMej) is converted to the spiroquasiphosphonium triflate (14) by the action of trifluoro-methanesulphonic acid <78JA5229> this in turn can be reduced by LAH to the phosphoranide (42 R = electron pair, Y = CMcj). 

Since no pK values are available for hydroxyphosphoranes, the pK of pentahydroxyphosphorane is estimated by the method of Branch and Calvin (1941) as 8.5. The application of the free-energy change for reaction (4) to... 

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